I have built the following electric guitar preamp which is mounted inside the instrument. The primary reason for doing is that there is a piezo pick up in the bridge with a very high output impedance which never sounds good plugged into the amp passively. I aimed for 0db net output gain with linear bandwidth within the 20-20k range so as not to impede the attack and brightness of the piezo. While the circuit is tested as functioning as intended, there is audible spurious HF interference noise, a +/- 6 to 8 kHz ring or whine mixed with static in the background, which becomes audibly loud during pauses when distortion is used in the main amplifier. All individual pickups have shielded connections to the main switch inside the guitar cavity which is additionally completely shielded with conductive paint. This was a quiet electric guitar without the preamp. The components are very tightly fitted on a 2.5x4 cm piece of hobby board with star grounding. The potentiometer terminals are connected via +/- 5cm wire jumpers all within the cavity.
- I might have gone overboard with the HF bandwidth. Will the reduction of the HF bandwidth by increasing the values of C3, C9, C11 achieve this and which of these capacitors are most relevant?
- I have read that introducing a modest +/- 1k series resister prior to the output can improve transmission through the guitar cable. Is this true and are there any other simple modifications to optimize the transmission through an unbalanced guitar cable?
- The tone controls are a bonus but not necessary per se. Would removing the tone stack make any difference to the problem?
- Any other suggestions are appreciated
BTW: I don't have an oscilloscope but I do have a loudspeaker testing setup. With tone controls set at unity, the circuit is linear to +/-40k with overall low distortion
This circuit is also public on CircuitLab: https://www.circuitlab.com/circuit/52475zw4edt5/opa134-based-guitar-preamp-with-tone-control/