[Things your Mom warned you about] Volume control vs power output

Things your Mom warned you about: do not exceed 12 o'clock!

Debunking audiophile myths - power output vs volume control position

Author: Lucio Cadeddu - TNT Italy
Published: March, 2022

Fact #1: the volume potentiometer of your amp/preamp and the gas pedal of your car are two completely different things.

The volume control of an amplifier - or of a preamplifier - is just a passive component (read: it does not add anything!) that interacts with the strenght of the electrical signal that passes through it. When you turn it clockwise, it just lets more signal pass through. In other words, it is an attenuator, more like the brake pedal than the gas pedal, if you wish. To make things even more counter-intuitive, there are two kinds of potentiometers (err...attenuators): linear and logarithmic ones. A linear attenuator is the dumbest thing on Earth: turn it clockwise till its half position (say, 12 o'clock) and it will let half of the signal pass through. And if the angle of rotation is ¼, then ¼ of the signal will pass through. In other words, its attenuation effect is linear, in the sense that it acts as moving along a straight line.

Fact #2: unfortunately, our ears are far from being linear! The linear scale, as in the example above (½ = ½, ¼ = ¼ and so on), is the most intuitive one, while the human hearing works better on a logarithmic scale, one which is much more powerful when you need to work with a large dynamic range. Have a look at the two sequences below:

Linear Scale        0   1   2   3   4   5    6    7     8     9      10     (+1)
Logarithmic Scale   1   3   9   27  81  343  729  2187  6561  19683  59049  (x3)

The first one covers all the values between 1 and 10 (+1 steps), while the second goes from 1 to 59049 (3ⁿ)! The 3 factor is just an example, of course. Human ears works this way, they allow us to detect the smallest volume variations, but are still able to react to huge volume variations. Hence, a logarithmic scale is pretty convenient to use, since it is a way of displaying numerical data over a very wide range of values, in a compact way.

For this reason volume controls are generally logarithmic, otherwise it could be difficult to precisely adjust voume at low listening levels.

Now, I think you've had enough of technobabble. Let's focus our attention on your Mom's wise words: do not exceed 12 o'clock position! and she wasn't referring to some Kamasutra figure. It turns out that, for the reasons above (and some other, more technically involved) the max power output of an amplifier is reached close to the 12 o'clock position. Above that limit it's mostly harmful distortion. Hence, don't be fooled by the position of the volume control! At 12 o'clock, chances are that your amplifier is running out of steam, being close to its natural limit. Anyway, as any wise mother knows well, there are always exceptions to any rule!

Exception #1: the output level of the source (say, a CD player) doesn't match well with your amp/preamp input level sensitivity. More precisely, assume that your CD player puts out 2 volts of signal, but your amp just needs 1 volt to deliver its max power output: you'll be in trouble adjusting your listening level! Small movements of the volume control will produce huge variations in perceived sound pressure. If that's the case, you might need to attenuate the output of the source. There are attenuated RCA connectors that are meant to cure this problem (Rothwell, McMantom, Harrison Labs, dBevolution and others). They contain small resistors that attenuate the signal (more or less like your volume potentiometer does) and the values of these resistors can be chosen appropriately, depending on the amount of signal attenuation you might need (-6dB, -12 dB and even -24dB!)

Exception #2: the output level of your source is too low, when matched to the input sensitivity of your amp. This sometimes happens when using a turntable with a low output cartridge: you may end up turning the volume control completely clockwise (!!!) and still not getting the max power output. In this case, you can exceed the dangerous 12 o'clock position and still live happy (read: nothing will explode). Moreover, you might need a preamplifier to increase the level of the line output of the source. Actually, this is why God created preamplifiers, after women, besides selecting different inputs.

Consequences

First of all, comparing two amplifiers by setting the volume control at the same position is technically wrong. The only appropriate way to compare two amplifiers is using a sound level meter with a test track: decide the SPL you desire and mark the volume position that allows you to reach that level. Then compare the two amps without varying the volume control position.

Secondly, the position of the volume control says NOTHING in terms of max power output. Considering the exceptions above, the amp could reach its max power way before the 12 o'clock position or even when the volume control is turned fully clockwise.

For these reasons, the volume control isn't like the gas pedal of your car: when you floor your car (the famous pedal to the metal driving style) you are sure you're asking your engine to deliver its max performance. Not so the volume control, that can be assimilated to the brake pedal instead: you push it harder to make car speed decrease quickly. If you turn the volume control counterclockwise you're actually braking your amplifier. Without the volume control, the amp will deliver its max power.

Conclusion

Was your mother wrong or right? As it happens, that depends on each situation. As a rule of thumb, full power is generally achieved near the 12 o'clock position of the volume control, but there are notable exceptions to this rule. You can turn the volume control fully clockwise and still not get the max power available or run into severe clipping distortion.

Do not miss our article on tone controls, in the same series Things Mom warned you about!