Big Muff Pi mod idea--your thoughts?

[Ben N]

This is kind of based on a reply of Doug H's (I think) to a question about making clean boost with a BMP-type tone control.  He suggested taking the first (non-clipping) stage of a BMP and have that feed the tone section and output buffer, skipping the two clipping stages.  It occurred to me first that that would make a good mod for a complete BMP, just by adding a toggle switch to bypass the clipping stages, or a 3pdt to selectively bypass one or both clipping stages.

Then I began to wonder whether you couldn't get that kind of flexibility with just the single Sustain knob, by using a dual blend pot.  Referring to JD Sleep's schem, use gang A of the pot where the original sustain pot was, except with the wiper connected to the output of the input stage (C3), one lug connected to C4 (the input of the first clipping stage) and the other jumpered to the junction of R10 and C7 (the output of the first clipping stage).  So turning down the pot, instead of shunting the boosted signal to ground, would route it around the first clipping stage.  Gang B would be hooked up in a similar way to bypass the second clipping stage.  (I'm not sure if the best place for that pot-gang is before or after the second clipping stage.)  The way these pots work is that each gang goes through its full travel in half a turn, and then remains at max (or min) resitance through the othe other gang's rotation.  If you connect the lugs correctly, then, at the minimum sustain setting both gang A and B are set to bypass their respective clipping sections, and you have a clean boost with tone control.  As the Sustain control approaches the middle, the second clipping section is gradually brought into the circuit, while the first clipping section remains bypassed--basically, a Muff Fuzz, I guess.  As the Sustain goes past the middle, the first clipping section comes in for the full 3 stages of BMP mayhem.

I guess this would take some fine tuning, in terms of pot values and such.

So, guys, do you think this would work?  I ask mainly because, with a new baby, I don't really have time to breadboard it now, but I would be interested in feedback.  If anyone is interested, I did kind of a rough schem by hacking up JD's.  I wont post that in view of his expressed wishes, but I suppose its ok to email it with an acknowlegement and disclaimer on the schem.
Thanks,
Ben

[Ben N]

OK, so maybe that post was a little too long.  :roll:  How about one piece of it:  The idea of changing a drive pot, like the sustain control on the BMP, from a variable votage divider to ground to a bypass or blend control, controlling how much signal goes to a following stage and how much goes around it.  Will it work?  Will it have bad side effects (like noise)?

[smoguzbenjamin]

Well I ain't sure if that'll create any problems as such, but congrats with your baby anyway

[mattv]

OK, so maybe that post was a little too long.  :roll:  How about one piece of it:  The idea of changing a drive pot, like the sustain control on the BMP, from a variable votage divider to ground to a bypass or blend control, controlling how much signal goes to a following stage and how much goes around it.  Will it work?  Will it have bad side effects (like noise)?

That's similar to the way a Fuzz-rite works. Noise shouldn't be  a problem.

[Mark Hammer]

Probably one of the smartest things you can do with a "plain vanilla" BMP is to stick a second Sustain pot between the two clipper stages to control how much the first stage drives the second.  Bear in mind that  the harmonic content seen by the second clipper stage is a function of how hard the first one is driven.  As it stands whatever you do to the first stage right now is also done to the second, with no way to unyoke the cascaded drive.

You will also note that the old Sola Jumbo Tonebender is essentially a BMP without the diode-pair-plus-series-cap in the second transistor stage.  That is,  there are two gain stages and one formal clipping stage.

So, stick a toggle between the diode pair and the base of the 2nd transistor to lift that path out of circuit, stick a second drive-level pot between stages 2 and 3, and you'll have a BMP that should be able to get a lot of different textures.

[Ben N]

Thanks to both of you!  I looked at that Fuzz-rite schemo, and that is kinda sorta it--a step in the right direction.  But would it work with the pot in front of the second stage?  Here's what I mean:  in the Fuzz-rite, it looks like the second stage is getting the first stage signal full-blast at all settings; the pot just controls how much of that fuzzed-out sound is in the final mix.  OTOH, the standard BMP sustain varies the amount of saturation of the second stage, and thereby the intensity of the fuzz.  I don't want to lose that control over intensity, just add a dimension to it.

Yoiur further respoonses are appreciated!

[Ben N]

OK, if I do it by putting the blend pot ahead of the stage that is being blended in/out, I get variable distortion, BUT do I also get oscillation in the stage when there is essentially no input signal to it AND the input is not grounded AND the output is connected directly to the next stage?

'Nother problem:  the  clean bypass signal is out of phase with the output of the stage it is bypassing.  Then again, that ought to be true in the Fuzz-rite, too  :?  Of course, if I skip the blend-pot thing and just bypass the entire fuzz section with the pot it ought to be ok.  Then  again, we could just go back to a couple of switches...

Alright, if one of you mavens doesn't get into the act soon, I guess I AM going to have to breadboard it (GDPITA!   )

[mattv]

I think breadboarding would be the next logical step.