I have a Fishman Loudbox Mini in right now that makes massive amount of hiss. Otherwise the amp works fine. I quickly isolated it to channel 2 and in particular pin 14 of U4D, which also had 0.47V DC, despite no DC (or noise) on either input pin 12 & 13. So I removed the opamp and checked for DC on the pads and all was well. Reinstalled a new opamp and back to exactly the same situation - no noise or DC on either input and DC/noise on the output.
To further isolated the problem I removed C71 and connected the output to my bench amp. Lots of noise which can be altered with VR7 and VR8. No noise on any other part of the circuit - just pin 14.
I had to hand a fresh delivery of TL074s so just out of interest switched the IC yet again to rule out the remote possibility of the first replacement having a fault. No change.
Can anyone shed any light on what I'm missing?
To further isolated the problem I removed C71 and connected the output to my bench amp. Lots of noise which can be altered with VR7 and VR8. No noise on any other part of the circuit - just pin 14.
I had to hand a fresh delivery of TL074s so just out of interest switched the IC yet again to rule out the remote possibility of the first replacement having a fault. No change.
Can anyone shed any light on what I'm missing?
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