For a while now, ever since the “Universal Preamp” was posted on the site, I’ve been getting steady requests for a good tube preamp to provide some color to solid state amplifiers. My initial answer to this problem was to design a good quality cathode follower for the Universal Preamp. There was one problem, the volume control.

ATTENTION: There is now a revised schematic for this preamp posted in the post titled “The 12AU7 “Color” Preamp with Improved Buffer”. If you are considering building this circuit, it is strongly recommend that you use the revised schematic.

I originally posted my design on the forum over on the DIYAudioProjects website. However, I was never truly happy with the design. The reason was this; in order to accommodate the cathode follower without adding multiple coupling caps, it was necessary to move the volume control to the input of the gain stage. If you read my post on 4S volume control placement, you’ll understand my problem. This placement seriously restricts the tonal shaping of the gain stage.

This problem surfaces in the back of my mind from time to time. Since I finished the wiring on the test jig yesterday, I was looking for a small project to work on today and this came to mind. Then it hit me, I can do exactly what I did with the Universal preamp and just scale it for the lower impedance. This is one of those ideas I should have had years ago but never did. I was always thinking about high input impedance amplifiers and never clearly thought about the problem of only driving solid state equipment.

It is simply a matter of selecting an output volume control that is large enough to not unduly load the cathode follower yet small enough that the peak impedance will effectively drive a solid state (low input impedance) amplifiers. To accomplish this I designed a good quality cathode follower stage with an output impedance of 575Ω. To this I added a 10kΩ output volume control. The peak output impedance of this pair is about 2.6kΩ. This will drive any solid state amplifier with an input impedance greater than about 10kΩ. This constitutes the vast majority of solid state power amps.

Here’s what the output impedance verses volume setting curve looks like.

In addition, if significant signal chain gain is not required, the end-to-end gain at the 50% volume setting is about 0dB (i.e. output voltage equals input voltage) and the output impedance is less than 1kΩ. This isn’t as good as the cathode follower itself but it should work very well. And because the volume control is on the output, the stage provides all the tonal shaping desired.

For the gain stage it would have been simple to just use the Universal Preamp design. However, I wanted the ability to provide good gain control (i.e use a lower gain triode like the 12AU7) but provide more color and tonal shaping than the 12AU7 does in the Universal Preamp circuit design. So I went back to the 12AU7 and played with some different loads and bias points. I finally settled on a design which halves the plate load resistance and doubles the cathode bias resistance.

This provides a stage with the additional tonal shaping I wanted to achieve yet increases the bias point so the preamp should never suffer overdrive when driven by normal line level audio equipment. So here’s the final schematic which was recorded in my design notebook.

So now I have a design to truly answer this need. The solid state class-D amps I’ve been using have input impedances between 22kΩ and 30kΩ. This design should drive that admirably. Note that this is not like the Universal Preamp where tubes of various gain may be used. This circuit is specifically designed for the 12AU7 triode. Both for the gain stage and the cathode follower.

I have some new Hammond GK88VG preamp power transformers which just arrived. If I can find a suitable chassis, this should make a quick and rewarding project.

As always, questions and comments are welcome.