Two bit controlled audio amplifier

A few years ago I had to repair a stereo, it was bad a digital volume control made by an unknown producer and no chance to purchase that from any distributor. I had to made a full replaceable circuit supplied from one positive tension. First choice was using a FET as resistor controlled but I had only n-channel FET (this need a negative potential for polarization) . After some think look what I had done:

FET acts like a resistor controlled ( this works ok only when Uds is very small ). C1 and C2 maintained  a galvanic isolation between dc polarization and ac signal. These capacitors are chosen with a small impedance for signal. C4 is a hold capacitor, this must have a very little leakage current.
C4 is charged through R7 ,R6 and cmos switch U1, when up button (connected to +16V) is pressed and discharged through R8 and cmos switch U2, when down button (connected to +16V) is pressed. Volume is changed with a variation give by time
 t = R8xC4 or t = R6xC4 ( R7 can be considered  negligible ). After volume is set, with a good C4 tantalum capacitor, it remains the same more than 4 hours. With a polyester or PMP capacitor, 24 hours of volume constancy can be achieved.
It's only another solution...

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last update: 01.10.2000