Signal Leaking

[GainFreak]

Hi to all,

I have the following situation. A SLO/Mesa DR type of DIY PCB preamp with a tube FX loop /Mesa type/ next /6 cm/ to the CF tube. When high gain channel's Volume is at zero and preamp's Master turned up there's signal leaking out of the preamp and I mean clearly audible. The signal is not affected by the tone stack /Volume still at zero/. When placing metal shield between CF and FX loop tubes and especially near the buffer input, the leaking signal level decreases but it's still there. My thoughts are since strong signals up to ~100V are present at the CF the FX loop tube grid maybe acts like an antenna picking up the signal from CF. What do you think? Is it possible to avoid this and how?
Any comments/advice will be greatly appreciated.

[stokes]

I've seen this in a guitar amp and it was just a case of the volume pot not doing its job well enough when turned to zero.New pot cured it.

[Enzo]

It is called crosstalk, and is a common phenomenon.

First, is it a problem? SOmetimes folks ask what to do when their amp make sound with the volume on zero, and I have to ask, how often do you play with the amp on zero?

Crosstalk occurs because a later stage has enough gain to pick up the signal from an earlier stage without a direct connection. it could be radiated, it could be coming through the B+ if insufficiently decoupled, it could even be from a poor ground scheme. Often it is solved through correcting the lead dress, shielding, and better circuit configuration.

[GainFreak]

SOmetimes folks ask what to do when their amp make sound with the volume on zero, and I have to ask, how often do you play with the amp on zero?

You're right - at higher volume levels this signal becomes too weak to affect the "main" signal but it's still annoying.

[Enzo]

I am not being a smart ass now, does it annoy you in a functional way, as in at low volume it interferes, or does it bother you in the sense that you just don't like that it does it?

Crosstalk and the control that doesn't go completely to zero are actually separate issues. You could have one or the other or both. Crosstalk though exists in all amps, but it is so much lower in level than the main signal that no one hears it until the main signal is interrupted. In the same way no one hears the air conditioning until the band stops playing - it was there all along.

[GainFreak]

I am not being a smart ass now, does it annoy you in a functional way, as in at low volume it interferes, or does it bother you in the sense that you just don't like that it does it?

When Master is at zero there's no main signal flowing. The crosstalk is heard as a harsh distortion that's being picked up before the tone stack /which means from the CF/ because TS doesn't affect it. When Volume control is moving towards max it blends with the main signal until it becomes inaudible. The only thing that affects the level of this leaking signal is some shielding around the buffer's input capacitor. Maybe if buffer is far from the CF it wouldn't interfere to such an extent.
After you say it happens in other amps as well I guess I'll be able to live with it although I would prefer to get rid of it completely if possible.