After successfully building the Sunn Beta (Bass) preamp thanks to the excellent layout by Alex, I decided to have PCBs manufactured.
I'm quite happy with how it came out BUT I have a very weird noise problem:
When the gain is quite high, I have a very loud squealing sound at the output (and not only at the output, as I've used a probe all along the circuit). I have noticed that when the body of the drive pot is not connected to ground (when the circuit is not boxed and I'm not touching the pot), there is no noise.
As soon as I touch the pot or when the circuit is boxed with the case grounded, the noise is there.
It's very frustrating because, I've checked the PCB layout along the circuit times and times again and didn't find anything... I've swapped all the ICs between the vero-based build and 2 of the PCB-based builds with no change...
the only thing I have not tested is replacing the PCB-mounted drive pot (dual, 6 lugs) with a non PCB-mounted one (as it is in my vero-based build)
apart from a mistake when drawing the PCB layout (which I do not exclude, though can't find after at least 5 full checks...), I do not have a clue... so if anyone has an idea or come across such a problem... I am all hears!