Grounded input. Why not grounded output?

[Solidhex]

  I was thinking about the regular grounded input true bypass switching scenario we often use for noisier effects. I was getting some bleed into the bypassed signal in a pedal recently. I had wired it with the general true bypass/ grounded input style you see in the General Guitar Gadget build instructions so I was sort of surprised. I tried connecting the output of the effect to ground while it was in bypass and the bleed disappeared. I rewired the switch to ground the output of the effect when in bypass instead of the input and all the noise was gone. Considering plenty of effects make noise whether anything is connected to them or not, why wouldn't we always go for the grounded output?


--Brad

[Paul Marossy]

Huh, that's interesting. Normally, you want to kill any input to the circuit to prevent it from oscillating or making other types of noise while in bypass mode. But I guess some circuits can still have a problem even with the input grounded.

[John Lyons]

Hmmm usually when you ground the input the circuit is dead quiet.
I wonder if there was another input source somehow, or possibly the
input was not truely grounded somehow and signal was getting to the input
and causing noise...?

[MikeH]

I've found that sometimes effects can have their own oscillations that won't go away if the input is grounded; for whatever reason.  And I've used the wiring scheme below to ground the input and output.  I don't use it on every effect though, because I'm not sure how much of a good idea it is.



Obviously the LED needs a current limiting resistor.

[aziltz]

you really tie the effect output to ground? I would think that would fry something as you are shorting it right?

[anchovie]

Why would it fry something? It's only an audio-level signal with next-to-no current. If the last thing in your pedal is a volume control and you turn that down to zero it doesn't blow it up!

[MikeH]

Why would it fry something? It's only an audio-level signal with next-to-no current. If the last thing in your pedal is a volume control and you turn that down to zero it doesn't blow it up!

Yeah that's essentially it.  If you ever have an effect that oscillates in bypass even with the input grounded (like I said, it's kind of rare- but it happens) turn down the volume control all of the way and you might notice it go away.  Same basic principle.

[aziltz]

Sorry I guess I was thinking about grounding at the output of whatever gain element is last, not after the volume where it would make the most sense. It's been a long day already.

[John Lyons]

Interesting. I'll file that one in case I need it.

thanks

John

[Paul Marossy]

I've found that sometimes effects can have their own oscillations that won't go away if the input is grounded; for whatever reason.  And I've used the wiring scheme below to ground the input and output.  I don't use it on every effect though, because I'm not sure how much of a good idea it is.

That is what I suspected. It's an unusual occurence, but I guess it does happen...

[Processaurus]

Why would it fry something? It's only an audio-level signal with next-to-no current. If the last thing in your pedal is a volume control and you turn that down to zero it doesn't blow it up!

There is potential to short out stuff in a bad way, because of the possibility of the output of an opamp or transistor stage being connected to ground directly (direct, from an AC signal's point of view) through the output cap, so if there is noise in the circuit, enough to warrant grounding the output, the last active stage would try to sink as much current into ground as it could to try and drive it.

Many commercial effects have a 470Ω or 1K resistor in series with the output to limit the current if it gets shorted out.  That would be fine for this grounded output idea.

Recently I had a problem with self noise from a hi gain distortion ending up in the bypass signal, because the bypass signal ran in a trace along with the effect output.  I thought grounding the input would be enough, but there was all this hiss there in the background that could be affected by the tone and volume controls.  That could be solved by better routing, but this could work too.

Some A/B switches, used as a 2 into one switch, ground the unused input.  That can really help to get that source quiet, though something like the small series resistor would keep any of those sources from getting damaged (if their designers didn't build in a current limiting resistor on the output).

[anchovie]

Thanks for that, I do like to learn these things!