Big Muff Pi Mods

[vanessa]

I know that there are tons of posts on here for Big Muff Pi mods. But I have done the usual suspects from the GGG site and had a few questions. What values would I tweak to give it more bass boost and more output volume?

I really don't want to change this pedal (GGG). I like it almost to the T. I just want more bass boost (and not effect the treble, meaning not mudding up) and higher output volume based off the GGG values, if not what values whould be a good start to experiment with?

Help!

:wink:

[Mark Hammer]

From your comments, it is not so much the quality of distortion that interests or bugs you, but the tone shaping, post-clip.  In general, the BMP seems to have as much bandwidth as is needed or reasonable, so there is nothing to change in the way of input or output caps, as is often discussed in threads related to TS-like or Dist+ type circuits.

Jack Orman has some excellent documents related to that in general and to the BMP in particular at his AMZ site (www.muzique.com) that you will find informative and chock full of ideas.

Bear in mind that the BMP tone control essentially pans between a crude highpass and crude lowpass filter.  The way it works, the more bass you want, the more treble you have to sacrifice, and vice versa.  Personally, I find that irritating.  Jack O details techniques for staggering the rolloffs of the high and low sections in a BMP such that you can achieve mid-humps, mid scoops and lots of other variations.

Having tinkered with broad notch filters a bit, I think you'll find them very useful for getting what you want.  In a sense, one appears to "boost" bass by simply getting rid of more mids.  What is left is heavy on the bass and upper treble.  The notch filter on the old Univox Superfuzz does a great job of this.  Sounds like doom.

You CAN achieve something like this on your BMP by increasing the cap value on the lowpass section (cap to ground) and decreasing the cap value on the highpass section (cap in series with tone control).  Lowering the bass rolloff and raising the treble rolloff effectively creates a bigger scoop in the middle, and the appearance/illusion of more bass, relative to mids.  When the tone pot is rotated either hard left or hard right, it will sound like crap, but in the middle you may have the exact tone you want.

Of course, the dilemma is how to vary the tone in useful ways.  On the Superfuzz, they used a simple switch to go between midscoop and unaltered.  Because the midscoop reduces overall volume, the "straight" position was a 2-resistor attenuator network intended to achieve an approximately similar volume level.

When I was tinkering with the pedal that eventually became the "Roseyray" (posted at hammer.ampage.org), I wanted something that allowed me to achieve mellower sounds, but also brighter sounds that were never too thin.  What I came up with was a slightly more complex tone circuit that essentially panned between a "rounded" tone (full bass and mids but only treble filtered) and a scooped tone, with full bass and treble sizzle but attenuated mids.  This is posted as flexiblemetaleq.gif.  Looking at the BMP schematic, you will note that the bass end of the tone control functions exactly the same as before (thought with a somewhat higher rolloff), but in place of the simple highpass filter there is now a notch filter.

If it interests you, for starters I'd suggest using the same component values as in the Superfuzz for the notch filter, and something like 10-12k and a .01-.022uf cap for the bass section, along with the existing 100k tone control.

This network tends to eat up more signal than the stock BMP tone circuit, so you will notice less overall output, but not a dramatic drop, and certainly without any impact on the degree of distortion itself.

Worth a try, but still, read Jack's articles, and let us know what you've done and how you like it.

[Mark Hammer]

Oops.  Just a followup.

In what is normally the "bass" position, you will get something that isn't bass-heavy, but takes the edge off the distortion for a "woodier" tone.  On the other end, you'll have something that sounds both bass-heavy and searing.  In the middle, you should get something that feels "full" but has just a bit of edge.