Bass Big Muff Pi Mods

[12bass]

Hey Everyone,

Last night I decided to see if I could improve the sound of my Bass Big Muff Pi. Although it sounded pretty good on its own, I found that it would get lost in a mix. Another issue was that it sort of "ducked out" when fed a hot signal at full gain. I suspect that the latter effect may have been related to shifting DC bias on the transistors.

Although I'm still tweaking, so far I've made these changes:

C2 - added a 1 μF film capacitor in parallel at output of Q4 to increase lows
C6 - changed to 0.068 μF in Q2 clipping loop to increase lows
C7 - changed to 0.068 μF in Q3 clipping loop to increase lows
C9 - changed to 0.004 μF using two capacitors in parallel (however this may have already been 0.0039 μF, so perhaps unnecessary)
C10 - changed to 330 pF at Q1 input stage to add a little more sparkle up top
C14 - changed to 470 μF Panasonic FM to mitigate hum (probably unnecessary)

I also spent some time messing with the tone section surrounding Q4 but ended up preferring the original values.

This page has a pretty cool diagram which shows the basic layout of the Big Muff and how the different components impact the sound:

http://www.kitrae.net/music/Images_S...IT%20GUIDE.jpg

From what I gather, the BBM is based upon the recent Russian Muff, but with a few modifications to make it work better with bass. The size of a few capacitors was increased to allow more low end, plus there's the addition of a blend circuit for adding fuzz on top of a clean signal (which is probably the role of the op amp IC at the bottom).

Haven't recorded any sound clips yet, but the end result is that the high-gain "ducking" effect is now gone and there's a fatter, fuller, response in the midrange and a bit more low end. I'm hoping that this will help it cut through better with the band.

Here is a picture of the board showing the various component positions:

[12bass]

Update:

Changed C10 back to the original 470 pF. The 330 pF ceramic capacitor I installed was microphonic and I found the original sounded better.

Added a 100 pF cap in parallel with C11 in the feedback loop of Q3. This smooths some of the raspy upper midrange/lower treble and give a nicer focus in the 600 - 1 kHz zone... a littler fuller and not as buzzy.

Another drummer/bassist friend of mine will probably try it out with his band in the next while.

[lawrence2]

Crazy... I will try, when I learn electronic a little more.
Thanks... more upgrades for any sound that cut through the mix better .
And... sound clips, ow yeah...

[12bass]

Here's a quick sample of the modded BBM. This was recorded directly into my Echo Gina24 and has no processing other than normalizing the waveform. The recording starts with my Pedulla Rapture 5 bass with Q-tuner pickup run passive and without effects. Next the BBM is kicked in, then later my A/DA flanger clone is added for good measure.

Modded BBM sample: link

Settings on the BBM: volume around 10:00, tone just past 12:00, and sustain at maximum.

Also, regarding C6/C7 above, increasing the size here actually lessens the bass somewhat.... well, actually what it does is allow for more bass to be clipped by the diodes. Still, I think that it sounds better and retains a little more midrange and doesn't "duck" like it used to, though it will still saturate with an active bass boost going in.

[G.Bisson]

Interested. Im in the process of creating a bass muff from a GGG russian kit.

one mod I've made is to split C1. Run two caps in parallel and blend between them with a B100k pot. Cap values are 10u tantalum and .01u film. This adjusts the amount of lows going into the front end of the circuit. It is identical to the 'depth' control in a Supercollider.

[mpdd]

cool thread, i was trying to get my father to do this when he retires, he is an aviation electronics technician with 40 years experience, i thought we could hang out and make some wild pedals back in PA

[karter2000]

Nice job!! I can't believe you were able to work with those ting SM components. What do you use for soldering?

[12bass]

Originally Posted by G.Bisson

Interested. Im in the process of creating a bass muff from a GGG russian kit.

one mod I've made is to split C1. Run two caps in parallel and blend between them with a B100k pot. Cap values are 10u tantalum and .01u film. This adjusts the amount of lows going into the front end of the circuit. It is identical to the 'depth' control in a Supercollider.

Although I don't have a BBM schematic, from the look of it, the three position switch on top does something similar, using a larger input capacitor in the "bass boost" position. FWIW, I found that changing the capacitors which are in series with the clipping diodes made a significant difference in the voicing of the distortion. The stock values were 0.047 μF which were increased to 0.068 μF. On mine, the clipping diodes are paired within a single small SMT package.

Originally Posted by karter2000

Nice job!! I can't believe you were able to work with those ting SM components. What do you use for soldering?

Really isn't that bad as all of the important components are on the visible side of the PCB, and the top part of the board follows the standard muff layout quite closely. I'm using a Hakko 936. It's a bit tricky getting the SMT parts off... sort of have to heat both sides in quick succession and slide them off while the solder is still molten; it does require a fairly careful touch. After that, there are a couple of pads underneath that are fairly easy to solder to. Just cut the leads on the new components to fit and solder them securely on top. In my mod I replaced some of the SMT capacitors with regular film capacitors. Film caps tend to sound "smoother", while ceramics are more "gritty". I'm also considering playing with the coupling caps at some point.

[12bass]

New sample after some more changes to bring out snarl in the mids, including replacing the original BC547C transistors with quieter-yet-gainier 2N5089s:

Click and save me

[dannybuoy]

Hmm, might have to pick up a second one of these to mod! Check this thread of mine out too, the feedback mod really brings out the snarl:

Bass Big Muff volume mod - TalkBass Forums

[12bass]

Originally Posted by dannybuoy

Hmm, might have to pick up a second one of these to mod! Check this thread of mine out too, the feedback mod really brings out the snarl:

Thanks.... already subscribed!

As mentioned above, the BBM design is very similar to the latest Russian Big Muff (except for a few component changes plus a blend circuit). There's a fairly standard muff layout of the top part of the PCB which isn't too hard to mod. If you're not happy with the current BBM sound, you can mod it to sound how you want.... change it into a Triangle muff... whatever....

IMO, the 2N5089s brought out more definition.... less "mushy" than the original BC547s.

[karter2000]

Again, great job! I sense a new project coming on!

[12bass]

Thanks for comments! BTW, the second clip was my 1982 Rickenbacker 4001. Note that the recordings were made straight in, while most bassists would probably run a Big Muff through an amp and cabinet, which would smooth out some of the buzzy top end.

[runmikeyrun]

damn, both those sound good! Might have to pick one up some day if i'm feeling up to attacking SMT components