Level and Gain control are not the same thing.

Gain control is where the actual voltage gain of the circuit changes as one rotates the control.

This is distinct from Level control where the control is a variable attenuator.


LEVEL CONTROL

A Level control has all the stages in a pre-amp running at a high level of amplication While they will adjust the noise level of the stage before the control, they have no effect on the noise introduced by the circuitry after the control. This sort of control operates by taking the signal of the first stage and attenuating it. 

The disadvantage is in that no matter the control setting, the point where the imput overloads stays the same. As well, the distortion below thr overload point stays the same. In mic pre-amps this means that the pre-amp needs a pad or a attenuator switch on the input to cope with higher level sugnals.. 

This configuration uses the readily available Audio Taper control. Known as a A taper from Asian suppliers and a B taper from suppliers elsewhere. It is also described as an "audio" taper by everyone. This has the advantage that the the control is readily available. 

GAIN CONTROL

A gain control is in the feedback loop of the Pre-amplifier stage, usually an op-amp.
This has the advantage that as gain is lowered, the noise and distortiom level of the op-amp also drops. As well the input signal capability increases. The further you turn down the gain the input overload margin increases.
 
In one Professional Mic-Pre I designed, when the gain control was at the top, we had 60db of voltage gain but it would overload with a -30dBu input signal.

But when the control was at 10% the gain of that stage would be about 10db. The noise level of the 0utput was over 40dB lower. As well we could drive the circuit with a 0dBu input signal and not overload the circuit.  We could plug line level studd directly into the XLR mic input. Things like wireless mic receivers, keyboards, and anything else we had. 

The are however, two catches in using this configuration. 

1. The taper is backwards and for it to work correctly, one needs to use a reverse audio taper potentiometer. Also known as a C taper pot. However these are not commonly stocked by suppliers . I was lucky in this as we were building a lot of these and we were able to order C taper pots direct from the pot maker in enough quantity to get a good price. 

2. The gain only goes down to unity. That is, there is always w little bit of output. I solved this by buying dual gang C taper control pots and using the second wafer as a conventional attenuator. Because of the C taper, in the upper hald of rotation, there is little effect on the level of the signal. But as one turned the pot further down the effect of the seconf wafer would come into play and then when the pot was turned all the way to zero, the second wafer also assurred there was no signal leaking through. 

The use of a dual gang C taper pot solved both these issues in Mic preamps.

We built thousands of mic preamps this way with true "auto-padding" input. This setup eliminates the need for a pad pot or pad switch on a mixer strip. 


In a stereo pre-amp which never went into production, we used a 4 gang C taper pot instead of a conventional attenuator.