Sorry I didn't mention this before, I'm using an electric guitar with magnetic passive pickups (no preamp in between). I'm testing with headphones and not with a guitar amplifier. With 10K on R6 it produce a little less noise. (Also thanks for the lutherie link, it's very interesting for me).
Anyway, as you suggested I tested the circuit with 2 9v batteries in series and... no noise at all! Neither at maximum gain! And it sounds great!
After that I tested 4 or 5 power supply and the best is one of an old a laptop, but still not noiseless.
Is it possible to eliminate the noise from the power supply or should I use always batteries? I'm just using a powersupply with multiple isolated output I thought it was a good choise.
ps, I'm posting the last schematic if anyone else from the forum needs it.
Some notes:
- The C3 and C6 do almost nothing for the (low impedance) ripple of the power supply; increase the impedance as I show on my schematic.
- The R1 and R2 can have ten times higher values ...
- The U1B, as it is used, does nothing useful! It supplies a few pA by loading the divider by ... a few pA!
- The C5/R6 values do not agree with a guitar pick up characteristics and an audio amplifier...
- The RV1, as it is connected, moves the operating point of the amplifier!
- It is generally a bad idea to change the gain of an amplifier, it changes also other characteristics of it.
- The value of C7 is very high unnecessarily.
- The value of R4 is low and consumes power from the output of the amplifier.
A little about the circuit I proposed:
- Instead of one amplifier with "huge" gain, the amplification is performed in two stages.
- The R8 and C7 constitute an additional filter for the sensitive input.
- The R2 and R5 set the bias to about the middle of the operating voltage.
- The C2 and C5 are blocking the DC, so as the DC voltage amplification is equal to one. They also affect the amplification at lower frequencies.
- The C10 is reducing possible radio interference and has to be adjusted according to the characteristics of the guitar's pick-up.
- The C8 and C9 are defining the upper frequency limit.
A quick test on a breadboard:
Differences from the schematic: C4=470mmF, R8=34k8, C10=0, RV1=560k fixed, R3=100k, C8=C9=22pF.
Results:
Amplification inside the passband: ~115
Passband limits (~0.7 of the voltage on 1kHz): ~12.5Hz ... ~34kHz
Maximum output voltage, before visible clipping: ~2V RMS
Regards,
Damianos