I originally posted a lot of what’s here somewhere else, but there was no real response and no discussion triggered. I realized I should have posted here really. Much of what I write is basic and obvious to many - but perhaps it’s of some help to others. All I’m really doing here is bringing together, and hopefully simplifying, some posts and articles on the subject I’ve come across online by a number of experts - above all the effects and DIY guru RG Keen. I like to simplify things if I can. (Should RG read this then I hope he won’t mind the lack of references to where each quote of his I use originates from - I’m afraid I didn’t record the exact location of each webpage where I found bits of info. A lot is from taken from RG Keen’s Geofex site, however).
Perhaps I haven’t got everything right and I also have a few questions that perhaps others here might have some answers or ideas about. So I hope the post might generate some discussion.
I’m ignoring LEDs for two reasons. First, I’m a bit lazy and don’t often add them to my builds... extra hole to drill, extra wiring
... I don’t use many pedals at once so don’t need LEDs to begin with 
. The second reason is that LEDs are a complicating factor that’s best looked at separately I think - as I say, I like to keep things simple.The basic idea behind using pulldown resistors to prevent footswitch pop
Let’s begin with a quote from RG Keen: “Almost all effects are powered by a single 9V battery, and this means that for DC biasing reasons they have both an input and an output DC blocking capacitor to carry the signal into and out of the circuit.”
However: “ No capacitor is perfect, and whenever the outermost end of either capacitor is open-circuited by a hard (mechanical) bypass switch, the leakage will allow some of the voltage to bleed into the outer terminal. When the capacitor is again connected, it no longer perfectly matches the average ground level of the signal, and this voltage is coupled into the signal path as a pop, the loudness of which depends on the size of the capacitor and the amount of the voltage change. The cure is simple - put another resistor on the outermost lead of the cap to pull it to ground permanently, even when the bypass switch is open. Leakage current from the capacitor is conducted to ground by the pull-down resistors and so no DC-offset pop can occur. For the purposes of curing leakage-clicks, the resistors can be quite large, from 1M to many megohms.”
And here’s a nice simple explanation of this at AMZ: http://www.muzique.com/news/pulldown-resistors/
Why use pulldown resistors of 1M or greater?
So that’s the basic idea. But pulldown resistors bring two issues with them as far as I can tell:
1. Impedance of the effect - you alter it. With input and output pulldown resistors then at both ends of the effect.
2. Signal loss - too low a value pulldown resistors may reduce signal too much.
Impedance I’m not getting into - that’s an issue I'm still learning about. I hope it’s ok just to say here that low impedance causes treble loss - tone suck. Usually that’s not desirable - though not necessarily always so.
Anyway - the standard 1M ohm pulldown resistor is a bit of a general, best guess value I gather. The lower the value of the resistor you use the more efficient it will be at its job of reducing pop - but then the issues of impedance and signal loss can become a problem perhaps. To let RG Keen explain:
“ We've found empirically that values between 10milliohms and 10 meghoms are all pretty effective at stopping clicks, but most people use a good, solid run of the mill 1M since the lower values tend to reduce signal levels too much. For stopping switching clicks, smaller is better. One inch of copper wire is a good preventer of clicks. No signal, but no clicks.
To keep clicking from happening, pulldown resistors have to be significantly less than the leakage of the capacitors. How big is that? We don't know without measuring, because makers of caps only say "it's less than xxxx." Therefore, it's not possible to calculate a priori what the theoretically right value is without knowing the specific cap's leakage.
As to having a bearing on the output impedance of the effect, yes, a pulldown resistor appears in parallel with the Norton equivalent output impedance of the effect, so it lowers the output impedance. It lowers the output voltage as well. How much? You gotta know what the output impedance of the effect is before you put the pulldown on. Generally, the output impedance of the effect is down in the sub-50K region, so 1M and above is comfortably big enough not to load things down much.
And on the input, the same is true. The pulldown is in parallel with the input impedance, and acts like a load on whatever is driving the input. So the previous effect, if it has a pulldown as well, has to drive both its own output pulldown and the input pulldown of the next effect, in addition to the actual input impedance of the effect itself. So from the standpoint of preserving signal and not loading things down by changing the input and output impedances too much, bigger is better.
Having thought through this once or twice, most people sigh, and just use 1M. I like 2.2M. But be creative!!”
When you might not need pulldown resistors
A number of vintage effects pedals wired for ‘true’ bypass that lacked LEDs had no pulldown resistors - and never needed them. Tone Benders, Fuzz Face et cetera. Here’s why I believe this is the case - please correct me if I’m wrong:
First, how the switch was wired - in this fashion:
This is how I always wire the DPDT footswitch - it sends the circuit input to ground when bypassed. As a result the DC blocking capacitor at the front of the circuit is not open circuited when bypassed - instead one end (the input end) is sent to earth... you effectively get RG Keen's “one inch of copper wire” here, the ideal pulldown ‘preventer of clicks’ at the input of the circuit.
Second - though here I’m not quite so sure I’ve got things right - the passive volume pot that follows the output DC blocking capacitor is connected to ground. Like in the Fuzz Face for example:
I read somewhere on DIYSB - the person who explained things I’m afraid I forget - that the 1k Fuzz pot in the Fuzz Face does two functions at once. It is a fixed 1k resistor to ground and a variable resistor connected to the 22uf capacitor to ground. Presuming that explanation is correct, then am I right in thinking that the passive volume pot at the end of the Fuzz Face (and other circuits that use one) functions to all intents and purposes as a ‘pulldown resistor’ at the end of the output capacitor as well as a volume control?
This point might be of significance if, say, you are building an old effect like the Dallas Rangemaster - which was never a stompbox with a footswitch of course, popping noises were not an issue the original designer(s) had to consider. If you build this as a stompbox, could replacing the 10k volume pot with a 10k fixed resistor and adding a 100k passive volume control, as Soulsonic suggested (https://www.freestompboxes.org/viewtopic ... r&start=20), have more benefit than Soulsonic realised (or let on)?
A few additional factors to consider
Leakage of caps are a factor to consider - less leaky, more chance 1M resistors will do the job... so cap quality, type, and age are factors... maybe film caps perform better than electros in most instances(?).
A bad Footswitch can cause some pop or crackle - the offending noise is not always necessarily due to leaky capacitors. Likewise, pop or crackle might occasionally be cause by DC from outside the box or defects within a circuit. (See RG Keen’s Geofex Tech Tips “Why does my stompbox pop when I hit the bypass switch?”). Nevertheless, I’d call these kind of issues niggly gremlins - and for that reason outside the remit of what I’m talking about
Johnny
