What purpose does this resistor serve on the big muff pi ?

[siam19]

I found this schematic online somewhere and it has this R1 resistor with a value of 1M ohms before the input resistor. I haven't seen this in any other schematics online. What does this do ?

[GibsonGM]

welcome to the forum, Siam  

Pull-down resistor...it ensures the input of the first stage stays at ground potential, which otherwise could drift around a bit due to leakage.

This helps minimize popping when switching the BMP on due to the presence of different levels of DC between the effect and whatever comes before it.

[stonerbox]

Hi Siam, it is there to prevent popping/clicks when turning on and off the device

[Mark Hammer]

Input and output caps need a way to drain/bleed off to avoid "switch-popping".

Useful to keep in mind that the 3PDT switches many have grown up with were a product of the '90s.  Prior to that, many manufacturers used DPDT switches, and often SPDT switches, neither of which included a means for switching status LEDs.  More relevant to the original question, use of a SPDT switch (common in MANY EHX pedals of the '70s) only switched between the input jack and the output of the circuit board, with the circuit board hardwired to the input jack.  Although this resulted in loading and tone-sucking, it also meant the circuit input was NEVER disconnected/reconnected so there was no switch-popping, hence no need for a bleed-off resistor to ground on the input.

Some schematics you'll find for "vintage" effects will add a bleed-off resistor, in anticipation of a builder using a modern bypass switch (that WILL result in switch-popping without that resistor), while others will simply show it the way it was drawn back then.

[Vivek]

Welcome to the forum !!

Here are links to some interesting reading on the Big Muff

https://www.electrosmash.com/big-muff-pi-analysis

https://www.coda-effects.com/p/big-muff-circuit-analysis.html

Maybe you have already come across the links earlier, but if you have not, they are very informative !!

[siam19]

Input and output caps need a way to drain/bleed off to avoid "switch-popping".

Useful to keep in mind that the 3PDT switches many have grown up with were a product of the '90s.  Prior to that, many manufacturers used DPDT switches, and often SPDT switches, neither of which included a means for switching status LEDs.  More relevant to the original question, use of a SPDT switch (common in MANY EHX pedals of the '70s) only switched between the input jack and the output of the circuit board, with the circuit board hardwired to the input jack.  Although this resulted in loading and tone-sucking, it also meant the circuit input was NEVER disconnected/reconnected so there was no switch-popping, hence no need for a bleed-off resistor to ground on the input.

Some schematics you'll find for "vintage" effects will add a bleed-off resistor, in anticipation of a builder using a modern bypass switch (that WILL result in switch-popping without that resistor), while others will simply show it the way it was drawn back then.

Thanks man this was very helpful