Pull-down resistors

[GGBB]

I have a few novice (I think) questions regarding bypass switching and circuit grounding that I can't find comprehensive answers to anywhere.  I'm thinking mostly about switching pop and to a lesser extent impedance.

1. Do pull-down resistors on input and output help or hurt anything if your switching scheme grounds the input and output anyway? 

2. Would I be better off with just the pull-downs and no switch grounding (or vice-versa)? 

3. Is it necessary to ground and/or pull-down at both input and output?

[amptramp]

I have a few novice (I think) questions regarding bypass switching and circuit grounding that I can't find comprehensive answers to anywhere.  I'm thinking mostly about switching pop and to a lesser extent impedance.

1. Do pull-down resistors on input and output help or hurt anything if your switching scheme grounds the input and output anyway? 

I can't think of any pedals that ground the output, but the resistor at both ends is a still a good idea.  If your leakage is significant as would be expected with an electrolytic capacitor at the output, a pulldown is a good idea here as well.

2. Would I be better off with just the pull-downs and no switch grounding (or vice-versa)?

If the effect is anything with a memory such as a delay, chorus (to an extent) or reverb, it will retain and output whatever noise is on the input unless the input is grounded.  In other cases, you may want to run the delay so that it is not "empty" when you switch it on, so the input should not be grounded or switched off at all.  Other effects are generally grounded just to eliminate random noise from driving a high-gain stage such as a tube screamer, where the noise may continue during switching as a switch pop.

3. Is it necessary to ground and/or pull-down at both input and output?

Input should be grounded but I don't know of any pedals that ground the output.  Since most pedals use electrolytic capacitors at the output and the level the output floats at tends to remain the same whether it is switched in or out, it is better to have a pulldown to absorb leakage current but not switch it to ground - ungrounding it could create a switch pop if the leakage was high enough.  It is generally considered a good idea to have the output and input near ground so that the electrolytic at the output can have the negative end going to the output and the input and output values are referenced around ground.  Thus, ungrounding does not change the voltage much.

[GGBB]

Thanks so much - I am much clearer on all this now.  But I have a follow-up question:

If your leakage is significant as would be expected with an electrolytic capacitor at the output, a pulldown is a good idea here as well.

Is the output pull-down still recommended in pedals where the output is coming from the wiper of a volume pot?  It seems to me that the volume pot itself would behave exactly like a pull down resistor.

[iccaros]

should not the output from the pot go to a decoupling cap (to remove any DC from the output) , at which the pull down is after the cap?

[GGBB]

should not the output from the pot go to a decoupling cap (to remove any DC from the output) , at which the pull down is after the cap?

I guess it makes sense that way, but I am thinking of pedals where the output cap is before the volume pot.  As in the EA Tremolo.

[boogietone]

Doesn't the Millennium Bypass require the grounding of the output to turn off the LED?

[R.G.]

Read geofex.

[GGBB]

R.G. - I have spent countless hours reading your articles - they are wonderful.  Thank-you so much for your efforts.  I'm sure the DIY community wouldn't be where it is without you.  But - what's that old saying?  A little knowledge is a dangerous thing.  Learning has only caused me to have more questions than I have time to spend looking for the answers.  If I was ignorant I would just build the darn things according to the schematics and be done with it - ignorance is bliss as they say.  But now I have to to tinker and question because that's how I'm wired.  I'm not the type to ask for help before I'm at the end of my rope.  I'm pretty sure I've read everything I could find from you (and others) on pull-down resistors, but nowhere have I found the answers to the specific questions I've asked, at least not in a form that someone like me with only a little knowledge can understand.  What may be an obvious extrapolation to many around here would probably completely escape some of us - we need as much help as we can. 

[amptramp]

Thanks so much - I am much clearer on all this now.  But I have a follow-up question:

If your leakage is significant as would be expected with an electrolytic capacitor at the output, a pulldown is a good idea here as well.

Is the output pull-down still recommended in pedals where the output is coming from the wiper of a volume pot?  It seems to me that the volume pot itself would behave exactly like a pull down resistor.

Yes, in this case the pot would behave like a pulldown resistor as recommended.  I geberally avoid designing anything with controls on input or output interfaces because the interaction between the control and the output impedance of the prior stage or input impedance of the subsequent stage may have an unintended effect, although I suppose a straight volume control would be less affected than a tone control.  A volume control may have its taper changed but a tone control may have changes in frequency breakpoints, scoop depths and all kinds of things that should be invariant.  If you have a control, it is best to have a buffer before and after it for isolation.

[GGBB]

Yes, in this case the pot would behave like a pulldown resistor as recommended.

Once again - thanks.  Clears up my confusion and I can have one less component on the board.