Switch Pop, Pull Down and Current Limiting Resistors

[Timko]

I've been spending some time this year away from building projects and focusing on understanding how circuits are designed.  Since I began pedal building 4 years ago, I've been fascinated with the Rangemaster and germanium based pedals in general.  So the first design I've been working on has been a layout for a Darlington (well, it's really a Sziklai) based layout using 2 transistors for creating Rangemaster tones with an additional output tone control on the end.  The eventual goal is to replace the tone control with some sort of synth style resonant filter, but I'm starting here for the moment.  My circuit has 4 controls.

* Input cap blend control
* Boost control
* Tone control
* Tone bypass switch

After working through the layout and through getting parts onto my breadboard that sounded good, I ended up with this schematic:
Schematic

And this layout:
Layout

However, after building it and connecting it to a footswitch I got an awful pop.  I spent the past week reading about the causes of switch pop, and I definitely have a 50ish mV voltage on the footswitch pin that corresponds to the effect output.  I noticed that my schematic (like most Rangemasters) lacks an output pull down resistor, so I clipped one of those into the circuit.  However, I noticed that the voltage only dropped when adding a smaller (100k-10k) resistor rather than the typical 1M / 2.2M often seen in modern circuits.  I know the Big Muff uses a 100k pull down resistor, and after reading through the diystompboxes archives, this seems to be an acceptable value.  However, I'm still at a loss as to why that's the case.  I'm not sure if my large output cap for a Rangemaster (220nF) has anything to do with it, but perhaps.

One of the other things I read about in my research was a resistor in series with the input / output as a current limiter.  I didn't find a lot of explanation on this (much of the focus is on the pull down resistor), but I have looked through a few schematics and have seen resistors after the tone/volume/buffer/last stage with a series resistor before the output with some sort of capacitor to ground in parallel.  I was curious if there are times you opt for this type of design and times you don't.  One of the other things that I'm not sure about is whether having a tone control after the boost section is a good idea.  Most of the time I see a tone control placed before the volume, but that won't work in this case.

Thanks for helping.  Circuit design seems to be a black hole - when you're looking for answers, you find more questions

[amptramp]

Maybe this will bring you a little closer to the event horizon.  A pulldown resistor is used to keep the open end of a capacitor which is attached to a bypass switch at roughly ground potential so there will be no difference in voltage when you switch between the bypass and active functions.  If you have, say, 30 megohms of leakage resistance in a capacitor and 5 volts on its input end, a 1 megohm pulldown will take that down to 161 millivolts which would make for a horrendous switch pop in most circuits, so capacitors should have over 100 megohms leakage resistance for it to even be moderate.  If you replace that pulldown with a 100 K pulldown, the 30 megohm capacitor pop would be just under 17 millivolts which is noticeable but not all that bad.

There are some schematics that do not work without series resistance on the output because the subsequent stage has either low input impedance or high capacitance (including cable capacitance) and this interferes with the feedback and causes oscillation.  It is best to separate an op amp stage from an output with at least 1 K of resistance and possibly more.  Similarly, the operation of a tone control stage can be affected by whatever is attached to the output.  If it is a low impedance, it could alter the response of the tone control so a bit of isolation helps maintain a uniform response no matter what is connected and if it costs some gain, no problem, you can always put a bit more amplification before or after it.

Anything I design has an input buffer and an output buffer and all tone (or other) controls between them so the response stays the same regardless of what you connect to it.  A tone control after the boost is too susceptible to differences in response depending on what you connect to it unless you can guarantee a high-impedance load.  The capacitance to ground at the input is usually added to prevent radio interference and the capacitance at the output is to swamp cable capacitance so the behaviour doesn't change wildly with cable length.