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?
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.
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. ;D
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.
Quote from: Beedoola on August 03, 2013, 12:43:39 AM
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).
Quote from: culturejam on August 03, 2013, 01:03:13 AM
Quote from: Beedoola on August 03, 2013, 12:43:39 AM
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.