Resistor values in Buffer circuit (math help)

[markeebee]

Apologies for butting in.......

Just wanted to say thanks to Knut, Merlin and Paul.  I spent a couple of hours reading, rereading and understanding this whole thread and I think I learnt more about buffers than I ever have from textbooks and stuff.

I have nothing to add to the thread, though. Carry on.

[knutolai]

OK! So I've boxed the circuit and the interference issues seems are mostly gone, they are acceptable. I'm building it in a plastic box as I couldnt find a metal one big enough to fit the whole project (also its a prototype).

The only issue that remains is a popping sound when I switch between bypass and signal-through-circuit. I do true-bypass switching with the input to the board connected to ground when in bypass mode. I got a feeling this has a simple solution, but I have never encountered the problem before Maybe you'll have something to contribute with after all Mark?

Here is the schematic for the circuit I soldered:


Edit:
I tried adding a 2,2MOhm and a 750kOhm resistor from the input to ground (not at the same time) as proposed in this article: http://www.geofex.com/Article_Folders/box_pop.htm
Didn't do the trick
Edit2:
I realize the circuit is very high gain (up to approx. 52). I tried reducing R4 to 10 k (max gain 27) and the problem is still there.
I also found this article: http://www.muzique.com/lab/led.htm I tried removing the indicator LED completely to see if it made a significant difference. It didn't.
I should also mention that as Im testing the circuit nothing is connected at the connectors named ADC and DAC.

[duck_arse]

the DAC point is not connected to the bypass is it? it could probably be pulled to ground with a high-value resistor without affecting its function if it is causing the problem. is your pull-down at the input connected hard-to C1, or is it switched in and out when bypassing?

[knutolai]

Added a 1MOhm resistor from DAC to ground. Didn't make a difference. The resistor at the input is always connected to the capacitor C1.

[duck_arse]

someone was having a popping problem recently, and it turned out to be the following circuit (an lm386), not the one he was working on. maybe you could look at what is attached to either end for problems.

[knutolai]

Good idea. However I have already checked that. I use a guitar and a amp in addition to the pedal. I tried substituting it with other true bypass pedals and had no noise problem.

[knutolai]

I've seen it mentioned some places that the big blue 3PDT switches cause a lot of noise when switching. Im using one of those. Should it still make a lot of noise when the input to the pedal is totally quiet? I get the popping sound even when the gain of the pedal and the guitar volume is turned all the way down.
Edit:
So I swapped the bypass switch for a smaller one, designed to handle smaller currents:

The issue is still there and the popping sound is just as loud. I've gone over the PCB board twice to check that everything that should be connected is, and everything that shouldn't isn't. Gaah! *pulling my hair out*