Electronic – In an amplifier, does the gain knob boost or attenuate the input signal

amplifiergain

I'm thinking specifically about guitar amplifiers, but I'm guessing this applies to all audio amplifiers.

I understand the practical function of a gain knob, but what's not clear to me is whether the gain knob is simply a potentiometer that attenuates the incoming signal, meaning if the gain knob is all the way up, it's simply letting the signal through unaffected, or if the gain knob controls some sort of boost circuit that actively increases the input signal strength before sending it to the preamp.

Or does the knob somehow directly affect the operation of the preamp making it provide more amplification or something like that?

Best Answer

It could be either when determined by marketing. Two common configurations are shown below.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1a: Fixed gain pre-amp with preceding attenuator. 1b: Variable gain pre-amp.

  • 1a is simple but has the disadvantage that the noise from OA1 is constant even with the input turned to zero.
  • 1b has the advantage that as the gain is reduced so is the noise.

(Gain in the example circuits is -R2/R1 or -R5/R4.)

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