Gain and Volume

Yep, it’s tech time again. There are numerous explanations of these two features of an amplifier, and all those I have seen don’t explain it very well. Mostly because they don’t use a model of an amplifier, which I’m going to do.

Picture your guitar amp. At one end is the guitar jack, which carries a tiny, tiny current from the pickups. If that was transferred across to the speaker, we wouldn’t hear a thing. Nothing like enough power. So we need some more power from somewhere - which is why the amp is plugged into the mains, to feed a transformer that feeds the amp’s circuitry. That feed is run through some kind of “amplifying widget”, which might be a valve, a transistor, some combination of both, or some other device.

This widget takes the guitar signal in one connection, the transformer feed in another connection, and combines them in such a way that the signal from the guitar affects the current from the transformer flowing through the widget. (See electronics textbooks for details.) If the widget works properly, the output will be a signal that looks like the input from the guitar, but on a large-enough scale to drive the speaker.

In summary…

Guitar input signal -> widget
+
Current from transformer -> widget
=
More powerful copy of the guitar signal from widget to the loudspeaker

Gain controls are on the power input side of the amplifying widget. Turning the Gain up increases the amount of power into the amplifying widget, and increases makes the output signal… Gain at 0 = signal direct from guitar with no increase in power, Gain at 10 = guitar signal amplified to maximum input power

Volume controls are on the power output side of the amplifying widget. Turning the Volume up lets more of that power pass to the speaker, so it gets louder… Volume at 0 = no output power, Volume at 10 = as much out as the Gain creates. 

Now here’s the thing. The amplifying widget will change how it responds as more power is applied to it. That’s why turning up the Gain often produces distortion (unless the widget has a kilometer of “headroom”). But when we adjust the volume, it won’t change the way the widget works, because the volume is on the output side, after the widget has done its thing.

However, adjusting the volume will affect the power going to the speaker, and that will affect the way the speaker reacts. Less power and it won’t be able to transmit the fine details in the signal loud enough for us to hear. Which is why a crunchy distorted tone at high volume turns to a nasty fizz at low volumes.

So that’s that.