Pedal bypass problem

brstma · #1 12-21-2008, 03:11 PM

I have an old Trace Elliot compressor pedal. It works fine when enabled but when in bypass the output is acting as an input and sending a signal back to the input stage.

I checked the bypass switch and there aren't any obvious signs of a bad component. I bought it used and the switch had been recently replaced. I don't disable it usually so it's not the end of the world but I would like to fix it if anyone has advice. It is a true bypass so the tips of the in/ out are connected directly in bypass mode.

niftydog · #2 12-21-2008, 03:27 PM

So has it been like this since you bought it? Sounds like a poorly done mod/repair to me.

Not so sure what you mean by "the output is acting as an input" - if it is true bypass then the two jacks are literally just hard-wired together and a signal would pass in either direction.

Can you expand on what's happening?

brstma · #3 12-21-2008, 06:14 PM

Yes, that's what I mean. The signal is going from the output to the input when in bypass. I noticed when I was turning on another pedal linked after the comp and there was a clip on the comp input.

So it's normal then? I guess I never had a true bypass pedal before

bongomania · #4 12-21-2008, 06:54 PM

It's normal to have a voltage spike occur when one pedal with a badly designed bypass switch is dis/engaged. A true bypass is, as nifty said, effectively just a straight wire from input to output (and obviously vice-versa). So when your one pedal with the bogus bypass switch is dis/engaged, it send a voltage spike into the pedals that it's connected to (on both sides), and that spike is seen by the detector circuit of the compressor.

niftydog · #5 12-21-2008, 07:58 PM

Ah! This seems like it isn't technically true bypass because the input of the compressor remains connected at all times - it's just the output that is switched.

This means that the signal present at the input jack will ALWAYS pass to the compressor circuitry, regardless of whether it's in bypass or not. Hence a big signal will cause the clip LED to flash.

Also, since the output jack is effectively hard wired to the input jack, and signals emanating from the input of the next pedal in the chain (such as a voltage spike caused by a true bypass switch, as bongo suggests) might also cause a signal to appear at the input of the compressor.

brstma · #6 12-22-2008, 04:06 AM

Thanks for the info. The bypass switch is after the input stage so that is the issue. At least I know it's not defective, just a bad design.

Does anyone know if it would help to put a diode in the circuit to keep the signal from feeding back from the output?

niftydog · #7 12-22-2008, 07:03 PM

No, a diode doesn't work like that. Personally I'd just live with it, it's not doing any harm is it? If you really want to change it, modding it to proper true bypass would be the way to go.