Well, I finally got tired of this. The two preamps I have, have this pesky problem. I decided to JUMP them switches out in the positions that keep the TONE CONTROLS ALWAYS ENGAGED, AND THE EXTERNAL PROCESSING LOOP (ELP) ON all the time, and installed jumper cables on the back, in case I want to add a processor later.
Now I don't have to bump the switches when a channel cuts out or sounds grungy. I measured the tone responses with the controls at the center, and its EXACTLY the same as when that button was left in the tone bypass mode. So Why have it? Flat response is center detent. The design has to many switches / connectors in the signal path, so now that's gone. I also added a heavy Ground wire from the shield / shorting wafer switch back to the back RCA jacks through the wall grommet. Unused sources are now many more dB down...much less crud gets through. The thin traces and connectors were not a great ground connection for that "shorted" unused input. Much quieter now.
I attached some pics of how to do this if you want to,
Now I don't have to bump the switches when a channel cuts out or sounds grungy. I measured the tone responses with the controls at the center, and its EXACTLY the same as when that button was left in the tone bypass mode. So Why have it? Flat response is center detent. The design has to many switches / connectors in the signal path, so now that's gone. I also added a heavy Ground wire from the shield / shorting wafer switch back to the back RCA jacks through the wall grommet. Unused sources are now many more dB down...much less crud gets through. The thin traces and connectors were not a great ground connection for that "shorted" unused input. Much quieter now.
I attached some pics of how to do this if you want to,