Humble Beginnings
The basis for this amp started out as a post in a thread on the DIY Audio Projects Forum site. One of the members had suggested building a Super Simple Single Stage Preamp (“4S” Preamp for short) and there was much discussion concerning various tubes, gain, noise, etc. and several designs were presented using various dual triode tubes. Then the proverbial gauntlet was thrown down with the phrase “switch from 12AU7 to 12AX7“. My answer was to design a universal stage that would work well with an entire range of tubes. Thus was the 4S “Universal” preamp born.
The Electrical Design
The design I came up with is a single circuit that can handle the following tubes with no changes: 12AU7, 12AV7, 12AY7, 12AT7, 12AZ7, and 12AX7. Dependent on the tube used (all these have the same pinout) and whether the cathode is bypassed, the peak gain may be varied by a large margin. Here is the circuit I implemented along with the tube rectified power supply.
The circuit is actually very simple. It uses a reasonably sized grid stopper to control clipping and bias excursions, a pure restive load, and a high impedance grid resistor to avoid loading virtually any driving device. The volume control is on the output to help ensure that the noise figure for this preamp is as low as possible. With these components (the output potentiometer is 250kΩ audio taper) and an optional 33µf cathode bypass capacitor the calculated performance is shown in the following table.
As can be seen from the table, the peak gain can be varied considerably by choice of tube and the distortion (virtually all second harmonic) is very well controlled. This is a well behaved preamp.
The Build
Deciding on the physical layout and build actually took some time. Up to this point, most of my tube builds have followed a fairly predictable pattern: wood base, aluminum top plate, polished finish, etc. For this build I wanted to go for two major themes: Utility & Compactness. So I started with a stock enclosure from Pomona, the 4226. This is an “E” size (8.3″ x 4.2″ x 1.74″) die cast aluminum enclosure with a sheet aluminum cover and internal card guides. Although small, I was sure I could fit the entire PS and stereo preamp in this box. Instead of trying to run wires from the top plate to components mounted inside (which would make working on the box and sealing it up problematic) I decided to mount all of the components to the top plate. Then I could build the entire amp and when complete, just slide it into the box. Here is the layout for the major elements:
The power components are all on the left had side of the unit and all the audio components on the right. Internally there is a metal shield between the two sections which is held in place by one of the card guides moulded into the cast chassis. One very important feature of this build is the ground lift switch. The entire chassis is grounded through the IEC connector ground pin. The signal ground is isolated from the case ground by the ground lift switch. This way, if I am driving an amp in which signal is grounded to the case, I can flip the ground lift switch and the preamp signal ground will be referenced to the amp signal ground via the audio cables. This avoids hum inducing ground loops when both chassises are grounded via the IEC connector. Here is a view of the component side with the unit all wired up and ready to go into testing.
Things are fairly cramped inside. I could have gone for a printed circuit board approach, but I really enjoy point-to-point build techniques for vacuum tube projects. Besides this, I’m not sure I could have designed a PCB that would work with all the bulky items protruding into the case (inductor, IEC connector, tube socket, potentiometer, RCA jacks, etc.). Once the build was done and all electrical connections triple checked, it was time to commence testing.
The Testing
Testing a device of this type is really fairly straight forward. The first step is to simply power up the unit and check all the major voltage points in the circuit. This is done with the unit open and sitting on a workbench. For this build all voltages were as calculated to within a reasonable margin of error. For this type of device I like to test using a signal generator with adjustable output, a dual channel AC volt meter, and an oscilloscope. Here is a picture of the unit as I was running it through its paces.
The signal generator allows me to test the unit at various frequencies and input levels, the oscilloscope shows me the input and output waveforms, and the volt meter allows me to directly calculate gain at any frequency without trying to get numbers off of the oscilloscope. So how did the unit do? Here is the gain and phase plots generated from measured data.
This is very good performance. The single dominant high frequency pole here is due to the input capacitance of my test equipment. The tube input capacitance is at least an order of magnitude smaller and is swamped by the test equipment and the nest of wires on the table.
Impressions
Of course, all this test data is nice, but it really doesn’t say much about how the preamp works and sounds. So once the unit was deemed technically acceptable, it was time to determine if it was sonically acceptable. This entailed hooking up the preamp to one of my other units. I chose the 6V6 Lacewood amp.
In the past, when driving this amp with just my iPod, I was disappointed with the overall performance. Not with the sound mind you, but with the attainable power level from the amp. The iPod output simply wasn’t enough to drive it to full power. So I inserted the preamp in front of the amp, turned the amp volume to max, and used the preamp control to adjust volume. Here is a picture of the setup.
In this picture, the iPod line out is driving the preamp, the preamp is directly connected to the Lacewood amp and it’s volume is at max. First, with no input and both preamp and amp turned to max, the setup was dead silent. I could hear nothing from the speakers even with my ear right next to the speaker cone.
From the first moment I started playing music I was amazed at how the pair sounded together. There was absolutely no problem with signal level or power. And the added effective dynamic range really let the amp open up.
This project was a great success. I now have a preamp that can be adapted to the task at hand simply by swapping tubes. It also allows me to do some tube rolling to compare various tubes in my stash. And it can be configured with sufficient gain to test almost all of my prototype power stages. I think this little unit is going to see a lot of use.
On one other note, like all my other tube gear, it looks pretty good in the dark as well.
Update 17 January 2022: I have made a blog post discussing the placement of the volume control on the output and the consequences of this decision. It may be found at the following link: https://www.cascadetubes.com/2022/01/17/about-that-volume-control/
Update 18 January 2022: I have added this link to my Grounding philosophy so that builders fully understand the purpose and usage of the Ground Lift Switch. It may be found at the following link: https://www.cascadetubes.com/2020/01/13/grounding-philosophy/
Note 1 July 2022: If driving a solid state amplifier, the output impedance of the 4S Universal may be too large. In these instances the 12AU7 Color Preamp is another option for driving solid state amplifiers. The 12AU7 color preamp has a low enough output impedance to properly drive most solid state amplifiers and provides a line level adjustment of -20dB to +20dB.
Hi matt id like to know ,what output impedance has the universal preamp thanks
The output impedance is dependent on the tube used and the volume control setting. The peak values are listed in the calculated performance table above under the heading “Ro (Ω max)”. But the variation with volume setting is non-intuitive. It is discussed in-depth in the following post: About that Volume Control
Let me know if you have any other questions.
Hello. I just salvaged a PT from a 80r Fisher tuner that was in rough shape. It is rated at 190-0-190 @ 65 mA. Would that be adequate for this project? Thanks
Yes, the specs are more than sufficient for the small B+ draw of this preamp. Make sure you use the correct secondary for the tube filament. One (secondary 3) is 6.3V whereas the other (secondary 2) is 21V.
Correct on the 6.3 and the 21v secondaries. Thank you for the reply. I am looking forward to the build. It will be next in que on my bench.
Hi Matt…I finished a version of this preamp about a month ago and couldn’t be happier with the way it pushes the amp I use. Amp is SE KT88 with either 6EJ7 or 6JC6 in front of them and doesn’t really need any gain to play well, but some builders have reported that it does benefit from the extra drive. So the basic grounded unbypassed cathode circuit would be perfect. I’m finding that simplicity has a lot of benefits. But I wanted to try something different for the tubes used. I’ve noticed a lot of the serious tube headphone amp users have moved away from using 6SN7’s and gone with an adapter allowing them to use a pair of 6J5’s. Apparently they like what they hear, so I found a pair of the pretty ST-12 shaped bottles, sat down with the 1940 RCA tube manual, picked a load line, and made it all happen. 5ma at 135 vdc with -4 v for the grid works nicely and puts the op in a good place. I decided to use solid state rectification and used a 280v toroidal transformer and the new Vishay SiC Schottky diodes. Simmed on PSUD and got very low ripple with 4 stages of filter caps. Your description of the how and why of putting the volume control on the output side made perfect sense, so the Alps went there. I decided to try using partial biasing with LED’s…one per tube. Laid it all out on a graphics program to see how it would all fit, got a chassis and the rest of the parts, and went at it. A day later I plugged it in, cranked the variac slowly, and watched the B+ go right to 360 vdc…just as PSUD said. Got the amp and CD player hooked up…and was shocked by how good it sounded. It was the perfect thing to drive the amp. A week later and I installed a Goldpoint attenuator and changed the filter caps to the standard Audio Note caps. A little pricey but I don’t regret doing that. I’m finding that single ended circuits seem to be sensitive to parts…even in the power supply. At this point, I can’t imagine that there’s much sound quality left to be extracted. Totally amazed here.
Matt.
Can I connect a microphone to the input ?
Do I need to lower the impedance ?
João;
That depends entirely on the type of microphone. However, this preamp provides no phantom power and lacks the gain required by typical professional quality microphones.
Matt,
I’m excited about the prospect of this little preamp. My plan is to build a separate PSU chassis so that I can build different signal boxes for different tubes (with different pinouts). For my first build, I plan on using the 12AU7 tube exclusively and will adjust anode and cathode resistors a little to put the load line more towards the center. My first question relates to the volume adjustment. I would like to use a Khozmo 10K stepped attenuator. I will be using this preamp for a KT88 SET amp and a VTL push-pull. Will there be problems using the 10K attenuator in place of the 250k output pot? Also, as a follow-on question, my DAC delivers 2.5 VRMS. Since this would result in about 7 Volts peak-peak on the grid, it would seem like a grid quiescent voltage of -4v would be appropriate. Am I thinking about this right? Thank you!
Reducing the attenuator on the output to 10kΩ will unduly load the stage and keep it from operating correctly. You could put the 10kΩ control on the input and just substitute a 250kΩ resistor for the output volume control. And remember, changing the bias and load resistors will mean the “universal” part is gone. These were chosen to support all the tubes listed.
2.5v-rms is about 3.5v peak. So a bias point around 4v would be fine to directly drive the input. If there’s a volume control on the input, it really doesn’t matter because you will be unlikely to run the preamp with the volume set to full.
Matt, thank you for the response. My only thought about putting the attenuator on the front is that is would lose the full harmonics of the full-swinging 12au7. I have a follow-on question. The type 76 tube filaments require the same voltage as the 12A*7 tubes. I am thinking about building a seprate PSU and building several signal boxes. I would really love to build this with a pair of Type 76 tubes. Would this work? I’ve built load lines using the same components as the 12au7 version and it seems to work well. Unless there is something that I’m not thinking about. Thank you for taking the time. I’m a relative newbie but really enjoy learning.
The short answer is, yes it would work. But the 76 triode is a far different beast from the 12AU7.
I usually run the 76 with a much lower load (warmer sound). If you set the PSU for a 300v B+ (which is ideal for the 4S universal, I would run the 76 with a plate load of 50kΩ and a bypassed cathode bias resistor of 2700Ω. This puts the quiescent operating point at Vp≈151v, Ip≈2.96mA, & Vg=-8v. The gain of this stage is -11 v/v or ≈20.8dB. This is ideal for a volume control set midline for unity end to end gain. Ro of this stage is only 9.7kΩ so it can drive all kinds of systems.
Personally I really like the 76/56 signal triodes. But they can get pricey sometimes.
Agreed! That puts the operating point right in the sweet spot. The anode dissipation would be about 0.45 watt with a rated power of about 1.4 watt. At only 32% of rated power, the tube would run unstressed and probably last a lifetime.
For the output, I presume that 220k 1/2W resistor would work. Correct?
Yes. 220kΩ is fine. Just make sure to calculate the output capacitor correctly. See the basic equations in this post. Just make Ru = 0Ω and Rl = 220kΩ.
Matt,
I’m sorry. I was referring to replacing the output volume control with a 250k resistor. I have a 220k 1/2 watt resistor on hand. I want to make sure this is OK in place of the 250k volume control. I will be putting a 20k step volume attenuator at the input.
I’m showing my lack of knowledge. Taking another look, it appears you understood my original question. Thanks again!
Hi Matt,
If the input signals from both channels are applied to the triodes in the same tube, would there be any cross-channel leakage?
Can I feed one input signal to both triodes of the same tube?
And then use 2 tubes for the 2 channels? Would the values of the resistors, capacitors be the same?
There is no appreciable crosstalk using both triodes in the same tube.
If you try and parallel the triodes in one tube then both the load resistor and the cathode resistor must change. This is not recommended. If you are really that worried about channel crosstalk (although you shouldn’t be) then use one tube for the gain stage and the buffer for each channel.
Hi Matt,
I’ve searched but couldn’t find what type of fuse do you use? In general, how do I choose a fuse? Fast acting, slow blow, amperage & voltage rating?
Thanks,
Chi
Here is how I choose a fuse. I calculate the total maximum VA of all the power transformer secondaries. (VA_t = VA1 + VA2 + … + VAn) Then multiply by 1.25 for transformer loss. (VA_f = VA_t * 1.25A) After that calculate the resultant primary current. Ip = VA_f / V_mains) Then I generally add about 25% for margin. (I_fuse = Ip * 1.25) Finally pick a fuse current rating that exceeds this value. Using fuses rated at 250v is sufficient for all mains voltages currently in use world wide.
For transformers always us slow blow type fuses.
Thank you for your reply.
Hi Matt, I have a pair of Pye Cambridge PF 91 mono amps which both use a PF91a preamp which use ECC40 valves which are getting harder and harder to source. I am thinking of changing to a newer set up stereo preamp to feed the two mono amps. I can’t upload the schematic of these two designs on this page, but would like for some guidance as to whether this universal preamp could be a good substitute. The sensitivity of the amps is .5v for 12 watts. Also would need advice for the correct input path to the ECC33 and ECC35 tubes. If you could advise as to how I can get these schematics to you would be appreciated if you are interested. These amps are circa 1954/55 with a frequency response of 2-160kHz and are really quite special. Thanks in advance. Bob
As a driver for your mono-blocks I believe that you would need a more purpose built unit. The PF91A includes a multipart equalization network, tone controls, selectable bandpass filtering, and buffered output. It is carefully tailored to the PF91 power amps. The 4S is just a simple gain stage. The ECC40 you wish to replace is a close functional match to the 12AV7. It is conceivable that the preamps could be modified to utilize 12AV7 tubes, but the circuit values would have to be carefully checked and potentially modified.
If you want entirely new preamps, the first step is to determine the sensitivity of your mono-blocks so you know how much drive is required. From the schematic they should not require too much drive. With proper interconnections there are a lot of possibilities for how to substitute other preamps.
Hi Matt,
Thanks for the reply. The PF91a preamps are modified to receive an input directly to the Volume control. The EQ and Device switching is bypassed altogether on V1 stage. I was thinking of bypassing the preamp altogether and running the line input directly to the mono block via pin 5 0f the octal plug connector. I could combine an external volume pot to attenuate signal level at the input . The sensitivity of the PF91 is .5v at the moment for 12 watts. Sounds like you have a schematic of this amp already so would appreciate your input to this approach. Thanks again. Sorry it has taken so long to get back to you. Cheers. Bob Hansel
The modification you describe to the preamp means the only functionality left is the volume control and the output buffer (cathode follower) formed by V2B. In this situation, I would simply use the amp unit standalone.
First off I would use pin 4 instead of pin 5. Pin 5 runs through a 0.25µf capacitor which serves as the DC block between the cathode follower formed by V2B in the control unit and the input of V1 in the amplifier. Pin 4 of the octal socket (on the amp side) connects directly to a grid on V1 in the amplifier. The grid leak resistor on V1 is 1MΩ so I would use a 250kΩ volume control on the input of the amp at pin 4 of the octal connector. This would be a master volume to control the signal into the amp.
Does this make sense?
Excellent.
The pin 4 is what I missed. Thanks Matt. I’ll do some mods and let you know how it flows. I’ve had these amps for about 40 years now and have loved their discreet and warmth especially in the early “harsh” sounding low sampling days of early 80’s digital CDs. That output transformer is something else. These days of high res sampling is something else again. Appreciate your time and response to this situation. Cheers mate. Bob H
All done. works a treat. Plenty of gain off the volume control and no hiss, spits or farts that used to come from the 91A. Thanks Matt.
Matt,
I noticed on the DIY Audio Projects page the schematic shows a 470K resistor on the preamp input. on this page, it shows 499K. Which would be preferred?
In actuality it really doesn’t matter. The grid resistor selection can be anything below 1MΩ and, in general, has little effect on stage performance. The difference between 499kΩ and 470kΩ is only about 6%. This little change will have no effect on performance of this circuit.
The short answer is use whichever you have. Or use something close.
Hi Matt,
Thanks for sharing this awesome project. I am going to build it but since I will use solid state AMP class AB I should consider output matching. during the research I visited Rod Elliot project ( https://www.sound-au.com/project167.htm ), have you experienced Mosfet follower for impedance matching? any recommendation?
tnx
In general, If I need a buffer in a tube amp I use a triode in a cathode follower configuration. You can see this approach used in the 12AU7 Color Preamp. If you study the circuit closely, you’ll see the topology is very similar to the MOSFET project to which you linked.
For the 4S there are two ways to deal with your situation. The first is to replace the volume pot with a fixed 250kΩ resistor and place a 100kΩ volume potentiometer on the input. The output impedance in this case will depend on the tube and will vary from about 11kΩ with the 12AU7 to about 38kΩ with a 12AX7. While higher than generally used, these numbers should not cause frequency response problems in the solid state amplifier input stage. The other option is to add a buffer to the output. You can use either your MOSFET approach or the buffer used in the 12AU7 amplifier. Just realize that you’ll have to watch levels carefully to avoid overdriving stages.
Thanks Matt for fast reply. Actually I need preamp for turntable and the solid state RIAA preamp that I made with gain 40 is overdriving the my preamp so 4S project with bypass cathode should do the job but I am mot sure about color preamp.
I just found your page today and not checked other projects yet but I think I start with color preamp and see the result.
Since my preamp impedance is 100K do you think first solution would be better?
Please note that the 4S does NOT provide the RIAA compensation curve for your turntable. If your RIAA preamp is overdriving your amplifier the answer is a volume control or pad on its output. The 4S won’t work as an RIAA preamp.
dear Matt,
thanks for info.
cheers
Amir
4S Universal Preamplifier for 12A*7 Tubes
if i use a 100-150KA VR ( input ) ?ohm plate load resistor to replace the P1
thx
joe N
I’m having a little trouble unwrapping your question. Are you asking the correct value of the replacement resistor for the output if you move the potentiometer control to the input?
If this is your question, then move the potentiometer to the input replacing the 500kΩ grid resistor, and replace the potentiometer on the output with a fixed 250kΩ resistor.
thank you very much!
hi Matt,
at last i buy a Alps 250KA VR and follow your diagram. Now with a amperex 12au7 it sounds amazing.
thanks again,
joe N
I’m glad it all worked out and you like the preamp.
Hi Matt..
Thanks for sharing the 4S Preamp project online.. Is it okay if my B+ is only 200VDC though? Is the 10k Gridstopper essential? What are the estimated Plate and Cathode Bias voltages for both the 5751 and a 12BH7, etc.. if I only have 200VDC B+? Thank you..
A B+ of 200V is very low for this circuit. It was designed for a B+ voltage between 250v and 330v. Having such a low B+ will greatly reduce the bias points. This will limit the maximum input signal the preamp can accept without overdrive. It will also increase distortion. The grid stopper is not that essential. It is amply sized to limit to possibility of high frequency oscillation if it is used with poor cable dressing. It can be decreased or eliminated if desired.
As for the bias points at a B+ of 200v, they will be low. For example, the 12AU7 at 200v B+ will have a cathode bias of ≈-1.7v and a plate voltage of ≈50v. The numbers for the 12AT7 are ≈-1.4v and ≈80v. I would recommend using a higher B+ if at all possible.
Hi Matt,
Thanks for the reply.. I see, if the max input signal it can accept is limited, then the preamp stage would easily clip from most modern sources.. I understand now.. Thanks again! Best regards….
I just came back to this project page to see some details that I might use in my build. I noticed on the schematic that the coupling cap is specified 0.47uF. I’m not sure if that was the value intended, but I used 0.047uF with good results. It might be a typo. FWIW, I later changed to 0.1uF couplers to take advantage of some high quality caps in my parts stash. Sound is good but haven’t bothered with measurements…I had to clear the bench for woodworking projects.
The reason was to get the low frequency support for the 12AU7 at high volume levels. The lower the output impedance of the stage, the larger the capacitor needs to be to support the low frequency response. Here is a plot of f-low verses Volume Setting by Coupling Capacitor Value using a 12AU7 and an amp with a 100kΩ input impedance.
The red line at the bottom is the -3dB response for the 0.47µf coupling cap. The green line is for the 0.047µf. You probably didn’t notice much difference because there is actually very little content down at the frequencies where this would make any difference. The 0.1µf is probably a nice compromise in this circuit.
Hi Matt,
Can I toggle between 2 cap values to switch between my tube amp (ARC VT-50 w/100k input impedance) and my SS amp (Parasound HC750 w/ 33k input impedance)? What would you suggest the cap values for the tube and SS amps? Thanks in advance.
You can do this. However, with the 250kΩ volume potentiometer and the 0.47µf capacitor you should be fine driving the 33kΩ amplifier. The only place you might see some low frequency fall off is if you turned the volume up beyond the 90% point. And even then, it’s just a few hertz.
If you want to be able to drive something with a really low input impedance, like 10kΩ, I might suggest a 0.68µ or even a 1.0µf but only for the really low input impedance amplifiers.
Hi Matt,
This is such a neat little amp! I just finished building it. It works great with only a little bit of hiss/hum. Since I made a few adjustments (input selector with indicator lights and a sub-out), I ran out of space for the ground-lift switch. What I am struggling with is more related to understanding why my plate voltage on the 12AU7 is so low. You list the B+ at the power supply (250V, I get 275V) but don’t show any voltages on the plates.
So, my B+ at the 100uF is at around 275V. Without a 12xx7 inserted I see 270V at the plate, after the 100K . But with the tube (12AU7) the plate voltage drops to 75-85V (measuring ground to plate). I tried three different tubes, no difference. This is not a complicated circuit, so I am probably missing something very simple. I checked all connections and values multiple times and cannot find anything wrong. The 12AU7 can work with 80V but this strikes me as very low. I should say that the amp sounds just fine, and I get the gain I need but I just want to understand why there is so much of a drop. Do you have any thoughts?
Here are some photos: https://photos.app.goo.gl/3g91MZHoXsdPzpmy9
According to the 12AU7 load line design, with a 250v B+, the plate voltage should be about 63V. With the 270 It’s probably right around your 70v figure. It’s important to remember that the current through the tube is about 2mA. This means that the cathode voltage should be about .002A * 1200Ω = 2.4v and the plate voltage should be about 270 – (.002A * 100kΩ) = 70v. So the math works out.
I saw this design was was the start of a Skunkie Designs Electronic YouTube project called, “HiFi Tube Pre-amp: Simple DIY Version!” Congrats on inspiring a video.
Matt,
What are your thoughts on using the 4S as a standalone preamp, then building the Marblewood or Lacewood or similar as a pair of monoblocks by eliminating the driver section and basically building 2 output sections. I understand basically doubling the components used in the original to build monoblocks instead. But would the values of components change drastically. I know the current draw on the power transformer would be less in this set up. But I would need two of them now. Two of everything now really. What’s your thoughts.
Personally, I would recommend against this approach. Much of the sound quality of these SE designs is tied up in the operation of the overall circuit (driver and power stage). By separating the power stages from the driver one risks improper interconnection impedances and poor operation.
Either the marblewood of the Lacewood can be built as traditional mono-blocks with excellent results but I would not separate power stages from drivers.
Yea I suppose your right. Why mess with it. But the lacewood as a pair of monoblocks would be cool. But I’m thinking of using a 6j5 single triode in the driver stage on each. Instead of the 6sn7 dual triode which, only one half would be used on a monoblock anyway. It seems that a 6sn7 is just two 6j5’s in one glass envelope for the most part. I’m guessing that I would bring the stereo pot out on its own. Probably with shielded cable. I think it would be an interesting project. Any pitfalls that you think I could run into? It seems like it should work out pretty well. But I’m no expert.
I’d just put a single pot on each mono-block. That external volume control is just more connections in the signal path.
HI,
Thank you for sharing this amazing project. What do you think about combine this project with Lancewood like you did but using the Lancewood power supply? My sound source have only 150mv RMS, so I intend to build a Lancewood with extra preamp stage. I appreciate any suggestion.
Not a bad idea. However, a word of caution about cascaded signal stages. It is very easy to overdrive the second signal stage with too much amplification. The Lacewood V2 requires about 800mV RMS to achieve full power and the signal stage will saturate at around 2.7v RMS. Therefore you only need 20*log(800/150) about 14.5dB of gain. As such I would recommend that you use a 12AU7 in the 4S such that, with the volume control before the 4S, the output will be neutral at the 12:00 position and approximately +20dB at the full right hand position.
Personally I recommend keeping the preamp in a separate unit. I find it easier to balance my system in this way. But the bottom line id be careful how much amplification is in the signal chain. it’s easy to saturate one or more stages with too much amplification.
I love the simplicity of this preamp. I added a number of separate inputs using a selector switch. But now I wonder how can I add a line level output that would send a line level signal to a tape deck for recording whichever input I select? Do I need to add a separate gain stage with a gain of 1? I am Nervous of loading down the preamp (and ch aging freq response due to loading effects) if I simply take signal between the input source and tube. Or taking a signal off the tube output prior to the volume control.
There are some things to consider. First, this preamp is designed to provide coloring to the signal. As such, I would think you’d want to take the signal off the output. Second, as you said, you don’t want to unduly load the preamp output and affect frequency response. So a buffer is probably in order both for isolation and for the “record out” output impedance.
If I was making this addition, I would add a second potentiometer in parallel with the output volume control, and feed the output of that potentiometer to a cathode follower that feeds the line out. The separate “Record Out” control lets you ride gain for the input to the tape deck without affecting the drive level of whatever the preamp is plugged into. And the very low output impedance of the cathode follower allows you to drive virtually any equipment; solid state or vacuum tube.
I am interested in this circuit. I rebuilt 2 Acrosound ULii’s which are driving rebuilt IMF RSPM Mkiv speakers. 60 watts is on the low side to power these beasts. I would like to add bass boost to this circuit for listening at lower volume. Any thoughts?
I retired from my business, Kandel Electronics, where we sold and repaired (NIST calibration lab) industrial electronic test equipment.
COVID-19 quarantine gave me the insentive to get back into my audio hobby. I purchased a Linn Sondek LP12 on eBay so I can listen to my vinyl collection I haven’t listened to in 30+ years.
My first thought is to build a version with a tone stack and buffer. I posted a good design for this over at the DIYAudioProjects website (Link – Baxandall circuit). This will give you the opportunity to tailor the frequency response to your particular situation. I would recommend starting with a 12AU7 in the second stage until you get a feel for the drive levels required.
Hi,
I came across your 4S preamp, and made one up in an old Conrad Johnson chassis.
Basically used the power supply, but regulated the heater supply. The Conrad had a solid state rectified supply for the tubes.
I used a 50K volume pot, and around 330K to ground on the input. Have a 2.0uf MIT coupling Caps, and used a couple 12AT7’s, but paralleled the 2 sections to have 1 tube per channel, so compensated on the resistors because of paralleling the 2 sections.
Lovely sound, I love it. Running it into a Pass Class A Amp Camp Amp modified, and use efficient open baffle speakers. Thanks a million for your efforts and sharing of the 4S. Think I will be listening to this combo for a real long time!
Best regards,
Den Foulk
Brinson, Ga.
Hi Matt,
I am slowly re-engaging in a life-long love of electronics, hi-fi audio in particular. A couple of years ago, after much internet exploration, I found your site when looking for a tube amp design, and decided to build your Lacewood V2. I’ve been intending to share my build outcomes with you since then (and will soon): the performance of that amp was and continues to be literally thrilling! I’m writing here because I am now preparing to build the 4S/Universal preamp, but am considering modifying the design to include the Boozhound Labs RIAA preamp in a semi-integrated manner. My first question is whether that preamp would normally (or ideally) provide input to this “universal” preamp or to the Lacewood amp, given that I will have both. If so, are there component separation and/or shielding issues needing careful consideration? The power supply would ideally also included within the same chassis to minimize the number of signal and power cabling. I’ve also been considering a source selector switch as a possibly desirable feature, but with separate physical inputs for phone and “other”, so that cabling would not need to be changed when switching sources. I am interested in your thoughts about this approach. Thanks!
I am really glad you’ve enjoyed the Lacewood V2. It is one to which I listen often.
As for the preamps, there are lots of things to consider. I will say up front that as a mater of course, I usually don’t combine anything with a good RIAA preamp (and the BoozHound is a good preamp). Since they are usually a just for a turntable I tend to let them standalone.
The most important thing to consider is that there is a voltage environment mismatch between a 4S and the BZ RIAA preamp. The 4S is a high voltage unit and the RIAA stage is a low voltage stage. The RIAA stage you also want as quiet as possible so just putting it in the same chassis with the 4S makes that difficult. Not only does the power have to be highly filtered for each unit, the layout would be very critical. I think that in an integrated setup, I would fully encase the RIAA stage in a µ-metal shield. This can be done, but it can be finicky to get a good result. And choose the biggest chassis you can, and then buy one half again larger. When doing this type of integration, volume and distance between parts are your friends.
Personally, I would build the 4S with selectable sources, but I would put the Boozhound unit in a small chassis with a switch and an LED and then use a good quality transformer based wall wart to power it. This unit can be placed near the turntable and then just run the line level cables to the 4S integrated with source selection, etc.
I hope this helps.
Yes, it helps a very great deal! …and more or less confirms my suspicion that it might not be a good idea to put both preamps in the same enclosure, though I was intending to pay attention to shielding the power sections of the two devices. My fallback idea, again aimed at eliminating one more separate chassis on the shelf along with the main amp, fm tuner and turntable, is to build a sub-base for the turntable, perhaps of the same wood I’ve used for the amp (and will use also use for the preamp) and install the RIAA preamp inside that… I would be grateful for your thoughts on that approach!
Regarding the second part of your response, it sounds like you don’t think the BHL RIAA preamp needs any additional gain, and already will provide a suitable input to the Lacewood amplifier? In that case, I might still use the 4S as a tidier destination for the cables from the RIAA, and place a selector switch on that chassis phono / tuner which essentially just bypasses the 4S when I’m using the turntable. Unless the switch itself is a potential weak spot in the chain?
Thanks Matt!
I think that a sub-base for the turntable to house the RIAA preamp is a great idea. And it has the added benefit that you can add another layer of vibration isolation to the turntable by choosing the proper feet. A small switch and a little LED would be all that you need to show with signal and power connections in the back.
The BHL RIAA preamp has a nominal gain of ≈40dB or 100v/v. This should be enough to get the signal up to a nominal line level. Personally, if I run the 4S in front of one of my amps, I almost exclusively use a 12AU7. The 4S/12AU7 has a gain of about 20dB and a typical volume control has about -20db at the 12 o’clock setting. So if the preamp volume control is left of center, it’s attenuating, and if it’s right of center, it amplifying.
I’d run the output of the RIAA to one of the selectable 4S inputs. One thing about vinyl, some disks are cut deeper than others and, depending on the album, you may want a little extra gain.
Hi Matt . . .
Although the pure ‘electronics’ part of my full-time working days were spent in consumer-level repair, a considerably less elegant, professional or knowledgeable environment than yours, my perspective on tube audio has evolved in a somewhat parallel manner. My contempt for thermionic emission grew over the years due to the self-destructive and generally cruddy, temperamental nature of tube TVs, so I happily embraced silicon for all it was worth.
Flash forward to now, and a busted Fosgate Signature headphone amp I’d gotten in trade last year for some handyman work, fixed, and became thoroughly spoiled on the “even-harmonic” (or whatever the hell it is) warm sound. Said unit croaked again a few days ago, and because I wasn’t impressed with the design and build quality, rather than overhaul the stupid thing again, I decided to strip it down to a bare chassis, rummage around on the web for a nice simple preamp schematic, and what should pop up but your lovely design, which I’d like to insert into the pre-out/main in loop of my Yamaha receiver.
Now that I’ve become addicted to “tubeness,” especially with a pair of halfway decent Sennheiser phones, and have a tiny hoard of 12AU7s and 12AX7s from various old Leslie speaker amps, organs, etc., the following questions will expose my shameful lack of basic circuit theory:
Could satisfactory coloration results be obtained by employing only one of these triode circuits per channel, since you prescribe against putting them in series? Or how about configuring the first stage in the manner you suggested to Scott Bartholomew, with the higher-value grid resistor and extra input load resistor? Would it still be compatible with different tube types?
My primitive reasoning for feeling as if there should be two stages per channel is that in order to attain an amount of “warm” timbre vaguely similar to what I’d gotten used to, which was produced via the two halves of a 12AX7 apparently in series, followed by some kind of goofy proprietary op-amp device and a couple of hybrid output ICs on little heat sinks, there should likewise be two stages of tube gain (unity or not) in the homebrew preamp..? Couldn’t the pot be used to cut the gain of the first stage down to something like unity?
Obtaining a nice clean B+ supply doesn’t seem too daunting, and I’m not sure yet whether I want to go the route of a big old honkin’ 5U4 and a double-pi-filter, or take the coward’s way out with one of these hybrid potted 250V PWM switch-mode widgets I’ve saved from various cannibalizing expeditions. Would it be utter blasphemy to use DC rectified / regulated from 15kHz instead of 60Hz?
Interesting note about the bizarre Fosgate device: the filaments were evidently operated at partial voltage, didn’t visibly glow, and the tubes were lit from underneath by orange LEDs!
Profuse thanks for the fresh project inspiration. And any additional feedback. 🙂
-e
Ed;
I don’t think you’ll need the series combination to get the level of color you desire. The key is to keep the stage amplification low so that you’re driving the stage along a greater portion of its operational range. In triode stages, the distortion (color) is roughly proportional to the drive level as a function of its total distortion.
To get good tube color with the 4S, stick with the 12AU7 with the volume control on the output. This will give you constant color and about unity gain with the volume control set to 50% (i.e. 12 O’clock).
Oh, that’s good news. Simplifies things greatly.
Mucho gracias. Before/after photos to follow.
Now more than a year later, I finally got around to finishing a semi-prototype. Surprisingly little hum, considering how hideously jury-rigged this first attempt is. I was constrained to using only extant junk-box parts attached to an old Leslie amp chassis, so I took the lazy coward’s way out: potted solid-state bridge rectifier, two old filter chokes (180Ω / 400Ω, inductance values unknown) in series, one of which came from a fifties-vintage Gulbransen organ, and is pretty hefty; a handful of filter caps (180, 600 and 750µF at both ends and in the middle (double-pi arrangement with the 600 & 750 being series-wired 1200 & 1500 µF 250V caps) and a 2 amp 6V switching DC power supply for the filaments. All power supply wiring is *tightly* twisted. Copperclad board material for shielding around the signal-handling bits. Omitted the ground lift switch (for now) and used common signal / PS grounding with a star connection near the physical center.
I tried to reduce the remaining hum by isolating the filament supply from ground, adding more shielding, adding a DPDT power switch to reverse line phase, lifting the earth ground, trying various different 12AU7s, and got no noticeable improvement. Could I trouble you for speculation on whether this could be better addressed by A: rewiring with isolated signal / chassis grounds, B: using your exact specified B+ choke/cap values, C: reverting to AC for the filament with the two 100Ω resistors per your design, or D: anything else I haven’t thought of? My funky old Tektronix scope hasn’t been fired up in so long, I don’t even remember if it still works.
If I could halve the hum by 50%, (right now it’s not the worst, only audible at fairly high volume with no signal applied) I’d be totally happy with the results, since my hearing isn’t the best anyway. BTW, I’m tickled pink with the coloration – exactly as you predicted. The “random pinkish noise floor” isn’t objectionable at all, and I assume it’s just the normal behavior of a hot cathode..?
My ugly bastardized version, “El Junko Cheapo,” is posted here:
https://onedrive.live.com/?authkey=%21AHmXBqXgE9zriV4&cid=AA324E6C93DF03F2&id=AA324E6C93DF03F2%212382&parId=AA324E6C93DF03F2%212381&o=OneUp
Thanks kindly for putting up with all my clueless noob questions!
Finally, success.
The combination of dumpster bits that works as hoped for has been found:
https://ydgqsa.dm.files.1drv.com/y4mgxUbMEV17BqiU3fD5xyv2BQArVh9QMV0qsSQjqlgWznOnkviFt-IlvOtqKIQd_Lc5RfFT3VBqY9QrPwnSOBkHMWUfIiC8_tUDUrEDLCMc0PLbiF7_itlDJRlfkzdV-oO75CHo2e36KWfr8FFAwRYU2lj3cIyw_4bwp_lt4WKFiB7R04GsaUIW0KUzNXFSn_NMiL8jHftlGBPtug9DqCq9Q/Cheapo%2012AU7%20preamp.jpg?psid=1
It now works so well, I’m tempted to build a “nice” one from new materials.
Matt,
I’m in the process of digitally archiving (24/96) a number of pre-1930 lateral cut and earlier vertical cut Edison discs. I need a preamp with flat response (un-equalized) from about 50-8000 Hz. Input will be both Moving Magnet Stanton 500, and Denon High Output Moving Coil cartridges. Is there any modification to your 4S Universal Preamp that would suit this application?
If you are looking for an all tube design that supports MM cartridges and RIAA equalization, the best I can recommend is the “Grove Watt” preamplifier designed by Bruce Heran at Oddwatt audio. He freely publishes schematic and design data and will help with build issues via a forum. Here are the appropriate links:
http://diyaudioprojects.com/Tubes/RIAA-Phono-Preamp/
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3545
If you don’t mind a well designed SS solution, the Boozhound RIAA preamplifier is also highly rated, easy to build, and has really been wrung out DIY-ers. Here are the links:
https://boozhoundlabs.com/collections/all/products/bhl-audio-jfet-phono-preamplifier-kit-2sk170-riaa
http://diyaudioprojects.com/Solid/JFET-Phono-Preamplifier-Kit/
I recommend you read up on these two solutions and decide if one meets your needs.
I hope this helps.
I built one of these and it sounds great. Had an old Telefunken 12AU7. Put it in between a CD player and my shop amplifier and it softened things up nicely, but still has a nice bite.
Thank you for the circuit!
Hi Matt:
you mentioned in a post here that this preamp isn’t suited for mic’s because:
1. too noisy, and,
2. insufficient gain
would running two of these in series for 2 stages of gain work? Regarding noise wouldn’t input and output transformers take care of that?
Thanks, David
I would avoid daisy chaining two of these simply because that’s something for which it was not designed and the gain would likely be excessive. As for the noise issue, the use of a step up transformer on the input is a relatively common approach to both controlling noise and providing some initial step up drive voltage.
If you have a microphone that is relatively low impedance, you can use a step up transformer that reflects the microphone impedance to the grid input at about 4 times the vacuum tube equivalent noise resistance. The noise resistance is approximately 2.5/gm at the chosen operating point. The restriction on this method is the requirement for a low impedance microphone. If you use a high impedance source, the a low noise active preamplifier stage is still required.
That makes sense, thanks very much, Matt. Appreciate you taking the time, learning a lot from this site.
Sounds like I’ll continue with the CF version of this circuit for hi fi and find a schematic for one that’s mic specific.
Hello Matt,
Have you ever tried a negative feedback for the 4s + CF preamp? I find the 4S + CF with 2 JJ Tubes ecc82 in the quiet range really sonically top. If it was louder, would probably the circuit for ECC82 be optimized? Therefore, I would like to use the ECC83 in the first stage! However, it has a very high gain and therefore I would like to incorporate a negative feedback. Switchable with a switch? Maybe in Kobination with a positive feedback! Is there already experience?
BR
Martin
Martin; The application of negative feedback in the 4S+CF circuit is a relatively simple matter. If using the ECC83 (12AX7) is the goal then the forward gain will be very high. As such the full feedback factor ( |Aβ| ) will be large and the total gain of the amplifier can be reduced to the inverse of the feedback network gain 1/β. To implement this, simply apply a voltage divider on the output of the CF and tie this point to the base of the cathode resistor of the 4S stage.
As an example, if you want a total forward gain Av ≈ 5 (14dBv) at max volume, then the voltage divider should have a ratio of 0.2. This could be accomplished with an 8kΩ and 2kΩ resistor in the divider network; the 8kΩ between the CF output and the 4S cathode resistor and the 2kΩ between the 4S cathode resistor and ground. This will significantly tame the overall gain when using the ECC83.
Thank you for the info.
1. An additional resistor with 2 kOhm between ground and cathode resistance of the 1st stage so the 12ax7.
2. An approximately 8 kOhm resistor from the cathode of the 12ax7 to the output of the CF after the output capacitor.
3. Should a leakage resistor from the grid of the 12ax7 to the connection point of the new 2kOhm resistor and the cathode resistor be installed or is it unnecessary with the potentiometer at the input?
BR Martin
You really don’t need an additional grid resistor on the 12AX7. As you indicated, the input potentiometer does this function just fine.
I tried it that way and it does not work the amplifier starts to oscillate. I think the phase is not right. Tomorrow I will measure the phase. Probably comes from the grid of V1 to the output of the CF about 40kOhm resistance. Or a resistor from the Cathode to the other Cathode. Let’s see.
There is likely insufficient gain or phase margin in the feedback loop. This is due to the large output capacitor on the output buffer. This is on e of the reasons I never implemented feedback in this design. The best way to limit gain in the 4S is to select a lower gain tube.
Yes
From gate v1 170kOm resistor to cf output. Thats good for 12au7 or 12ax7 in the first section.
Basically, I also prefer circuits without feedback. With the 4s + CF, the noise is too high due to the setup with the potentiometer at the input, at least in my setup. Hence the feedback. From a technical point of view, it looks good with the 170kOhm resistor from the grid of the S4 to the output after the capacitor. Also, I think it gets even more linear because the capacitor at the output of the CF makes the bass more linear. Whether the whole in the hearing test is good or bad I will like to report next week.
BR Martin
After 2 weeks of probing I can highly recommend the feedback with 170 kOhm resistance.
My favorite tube setup is the JJ 5751 in the s4 level and ECC82 in the CF.
Hi Matt,
could i use in the Power Supply a Hammond 156M (3H, 100mA, 86 Ohm, 400V) instead the 154M?
Yup. This should work great. A little better filtering even.
Hi,
I am attempting to build this amp using a Stancor 235-0-235 117v transformer. It is my first build and I have played out the power supply and one channel on a breadboard. My transformer is putting out 269v and 7.4v on the heaters. I am getting 123v from the wall. I was hoping you could explain the best way to go about correcting it. And thanks for the schematic 🙂
I am assuming that the voltages you are quoting are with the breadboard. So 269v is the B+ voltage? The filament voltage is a little high. Ideally you want to keep a 6.3V filament between about 5.7 and 6.9 volts. A small power resistor in series should fix that. (7.4-6.3)/(1.0+0.3)≈0.8Ω; (1.3^2)*0.8≈2.1W so use a 0.8, 5W power resistor like this one: ACS 5s R8 K T&R
Does this make sense?
Yes got it!
Thank you Matt!
And yes the 269v is my B+
That should be ok?
Thanx,
E
That’s fine. The 4S performs just fine with B+ anywhere between about 250v and 310v dc.
Ok..thanks a lot.i am looking forward to the build. You do very nice work 🙂
Thanx again
e
Hi Matt, I have sent you an email.
What would I need to do to enable the 4S Preamp to work with a Quad 606 Power Amp? The specs are below.
Quad 606 Power Amplifier Specification
Power Output: 140 watts per channel (8 ohms)
Distortion: < 0.01%
Input Sensitivity: 500mV
Input Impedence: 20Kohm
Crosstalk: -100db (100hz), -85db (1 khz) and -65db (10khz)
Hum and Noise: 105db
Voltage Rating : 220-200V AC (110-120V via simple power board modification)
Dimensions (WxHxD): 321 x 140 x238 (mm)
Weight: 12kg
If it is not possible then can you recommend a good quality preamp tube circuit to build instead? possibly using a 6H30Pi EH or similar. This will be my first-time tube build.
Thanks
To drive an amp with that low an input impedance, I would recommend an output buffer. You could use the circuit documented here: 4S Tube Preamp with 12AU7 Cathode-Follower
Hi, Matt.
Thanks for providing the schematic, finally finished.
Tubes: Ez81 Tungsram and Telefunken 6211 Gold Pins.
Power Supply mod.
https://www.dropbox.com/sh/s9wt3acclrz5cjp/AACQ8tbk-rh5i86qDbSxPxN7a?dl=0
Very cool build! How do you like the sound?
Hi Matt,
thanks for sharing this Project. I want to build this one and have some Questions:
1) this is a single stage pre amp, so the absolute Phase is 180° inverted, isn`t it? So i have to change the + and the – Loudspeakercable. Right?
2) I want to use the PGA 2311 to Control the Volume, so the cap on the Output to the PGA must be another. Right? The PGA is 100k in and < 1k out.
best regards
Patric
To answer your first question, yes there is a 180° phase inversion. However, you do NOT need to swap your speaker wires. There is no phase coherency in recorded music. The only reason you need to think about the phase inversion is if you are embedding the preamp inside of a feedback loop.
To answer your second question, as for the PGA2311, this is not an appropriate volume control for this preamp. The input impedance is only 10kΩ and the preamp would not function properly with this control. The natural output impedance of the stage can be as high as 39kΩ meaning that any volume control used should have an input impedance of at least 156kΩ. This is why a 250kΩ control is used in this design.
Thanks for the reply. The volume pot brings me in trouble, because i use class D amps with a input impedance from 10k and 100k. What can i do?
Best regards
Patric
Cathode follower output stage. One 12AU7, a few resistors, a couple of caps, and you’re good to go.
See here: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5186
Thanks Matt,
i build the Cathode follower Output stage. Does the value of B + apply with the tube inserted or without a tube? With the tube inserted the B+ is at 280V, at the anode 80V and at cathode of the preamp stage 2,4V. The Filament Voltage is 6,9 V. Are these values ok?
These voltages are in the ballpark. And don’t worry about the B+; 280v is fine. Is this with a 12AU7 or a 12AV7?
a 12AU7
Hi Matt,
Can you describe me please the difference in Sound between the original 4S and the 4S with Cathode Follower. Because i can not hear in direct competition but i build first the 4S+CF and then the 4S original with pot at front (100k) and like the 4S without CF much more but don‘t know if it is the sound or the not perfect match with the classD amplifier with a impedance of 33k.
You also talk in the diy forum about a circuit with the 12au7 and a output impedance of about 6,5k. I think this is also ok for my 33k classD amp. So could you please tell me the difference in Sound with and without CF and maybe sjow me the circuit from that 6.5k preamp?
Sorry for my english
Thanks
Patric
There shouldn’t be any differences in sound between the two versions. However, the differences in output impedance likely have a large effect while driving lower input impedance amps like the class D amp you mentioned.
The 4S with a 12AU7 and the volume control on the input, has an output impedance 11.5kΩ. With the volume control on the output, it has an output impedance that varies between about 2.5kΩ to 65kΩ depending on volume setting. The 4S with the cathode follower output buffer has an output impedance of 575Ω.
My hypothesis is that the low input impedance of the class D amp coupled with the action of the 4S drove up the 2nd order harmonic distortion and this is what you are hearing. The even order harmonics (especially the 2nd) are called the choral harmonics because they are prevalent in music and reinforce the natural hearing process. As such they tend to add a pleasant “fullness” to the music. I think this is what your are hearing with the basic 4S.
Hi Matt,
i think your hypthesis could be right. But is the relationship 11.5KOhm Output from the 4S to the Input impedancce of 33KOhm of the classD amp not good enough? It´s not the ideal of 1:4 but is 1:3 so i thought ist ok. Also i`m interested in your layout with the 6.8 KOhm 12AU7 preamp. This is a 1:4 relationship. Maybe you can send me per Email?
Thanks Matt for all your help and engagement
Hi,
is ground lift switch SPST type?
Do you recommend a specific make?
thanks,
Jay
The ground lift switch is really set during system setup and won’t need to change. As such, any good quality small switch will suffice. I like to use ones like these from NKK: NKK Switches M2012SS1W01
Regarding using the 12BH7 tube if your heater supply is capable of putting out a full 600 mA into the filament- If I built my preamp using the Hammond transformer per your schematic- then I should be able to use 12BH7 tubes?
Thanks for this great design. I’m able to roll different 12A*7 tubes galore!
Dan
Yes. The Hammond 296EX (or the 369EX if you need a 240v primary) has a 2.5A heater winding. So you should have plenty of filament current available for the 12BH7.
May I request the rectifier tube pin schematics? I want built my first tube amp. I find your universal preamp friendly for novice like me. I have built a simple op amp and that is as much I know about building an amp.
You can get the 6CA4 data sheet,including pinout, at the following link: https://tubedata.altanatubes.com.br/sheets/093/6/6CA4.pdf
Thanks Matt. I have built the preamp. I must say it great audio engineering by you.. so quiet. I am wondering how I could manage a lower gain… it’s loud even with a 12au7 at 8. The only change for my built is the 47uf instead of the 33 uf cap after the 6ca4. Hope you can help.
The simplest way to do this is a restive pad behind the preamp. The 12AU7 is ≈20dB of gain so maybe a -10dB pad would be sufficient? A simple voltage divider with a 24.9kΩ resistor in-line and a 10kΩ shunt to ground should give a -10.8dB reduction in output level. This would mean that the 12AU7 would give about 9dB of gain at full volume.
I did a voltage divider, it somewhat reduced the volume which I m happy. Thanks! I m wondering if the power supply of this preamp can be used for a phono preamp with 2 12ax7? The B+ required is + 250v.
Hi Matt
I would like to build a phono preamplifier. Can I use this circuit to modify the 4s universal pre-amp?
Here is a setup scheme:
https://www.dropbox.com/s/v4llkgp1r7miq37/4S-Universal-Tube-Phono-Preamplifier-Schematic-Riaa.png?dl=0
Thank you.
I haven’t run a simulation to see how close the poles are to the RIAA standard, but I am skeptical that this is going to work very well. You really need a buffer on the output otherwise, the equalization is too easily affected by the load and volume setting. Personally, if you want to use tubes, I would suggest the preamp from the old 1975 RCA manual. I have built it before, it works well and is easy to assemble. Just don’t try to make it too small.
Excuse me, I am a newbie
How do you connect the oscilloscope, the voltmeter and the signal generator to the preamp to do the testing? Thank you
This testing is done one channel at a time.
Lets start with the input. The signal generator should be connected directly to a single input (left or right). The cables you use are dependent on the test equipment you’re using. One of the two oscilloscope channels should also be connected to the signal generator. This way you can see exactly what signal is feeding the preamp. If you have a stand alone AC voltmeter, you can connect it to the signal generator as well.
On the output (of the same channel, left or right), you need to connect the other channel of the oscilloscope. This will allow you to see the output waveform. You may also connect an AC voltmeter to the output but you need to be careful. The AC voltmeter needs to have a relatively high input impedance (>100kΩ). Otherwise it will unduly load the output and distort the waveform. For example, the Leader LMV-195 shown in the test setup picture only has an input impedance of 600Ω. So it is ok to have it connected to calculate ac gain (with small signal inputs), but it must be disconnected from the output to measure distortion. If you have a modern digital oscilloscope, you can dispense with the AC voltmeter and just allow the oscilloscope to measure the voltage.
I hope this helps.
Hello
Just starting a preamp project to feed my power amp. I never worked with tubes before. My source is a DIY network player based on Volumio and a dual mono Buffalo IIIse DAC from TwistedPear Audio.
The preamp I want to build need to be balanced to fit the source and the power amp. After carrefully reading this post and its twin on DIYAudioProjects, I wonder if a simple 4 multiplication will fit my need ? I mean 4x 12AX7 tubes, 2 per channel to act as a preamp ? Here a link of what I have in mind : https://drive.google.com/open?id=1OnnT65jnonsf9zSDV3NhKnjpsSavQcqV
Technically, the design you drew could be considered a differential preamp, if the tubes were perfectly matched, and all the load and bias components were perfectly matched. However, in the real world, attempting to build a differential amp like this causes problems. The problem is that it’s not really a differential amp, it’s just two separate amps, one for each side of the signal. As such, it suffers asymmetry problems and has zero CMRR response. You might get it to work, but it won’t work well.
There are really two ways to go. The first is to use center tapped transformers on the input and output. This means that the input cable always sees a balance load, you can provide amplification with a single common cathode triode, and then the output transformer is used as the load for the preamp triode and the center tapped secondary provides a balanced output.
The other option, if you want to avoid transformers, is to use a mixer tube (like the 6J6) on the input, tap one load to drive the preamp tube, then couple to the output with a concertina phase splitter. This is a little more complicated, but will also meet your balance requirements and provide good CMRR. Note that this will only work for a high fidelity amplifier. The concertina has a massively unbalanced output impedance when overdriven so it should not be used as a driver in a circuit expected to be operated in overload (like a guitar preamplifier).
Thank you for you reply. This is much appreciated.
To sum-up my understanding :
– The design I had in mind is driving me to a no properly working system. I’ll avoid it then
– The second option is to unbalance the signal though transformers. My DAC has already a transformer output. I should think about it. My power amp is better behaving with balanced signal.
– The third option, if I understand properly, is using a 6J6 tube per channel to mix the 2 phases, using half a 12AX7 to act as preamp and use a concertina phase splitter to get back the 2 phases. This is not anymore an 4S preamp 😀 😀
This is higly interresting as I measure the way to go before finding a solution. As a new person to the tube world, the last option sounds interresting but I am far away from being able to calculate all values needed : capacitors, resistors, biais, gain, HV rails…
Hi,
It will work ok with a 6H30PI Sovtek ?
will that be ok ?
If I use an 6H30PI Sovtek are there any changes that I need to make from the original schematics?
Thank you.
The tube should work with the same component values. However, the filament of the 6H30PI is on pins 4 and 5 ONLY. This means that the socket needs to be wired differently. On the 6H30PI pin 9 is an internal shield. If connected, this pin should tie directly to the signal ground.
thanks for the help
Can I leave the PIN 9 disconnected ?
Yes. Just leave pin 9 of the 6H30PI disconnected.
can i use a tantalum 33uf 35V bypass cathode capacitor?
it is advisable ?
Yes.
But a word of warning. Tantalum capacitors have a very bad failure mode where they fail shorted. This can lead to the capacitor destroying the tube if it fail in this manner. Also, if by some manner it sinks enough current, the tantalum capacitor can catch fire and burn like a miniature road flare. When used in aircraft avionics we are required to place a fuse in-line with every tantalum capacitor just in case they fail shorted.
I would recommend getting a standard electrolytic if at all possible. The higher performance of the tantalum is not required and the risk to your amplifier is not worth it, in my opinion.
If I use an Edcor XPWR177 transformer are there any changes that I need to make from the original schematics?
No. The B+ will run a little bit higher but the 4S Universal actually works well with B+ voltages up to about 310V. With the Edcor XPWR177 your B+ will probably be in the range 270 to 280 volts. This is more than adequate for preamp operation.
Hi,
It will work ok with a 12BZ7?
Thank you.
Potentially. Just looking at the curves I’d estimate a bias of about a volt. However, the gain will be VERY high. You need to make sure that you really want that much gain.
Hi Matt,
What is the approximate anode voltage and cathode bias voltage? Just want to be sure that I am doing the right thing.
Thanks!
Francis
The bias voltages are given in the table above under the column labeled ECo. The plate voltages run from about 65v for the 12AU7 to about 125v for the 12AX7.
Hi Matt,
Thanks for your reply!
May I ask one more question.
If the preamp is built as mono-blocks, please advise which connection is better:
1. Use one section only, or
2. Parallel two sections.
Thanks,
Francis
If you only want one channel, just wire up one of the triodes and leave the other disconnected. Do not wire the triodes in parallel as this would require all different component values.
Why not parallel?
Wanted to rebuild the amplifier for a long time!
However, I would be interested in a parallel connection.
If it is well constructed, the dynamics should be improved by up to 4 dB and the anode current could even be reduced. Has anyone already taken a test?
Hi Matt,
The cap following the power tube is shown as 33uF@450v. I’ve got some 47uF, will that be ok ?
Short answer is yes. Just be aware that if you are using the Hammond power transformer, it will increase core heating. But that transformer gets pretty hot anyway.
The Hammond 269E doesn’t seem to be available anymore.
Any suggestions?
I just checked and the hammond 269EX is still listed on their website. (link: High Voltage (Plate) & Filament 200 Series) Alternatively you can also use the Edcor XPWR177.
Hello,
Like you I am also an electrical / electronics engineer who builds audio circuits for a hobby, and sometimes a little profit. I primarily work with discrete transistors, but way back when I did build a few tube amps.
My primary question involves how you are taking the THD measurements.
In my case I don’t have an audio analyzer, but use “clean” recorded tones from an iPod or from Audacity on my laptop. I then use the FFT function on my oscilloscope to view and measure harmonic content, then plug the numbers into some math and come up with a %THD number.
I also occasionally use a PC oscilloscope with a built in function generator to perform this measurement also – the one I have can give a direct %THD readout, but it is “suspect” – the function generator running in sine mode in and of itself has a THD of almost 0.5%, and I know to be truly accurate, a good audio generator with something like .001% THD or less is really required to get a good THD reading.
I am just curious as to how others with a “basic” basement lab are getting their THD measurements.
First, my %THD measurements are made much like yours. In pure tone testing I use the FFT function on my digital oscilloscope to determine the dBv levels of the harmonics, I then convert to absolute, and RSS them to get a total. This is a reasonable way to generate a THD.
Second, depending on your signal generator, 0.5% THD may well be perfectly reasonable. For one harmonic this would be about -46dBv down from carrier which would be right in the ballpark for a lot of wien-bridge oscillator signal generators. My title TENMA audio oscillator has a THD between 0.1% and 0.5% (-60dBv and -46dBv) in the audio ranges and it’s pretty typical. My Leader LAG-1208 has distortion specs between .05% and 1.5% (-66dBv and -36dBv) depending on settings, but getting down to below -60dBv is finicky on this particular unit. And my Precision E-310 vacuum tube signal generator is specified at <1%. That's only -40dBv.
Forget 0.001% THD. You'll likely never get there and any signal generator advertising such a value is blowing smoke. That's -100dBv. For those measurements your talking conditioned lab power, Faraday cages, and high end equipment. It's the same reason that the advertised THD of most SS amplifiers is total hooey. They are theoritical numbers or very controlled lab tests under very clean conditions, not real world operation.
Thanks for the reply, I’m glad to hear that the FFT and RSS method is being used others. Do you do anything to subtract out the distortion of the generator when you measure THD?
Today’s audio equipment ratings – more than “hooey” – nowadays a lot of “mainstream” audio ratings are pure bald-faced lies.
Low distortion generators – Rohde & Schwarz has a few with THD at -120 dBv, but one problem – they cost more than a new car, a little rich for my blood.
And, I think the reality of harmonic distortion is that it is impossible to hear <1%, most of us can't hear 5% in music or speech, and through some unscientific testing, I really can't hear anything wrong with the sound until I see clipping of music or speech on my scope.
As to the published specs of some of today’s amplifiers, it is important to realize how this is commonly done. What the manufacturers do is the publish a “referenced” distortion number.
Let me explain. The normal modern audio design approach is to produce an amplifier with very high open loop gain and then feedback a significant portion of that signal gain to cancel noise and distortion. So lets say the designers need 50W from a line level signal (i.e a standard power amp design). At 8Ω, 50 watts is 20v rms. A line level signal is +4dBu which is 1.228v rms. So the actual end to end gain they need is 20*log(20/1.228) or ≈24dBv. So they design an amplifier with an open loop gain of something large; like 80dB. Then the approach is to use 56dB of negative feedback to get back to 24dBv end-to-end gain. Now in addition they want really low low distortion, like the 0.001% figure you mentioned above. Thats -100dBv in all one harmonic (i.e. the dominant harmonic). But achieving and measuring that 0.001% distortion is very difficult. So how do they accomplish this?
Well, the theory of negative feedback says that the reduction in distortion is the same as the feedback factor. And the feedback factor (in our example) is 56dBv. So what they do is target the open loop distortion to a number they can achieve. In this case -100dBv + 56dBv or -44dBv. This is only 0.63%. Now the other thing that negative feedback does is significantly increase the bandwidth. So what they do, is design a high gain ≈80dBv but narrow band amplifier with a distortion factor of <0.6%. This is actually an achievable goal and a rather straight forward process. Then they measure the "open loop" distortion, convert to dBv, then subtract the feedback factor (in this case 56dBv) and convert back to %.
So lets say that the amplifier as designed has an open loop distortion of 0.57% (i.e. below the target of 0.6%). This measured distortion equates to -44.9dBv. Then they subtract the feedback factor of 56dBv to get a "theoretical" distortion of -100.9dBv. This corresponds to 0.0009% THD. And THAT is the number they publish even though the real measured distortion was about 1/2%.
The problem with this, of course, is you NEVER even come close to this theoretical distortion number in actual usage. But that doesn't keep the manufacturers from publishing it. This type of mathematical masturbation has been accepted practice in the audio equipment industry for decades. But once you understand what's actually going on, all those super small distortion numbers become just so much advertising foolishness.
Thanks for the interesting insight on THD measurement and calculation. Come to think of it I have seen a places on the net. That some builders do say that the “real” distortion is actually the measured number minus the calculated reduction due to NFB.
Amazing, what you can twist with math.
BTW, the pictures of your builds are beautiful. Your finished product looks the same or better than most of the commercially available tube gear out there!
Matt,
I have recently started utilizing Ltspice to try some modeling in order to learn how various component values may effect an amplifier response. Everything went well until I put a model of a speaker on the output of the preamp circuit during simulation. As you mentioned, your testing is done with an oscilloscope and a.c. voltmeter. I’m not sure what the final impedance is seen by the amplifier. Have you ever looked at results utilizing a speaker as a load? Is this something I should be concerned with during design and testing? As always, I greatly appreciate your time and consideration towards the DIY community.
In general, amplifier circuit testing or modeling with speakers is not done. This is because speakers present a frequency dependent, complex impedance which tends to mask circuit operation. Using a resistive load (i.e. a constant real impedance) elminates variations in circuit operation due to load nonlinearity. Speaker impedance is also very difficult to model correctly due to the variables involved. Models of speaker impedance seldom accurately match real world response.
Take a look at my blog post here Sizing up a new Power Triode and you’ll see that I use a resistive load (the unit with finned heat sinks in the back) so that I can accurately determine how the tube is performing.
About the only time I do testing with speakers is when I am measuring SPLs. And this is only for speaker testing and not amplifier testing. In rare instances it may be required to monitor speaker impedance referenced to the amplifier across frequency when employing a compensation circuit in an amplifier. This is not something I do regularly.
Hi Matt,
I built this pre and got it working over the weekend. Simple and clean sounding. I am really happy and I am sure it has some more burn in to go. I had read that the gain on this little guy can be more than some would prefer and I am afraid I might fall into that camp. Here in my office with a 12au6 in place the usable range of the volume control is between 6 and 7 o’clock. Is there any mods to this circuit which would limit the gain some.
Thanks for the design and the knowhow. It has been a pleasure to construct this.
Scott
As a follow up, would a stunted volume pot like this work?
http://www.world-designs.co.uk/forum/showthread.php?t=5795
Thanks,
Scott
The 12AU7 provides about 20dBv of gain. If you want lower gain what I recommend is a -21dBv pad in front of the amp. It is like the diagram in your link but replace the pot with a fixed resistance. Make the series resistance 100kΩ and the resistance to ground 10kΩ. And eliminate the 500kΩ grid resistor on the first stage. Use good quality metal film resistors for minimum noise.
Matt,
Thank you for the response. Please excuse my ignorance, I am a newbie. Is this even close to your instructions above?
Thanks Again,
Scott
That’s exactly what I described. This will give you a -21dBv pad prior to amplification. This means that with the 12AU7 in the preamp, you’ll get approximately unity gain with the volume at full.
Hi Matt,
I was wondering if this amp could be used as a stand alone amp if the output stage includes an impedance matching transformer to drive speakers?
Thanks, John
Unfortunately, no. This is a voltage amp and not a power amp. Even with an impedance matching transformer it will not provide the power necessary to drive speakers. If you are looking for small power amp, I highly recommend the 6CY7 amp. It is inexpensive, easy to build, and sounds great.
Matt,
First of all, thanks for sharing your designs and all that stuff. Now, my question tags on the March 15, 2014 post from Arturo. My son would like me to build a mic preamp. He is using Shure SM57 and 58s. Can this circuit be used for that? Also, he may occasionally want put his electric guitar signal through it. Any help is greatly appreciated. Thank you for your consideration in this matter.
MSgt Brian E. J. Taylor, USAF (retired)
Brian;
I would like to be able to say yes but unfortunately, this preamp just doesn’t have sufficient gain to serve as a microphone preamp. Microphone preamps have two basic requirements. One is very low noise, the second is a gain of between 50 and 60dBv. While this is a good preamp design, it really isn’t low noise enough to give good results with a microphone and it definitely lacks sufficient gain by a significant margin.
The Sure SM57 has a sensitivity of -56dBv/Pa. This means that at a sound pressure level of about 94dB it has an output of -56dBv. In order to get to a line level signal to drive a mixer or general purpose amplifier will require a preamp with a gain of abut 60dBv. For microphones, you might want to think about something like the Behringer TUBE ULTRAGAIN MIC100 Preamp for the microphone use.
As for the guitar, with a 12AU7 this can make a really nice little boost amp for an electric guitar. Of course, you only need one channel, or you can just build a two channel unit with separate volume controls.
Thanks Matt for the drawing and all the idea.
Could I use copper chassis to build this pre-amp?
Furthermore, I assume that the output of this pre-amp is ok for the 6sN7 pre-amp stage of my amp (6SN7-300B SE)
Dimitrs
Copper makes a great choice for chassis material. It’s easy with which to work, provides great shielding, and can even be soldered. The only down side is cost.
If you look at the calculated performance table you’ll see that the 4S has a varying output impedance with peaks ranging from ≈65kΩ to ≈80kΩ. In general the input impedance of the amp should be more than four times the driving impedance. so your amp should have an input impedance greater than about 320kΩ. So it really depends on what the input of your amp looks like. You would probably be ok if your input volume control is 250kΩ or greater. Otherwise I would abandon one of the volume controls and rework the interface.
Hi!
Do you think this pre amp will be capable of driving my power amp (LM1875 based on non-inverting design of National Semiconductor datasheet which includes a 22K resistor to ground at the (+) input of the chip)?
I currently use a 10K potentiometer in front of the amp as my volume control and it works well instead of higher potentiometer (I tried 50K and 100K and they sounded worse).
Thanks.
This will probably not drive your LM1875 very well. The input structure of the LM1875 is such that the input impedance is simply too low for effective 4S drive. The best way to correct for this is to insert a buffer stage between the 4S and the LM1875. The buffer needs a high input impedance and a (relatively) low output impedance. This could be done with a vacuum tube cathode follower, a transistor emitter follower, or a solid state op amp properly configured for buffer operations. Just remember that the buffer input impedance should be at least 250kΩ for proper 4S operation.
Hi Matt, thanks for sharing this wonderful design, the circuit is fairly simple, and
I have a silly question about this pre-Amp.
Is it possible to use it as a headphone amplifier?
Justin;
This is a line stage preamp and the output impedance is far too high for headphone use. For headphones you need a much lower matched output impedance.
Take a look at The Recovery Amp for a good design for headphones. I have used this amplifier (actually the “Rebuild” version) five days a week, eight hours a day for a couple of years and it still is performing flawlessly.
Hey Matt, thanks for sharing that desing with the community. I love doing my own audio setup and i like to keep it simple, so your little project fit really well. I’m in the part scavenging to build one of these, but i chosen to use a 5y3GT and a 6sn7 from and old 6v6 GE monoblock, so i guess i remove the universal but keep the 4S. I think i will add an input selector to. Keep working, you got an another “suscriber”!
Have a nice day.
Frank
Glad to hear you like the web page. If you’re using the 6SN7 you might want to consider this circuit. “6SN7 4S Circuit”
Thanks, i build it around this schematic, i’m building a wood box and a aluminuim top plate for the retro look, will it be ok to drive a 10k ohm SS amp input?
The output impedance of this preamp is far too high to effectively drive a 10kΩ amplifier input. This would require a buffer stage (ss or tube) to properly match the impedances.
I got a little question again, when verifying the voltage, do i need to put a load resistor? Or i got a little problem where the b+ split to plate, one got 60vdc and 80vdc on the other.
Frank:
As to your first question, no you do not need any output load while checking voltages. As for the other problem of varying and low B+ voltages I’ll need a little more information. If you could go over to the diy audioprojects forum thread for this project (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628) and post your question along with a schematic showing exactly which voltages you are measuring and what tube you are using, that would help a great deal.
Thanks.
Matt
Hello, Thanks for adding the 12bh7 option. I am going to build this version. Also do you have any opinions on external power supplies, where the power supply is in one box and the line stage is in another box?
There are two conditions where I like separate power supplies. The first is in large builds where it can control the layout so there isn’t a large or unwieldy chassis housing everything. Personally if a chassis layout footprint is getting much above about 10″x16″ I begin to think about splitting the setups either in a PS/AMP layout or in mono-blocks. The other place where I like to split things sometimes is in headphone amps. This way a small amp can sit on a table next to a chair for easy volume control and the PS can sit below so there is still a place on a small side table for a mug and or a book.
For a project like this, it is so small already, I would not even consider splitting the chassis. I just don’t think that it’s worth the extra effort.
I’m thinking about building this preamp but I’m new at this. I want to use it as a phono preamp. I’m curious if this preamp would provid the necessary equalization for records. I think they call it RIAA.
This preammp is a line stage and, while very good, it lacks the RIAA equalization curve you’ll need for phohograph playback. There are several good solutions for building such a preamp but the one I would suggest for your first RIAA preamp is the JFET Phono/RIAA Preamplifier kit from Boozhound Labs located here (http://boozhoundlabs.com). This kit performs very well and still allows you to do your own power supply and case.
Hi Matt,
interesting little preamp and so simple.
I’m amazed at how much can be achieved with so little in valve amps.
I would be interested in building one of these as i have always wanted to see if I like the sound of a valve system.
I have a turntable with a AT-95E cartridge connected to a cheap($30) ss preamp and a cd player currently connected to a pioneer elite av recvr. I take the pre outs from the pioneer and connect into a ss 2 ch yamaha pc2602 power amp. I would like to connect the cd player thru this little preamp of yours directly to the yamaha. Do you think it would sound better than thru the pioneer elite av rcvr ?. thanks in advance Phil Murray
Phil;
“Better” is always a subjective assessment. However, I would hazard a guess that the CD player fed through the 4S with a 12AU7 in the preamp should sound pretty good. There is one catch however. The Yamaha PC2602 only has balanced inputs and this preamp has unbalanced outputs. An electrical translation will be required. Provided you make this translation it should sound great.
Matt
Hi Matt
i see you only used 1 side of bottle , i was thinking ,i could just duplicate the components and use the other side of the tube ,to make a push pull version.
Could i strap a transformer on the back of it ? or is best to use the cap output.
cheers
den
Actually, I did use both sides of the dual triode because it’s a stereo preamplifier. I simply showed only one channel in the schematic.I’m not sure how duplicating the components would allow you to make a P-P version. As this is a voltage amplifier and not a power amp there really is no need to construct a push-pull stage.
cheers
den
hai.. i’m want to make preamp class A
Amazing tube preamp. Do you have any schematic or something to build it in PCB?
I have only done point to point versions of this preamp. However, If you contact the author of this post (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266&start=256) then he should be able to help. If not, post a request on that forum thread.
In your excellent write-up on power supply design you go through a calculation and arrive at a 250v DC supply output from a 275-0-275 transformer. In this preamp design you show a 250v supply being generated from a 190-0-190 transformer. The math doesn’t seem to add up. According to your analysis the 190 transformer should provide a DC supply voltage of 142 volts or so.
I believe what you are missing is the different loss in the rectifier and filter circuits of the two designs. The power supply design in the paper for the “Ghost Amp” has a peak rectifier current of 260mA. The rectifier drop in this design is around 28v (Er≈0.74) and the filter loss (with a 130mA dc load current) is over 40v. In the preamp design, the peak rectifier current is about 11mA with a drop of around 3.6v (Er≈0.96) and the filter loss is about a third of a volt.
So, the ghost amp design has about 68v of loss whereas the preamp loss is only about 4v. Also the transformer secondary waveforms turn out to be quite different which affects harmonic generation and conversion efficiency. I know that at first glance it doesn’t look like it makes sense, but this is why I wrote the power supply design paper in the first place.
Yes, that’s clear, thanks. Makes perfect sense. Your comment about the secondary waveforms differing and their effect on harmonic generation; I’ve never heard this discussed. Can you point me to a reference? I’d like to read up on it.
thanks for the help
I don’t have a reference for harmonic generation, that’s just how it turns out when you do the math.
You can think about it this way: In a light load condition the first capacitor is going to discharge very little and the rectifier current waveform is going to approximate a square top pulsed waveform. But in a heaver load condition, the cap discharges more and the waveform starts to look more like a sawtooth waveform. When this happen, more harmonics are added. The filter is made to dump everything above the first ripple frequency so the more harmonics you have, the higher the loss in the filter. Make sense?
Hi Matt. I have a realy silly question about pre amps (I am new at this):
what would happend if I use this configuration:
iPod or a apple laptop – your S4 tube preamp – an ADCOM 545ii (100 WATTS p/ch) AMP – and a great pmc twenty.22 speakers?
My question, other than if this configuration is posible, is that if the final sound would be nice.
I’m tired of seen preamp prices over thousand dollars or more, so, should I invest in little creators as yourself or some DIYers?
Thank you Matt, realy impressed with what you have done here.
I think the answer to your question is that it should sound good. I checked the frequency response and you should be fine with the ADCOM 545ii’s 50kΩ input impedance. You don’t need much gain since 1.2vrms drives the amp to 93W output. So I would recommend using the 12AU7 tube and keeping the iPod output relatively low (less than 50%). Then use the preamp volume as your main volume. The 545ii is a nice amp. It should make for a very nice sounding system.
Hi Matt, is there a step-by-step type instruction for this pre-amp and what would it require to crank up the gain to line level?
As for a step by step guide, I really don’t have one. I would however suggest that you look over the “Super simple single stage tube preamp” thread over at the DIY Audio Project vacuum tube forum. The discussion of the 4S starts here “Super Simple Single Stage Tube Preamp – Post 109”
As for your request to “crank up the gain to line level”, I’m not quite sure what you mean. The preamp has gain figures from 19.9dBv to 34.6dBv depending on configuration and output voltage swings in the range of tens of volts. As such +4dBu (1.228 vrms) is no big deal. The real question is how much gain do you need?
Hi, I am looking for a Mic Preamp. Does this one delivers enough gain to get Line level (+4dBu) from mic level (-40dBu aprox)?
If you look at the gain table you’ll see that if you use the bypassed cathode configuration and a 12AX7 tube, you should get around 34.5dBv of gain. This will take a -40dBu level to approximately -5.5dBu. So not quite. But, whether it will be useful or not is really a function the system you are driving. If it’s going into a consumer amp, this may be just fine. If you’re driving the input of a commercial mixer board it depends on how much gain you can get in the channel off the board.