Mid boost toggle for EHX Big Muff Pi

[medio-core]

Hello, and I'm sorry in advance for the dumb question. I have been building pedals from kits and done a couple of builds on Veroboard, but I'm still pretty green to all this and greatly appreciate any help I can get.

I'm looking to add a 3 way mids toggle to a Big Muff Pi (EHX, not a vero or customer build, existing PCB). I'm looking to have the stock scooped mids, flat mids or boosted mids. While I know this can and has been done many times I'm having trouble finding a clear guide on how to do it. I'm growing more familiar with schematics but not quite as good as I would like to be, which also makes it tricky and it's hard to weed through so many web pages mentioning BMP mods. I know there are clones with a mids knob but I have a lot of history with this pedal and would like a switch to keep the core tone of the pedal always on tap.

My conjecture is that I would need to mod the pedal to essentially have boosted mids, then use a DPDT on/off/on with caps on it to lower the amount of mids with each setting? But I didn't know how doable that was with the high pass/low pass configuration of the BMP's tone stack.

Any help, photos or documentation on this would be amazing. Thank you for your time and I really, truly appreciate the help of this awesome community.

[theehman]

Is this a big box reissue?

[nocentelli]

Your stock sound is the scooped sound, you could add a small cap in parallel with the 4n7/tone3 cap to give flat eq, or add a bigger cap in parallel to let even more mids through for mid boost

[medio-core]

Yes, it is a big box reissue.

[medio-core]

Nocentelli- in the example you posted would that be an on/off/on switch? And I assume I would be removing the existing cap on the PCB, running wire to the dpdt switch and all caps impacting the mids would be there, correct?

[EBK]

I did a similar mod once and very much regretted putting a toggle switch on the face of the enclosure for the very subtle difference it provided.  Your mileage may vary, but consider breadboarding this first before you fully commit. 

[nocentelli]

Nocentelli- in the example you posted would that be an on/off/on switch? And I assume I would be removing the existing cap on the PCB, running wire to the dpdt switch and all caps impacting the mids would be there, correct?

Yes it is an on-off-on switch, no you do not need to remove the cap. Check out your board and report back the value of the cap that is connected directly to the trebliest end of the tone pot. I would guess it is 4n or 4n7. The switch either does nothing (off position, scooped mids), adds 4n7 in parallel with the on-board stock 4n/4n7, doubling its value and allowing some mids through (on, flat), or adds a 10n in parallel, allowing loads of mids through (on, mid boost)

Your mileage may vary, but consider breadboarding this first before you fully commit.

You could get a switch and the caps and a few bits of wire and just try it out by crocodile-clipping the switch to two points on the board: Tone lug 3 (one end of the stock cap) and the other end of the stock cap (which should be connected to the output cap from Q3's collector). If you have/obtain a breadboard or have the patience to resolder, you could get a few other cap values (e.g. 6n8, 8n2, 15n) and set up a little test rig to find the most useful values in a totally non-destructive manner.

[medio-core]

This is great stuff, thanks everyone!
The cap connected to the outer/treble lug of the tone pot is a 3.9nf.

I think im getting it, I would want to connect the switch between the cap and lug 3 on the tone pot, right? Then when bypassed its doing nothing, when engaged its stacking the caps. Am I on the right track there?

The last mod I did was a diode mod so I guess i still had it in my head to wire the switch in lieu of something else.

[medio-core]

Sorry for my last post, i re-read your instructions right after posting. Going from C11 and lug 3 on the tone pot worked like a charm! I need to play with cap values a bit but it totally works.

THANK YOU SO MUCH

This is something I've wanted to do for ages, im really stoked.