Footswitch POP

I’m getting some footswitch pop on my reverb now that I have most of the other problems licked.  I’m using a “half” true-bypass to allow the reverb to trail off naturally instead of cutting off abruptly with the footswitch.  I’ve not seen much on the Google about achieving “trails” (or “tails”) with reverb or echo units.  I see where people are attempting to implement it, claim it works, then take it out because it’s not working.  Of all the schematics I’ve found, none are too clear about the switching circuit itself.  Most schematics leave the switching out completely so it’s hard to know exactly what’s happening.  But I’ll leave that to another post.  What follows is a (mostly) comprehensive discussion of why switch pop happens and how to prevent it.

First and easiest thing to do, add a 1MΩ.  Tie the resistor across the input and output of the pedal  The theory is that any DC bias that tries to feed the input will be leaked to ground through this resistor.  There is also a capacitor in series with the input buffer that blocks DC.  Leaky capacitors can cause switch pop sense all of the DC is not blocked.  Electrolytic caps tend to be leaky so always use film caps at the input and output of your circuits.

Another cause of pop can be the LED that tells the user the effects circuit is on.  This happens because of the rapid current inrush.  A wire (or PCB trace) with a large change in current causes a magnetic field that induces a change in surrounding wires (traces) which can result in popping.  Be sure to use an appropriate resistor in line with the LED to limit current.

Leaky capacitors can also cause a voltage potential difference that gives pop.

Power supply noise and ground noise is what the “LED tick” is doing. The LED current is suddenly changing either the power supply or ground voltage at the amplifier’s sensitive points by suddenly changing. It’s usually a wiring or wire routing problem.

Any sudden change in current in a current loop also broadcasts itself as either a magnetic pulse to be picked up by another current loop, or a radio pulse.

In the image below, you can see R1 is the input resistor to ground that drains any C1 capacitor voltage that leaks across.

input buffer

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