Rafa,
How do you want to use it? Line amp, make up gain, mic pre?
HiFi designers make amplifiers that are optimized for a particular gain setting like 20dB, 40db, etc. depending on the application. The concensus being that variable gain alters the sound of the amplifier which is bad. Studio engineers do not have this luxury because the noise from microphones starts adding up when mixing multiple tracks to levels that are not acceptable today. So gain has to be variable for optimum noise performance at the price of changing characteristics throughout the gain range.
If you try to use amps that have not been tested for the gains you need at lower gains they may become unstable (oscillate) or at higher than intended gains they may loose their good sounding characteristics. If you build it you will have to do some experimenting to make sure it works for your purposes.
Cheers,
Tamas
