I’d like to take a few minutes to talk about about required amplifier output power. People who advocate low power single ended (SE) amplifier designs often get a bad time about the low output power of their amps. Due to the nonstop marketing forces of the consumer audio equipment industry over the last 40 years, most people think that 25W to 50W per channel is the absolute minimum acceptable power for any “decent” audio amplifier. They have been told repeatedly that this type of power is required to get any volume out of your system. But is this really true? Most of the are SE tube amps are from one to four watts peak output power. This seems totally inadequate to the “well marketed” masses but in reality, it’s much more power than you’d think. Matched to appropriate speakers, in a normally sized room, these amps can put out a surprising volume level and very dynamic sound. So let’s investigate how this could be.
First, it is important to understand sound volume and how it actually relates to power. Sound or music volume follows a logarithmic scale. This is why we always speak in dBs, it’s a logarithmic scale. In this type of scale, an increase in ten units (e.g. dBs) actually means multiplying the absolute number by ten (e.g. x10). So if 1 is 0dB then 10 is 10dB, 100 is 20dB, 1000 is 30dB, etc. Now, most music also has a fairly large dynamic range, usually at least 30 to 40 dB; and in general the average level of the sound is going to be between 1/3 and 1/2 of the dynamic range scale in dBs. But what does this actually mean? Lets talk some numbers.
Here is a table of typical sound pressure level (SPL) values of some common sources. Notice that some of these are inside and some outside; and this makes a difference. If you think that chainsaw you are running is loud standing outside next to a wood pile, imagine running it in your living room!
As you can see, some of these numbers are quite loud. When you add to this the fact that most music is played in a typical room of somewhere around 12 by 16 feet, that 100dB number is very loud. Frankly, most music is listened to with an average SPL somewhere between 60 and 80 db. So splitting that 40dB dynamic range referenced above a typical 70dB average volume track may have a peak volume around 90dB and minimum volume around 50dB. These numbers can be related to the examples found in the table. So now that we have an understanding of sound pressure levels, lets discuss how amplifier power relates to it.
Amplifiers are rated at their peak continuous output power. You can think of this as a single frequency sine wave at the peak output power or even continuous white noise at one volume level. But this isn’t how music sounds. Music has a dynamic nature to it with some parts louder and some much quieter. This is why I talked about the average SPL and dynamic range. Now to reproduce the sound, two elements are required, the amplifier is one, the other is the speaker.
Speakers are rated by their sensitivity. This is generally given in SPL in dB, at one meter in front of the speaker, per watt of input power. This means that a speaker rated with a sensitivity of 90dB will produce an SPL of 90dB, at a distance of one meter, when one watt is applied to the input. Sticking to this same speaker, with two speakers about 4 feet apart angled in slightly and listened to from 1 meter away, the overall average pressure level will be about 3dB higher or about 93dB. So at one watt, the peak will be about 93dB and the average will be about 20dB less or 73dB. And at 73dB, the amplifier will be outputting on average about 10mW per channel (~1W peak). If we increase to an average of 83dB (fairly loud), the amplifier will still only be outputting about 100mW per channel on average (~10W peak and ~103dB). So, on average, the amplifier is actually putting out very little power (less than a watt) and the extra power is really there to provide headroom for very loud passages.
The other thing to consider here is breakup characteristics. This is how the amplifier sounds when the input level causes it to exceed it’s max power. Transistor amps break up very suddenly producing very pronounced 3rd order harmonic distortion. This is very unpleasant so people tend to back down a transistor amp so that it never reaches this point. Now, if there are loud passages that peak really high, say +25dB above average, then assuming the loud average listening level of 83dB average; then the amp will need to cleanly go to 108dB. In our example this will require an amp with a peak power of 15dBw or about 32w. And the listener will reflexively reduce the volume so that the amp never gets to this level because when it does, it sounds really bad.
But what about the vacuum tube SE amplifiers. Well, they breakup in an entirely different manner. When you begin to overdrive the power stage of an SE amp, it gradually slides into compression which slowly adds increasing amounts of 2nd harmonic distortion. This type of distortion actually is not at all unpleasant to the ear and often goes completely unnoticed. This means that a typical user will often drive a vacuum tube SE amp as much as 5dB into compression and not even notice. This takes that very loud listening condition above and reduces the peak power level requirement to only 10W for the same average volume level. And if the music does not have those extremely loud passages and only peaks 20dB above average, then the peak power requirement for the SE amp drops to 5dBw or about 3w whereas the transistor amp still requires over 10w (and will probably be adjusted downward to avoid any breakups). This is why so many people comment that SETs just sound louder. It’s because they are much more efficient at producing real music. This is something that never shows up on a spec sheet but does manifest itself in real listening tests.
The other side of the SPL discussion is about increasing power levels, It takes about a 3dB change in SPL to register in the human ear. So lets go back to that speaker/amplifier setup I mentioned above. If 1w produces 93dB, then 2w will add three dB and make it 96dB. Now here is the rub concerning logarithmic scales. It takes another doubling to get an additional 3dB. So to get 99dB takes 4W. I hope you can see where this is going. It takes 8W to get to 102dB and 16W to get to 105dB. So by going from 1W to 16W (a 1600% increase) we have only accomplished a 12dB SPL increase (an ~12% increase). I hope you can see from this discussion why most of us feel that a few watts is more than enough in our SE amps. Granted, sometimes more headroom is really desired, but most times it is really not required.
Questions or comments? Drop me a note and let me know what you think.
Thank you Matt, now I am even more eager to start work on Lacewood amplifier
Matt thanks for the informative section on power and music listening relationship above. It makes me appreciate my SETs far more and goes someway to explain why listening to them is so enjoyable.