why different pulldown values?? and a bypass ?

[csmatt45]

Just wondering. I know the "standard" is often 1M, but I see on some of Justin's (fuzzcentral) schems he'll use a 4.7M on the in's and out's (on the Tycobrahe).  why the difference?

also, with a grounded input true bypass, am I correct in assuming the input no longer needs a pulldown for "pop" prevention?

thanks in advance.

matt

[R.G.]

I know the "standard" is often 1M, but I see on some of Justin's (fuzzcentral) schems he'll use a 4.7M on the in's and out's (on the Tycobrahe).  why the difference?

Phase of the moon? Designer's mood when he wrote the value down?

The purpose of a pulldown is to pull any current that leaks through the insulation of the input and/or output cap to ground.

While the effect is being used, the inside terminal of the cap is charged up to the DC bias voltage of whatever it connects to in the effect. This is often half the supply voltage or more. The outside is pulled to ground by the guitar or whatever else is attached. When the effect is bypassed, the connections to these capacitors are opened, and the capacitors have the residual voltage across them. If they did not leak across their own insulation, they would not pop when reconnected.

But they do leak, a little. How little depends on the kind of insulation in the cap. Electros leak a lot relative to mylar, polypropylene, teflon, and polystyrene in that order.

The pulldown resistor keeps the end of the cap pulled to ground. To do this, the resistor has to be low enough to pull the leakage current down to an insignificant fraction of a volt. As you realize "insignificant" is not exactly a technical term. 10mV is pretty low compared to a guitar signal, so I'd say it's got to be well under 10mV. Since caps don't leak much, then the resistor can be big. What current will a 1M resistor pull to 1mV? 1ma. That's pretty good. Even most electros don't leak 1ma. 10M? That keeps 100uA to under 1mV

So far, we could just say make it 1M - or 100K, or 10K - and be done with it. But this resistor loads down the guitar signal and eats treble as it gets lower. 1M is kind of a minimum acceptable level for that. Lower resistors also tend to eat the switch transients from other things better.

As you can see, there is no definite answer to "what is the right pulldown resistor?" The answer is "big enough to not load the guitar, so 1M or over, and low enough not to let you cap leak more than 1? 2? 10? mV from being open, which changes with the kind and age of your capacitors."

1M's enough. 10M's OK. 100M's probably too big for some caps, not for others.

also, with a grounded input true bypass, am I correct in assuming the input no longer needs a pulldown for "pop" prevention?

Maybe. The cap doesn't have much time to leak while the switch contacts are moving from one position to another.

But a 1M 1/4W resistor costs about $0.03. It's awfully cheap insurance against that "other stuff".

[csmatt45]

Geez, I was hoping for a detailed answer. Can someone else help me?

Of course I'm kidding. I always enjoy getting a reply from you RG, and I always learn something.

Thanks,

matt

[dosmun]

What if it screws up the input impedance?

[markm]

Great explaination RG, Many Thanks.

What if it screws up the input impedance?

Look Here;
http://www.diystompboxes.com/smfforum/index.php?topic=49467.0

[csmatt45]

Oops, it was the GGG tyco schem, not Fuzzcentral.
sorry justin!

matt