pulldown resistors: how low can you go?

[Beedoola]

Is there like a limit to how low of value a Pulldown resistor should be? I built a BYOC Reverb 2 on perf and I have some switch popping when the pedal is turned On. I tried a 1M with no luck, 100k helped but still some pop. I tried a 68k and that seems to do the job.

Just out of curiosity, whats the lowest value one can use to eliminate switch pop?

[culturejam]

Well, the lower you go, the lower the input impedance becomes. I doubt you'd hear a tonal difference until you get down 200K-ish or lower (depending on the pickups), although there will be some loading.

At 68K though, I'll bet the loading on the pickups is more prominent.


Just to do a little math (yuck!), the pickup's output impedance will be in parallel with the pedal's input impedance. So if you have a 15K humbucker in parallel with the reverb @ 68K, you'd end up with an effective impedance of about 12K. And it will change the resonant peak of the guitar as well (rolling off some highs).

And that's not a huge hit, but if you have multiple pedals, that drag on the top end sparkle can add up.

It depends on your signal chain as well. It may not be a big deal with 68K, or it might be tone killing.

[culturejam]

I should add that my above calculations assume a very high input impedance from whatever is the first amplifier in your circuit. So with op amps, the above numbers hold fine.

A BJT amp will have a lower input impedance, so you'd have to know that and then calculate for the pulldown's parallel value, and then parallel that with the pickup's impedance. At least I think  I have that right. 

[Beedoola]

do you mean the bypassed impedance may be affected? If so, I measured the impedance of the pickup to cable and then on the output of each effect when bypassed and the impedance stayed the same - 15k (what the bridge pickup is). I actually used 100k for both PD, not 68k as I thought.

[culturejam]

do you mean the bypassed impedance may be affected?

No, I mean the impedance when the effect is in the signal chain (not bypassed). If it's bypassed, it's bypassed.

68K is definitely low for a pedal input impedance. But it's not necessarily the end of the world either.

On the other hand, you might want to try a different input cap. Not a different value necessarily, but a different cap (of the same value).

[Beedoola]

do you mean the bypassed impedance may be affected?

No, I mean the impedance when the effect is in the signal chain (not bypassed). If it's bypassed, it's bypassed.

68K is definitely low for a pedal input impedance. But it's not necessarily the end of the world either.

On the other hand, you might want to try a different input cap. Not a different value necessarily, but a different cap (of the same value).

should a circuit provide a fixed impedance reading when the circuit is engaged? I don't get a stable value, it fluctuates.