Hey everyone, I got an old Little Big Muff Pi from a friend. It wasn't working (there were open circuits and corroded wires/connections), so I decided to re-build it on a perfboard and put it back in the original enclosure. I just got everything soldered, and once I found a battery that wasn't half-dead, I tried it out.
So it makes noise, and the overdrive sounds awesome. However, this thing is absurdly loud. I turn the volume pot like 10 degrees and it's already LOUD. And the buffered bypass definitely has a gain of more than 1.
Here is the schematic I am working with: http://www.diystompboxes.com/pedals/lbm.gif
After pretty close inspection of the old circuit board, it looks like this schematic is pretty accurate. So my question is what mods could I do to get this thing a little quieter? My first thought is to take that feedback resistor on the first stage down to like 100k.
I also thought about making it true bypass, which would solve that problem. However, I still would need to figure out how to tame the gain when it is switched on. Putting that Fuzz pot in would help control the gain of the second stage, but I'd prefer to keep it in the original enclosure. Maybe a different volume pot would quiet it down a little bit? Or shorting a couple diodes in the clipping section.
Any ideas on the best course of action?
So it makes noise, and the overdrive sounds awesome. However, this thing is absurdly loud. I turn the volume pot like 10 degrees and it's already LOUD. And the buffered bypass definitely has a gain of more than 1.
Here is the schematic I am working with: http://www.diystompboxes.com/pedals/lbm.gif
After pretty close inspection of the old circuit board, it looks like this schematic is pretty accurate. So my question is what mods could I do to get this thing a little quieter? My first thought is to take that feedback resistor on the first stage down to like 100k.
I also thought about making it true bypass, which would solve that problem. However, I still would need to figure out how to tame the gain when it is switched on. Putting that Fuzz pot in would help control the gain of the second stage, but I'd prefer to keep it in the original enclosure. Maybe a different volume pot would quiet it down a little bit? Or shorting a couple diodes in the clipping section.
Any ideas on the best course of action?
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But there IS a volume control. It's also last element in signal path and should attenuate the signal rather consistently throughout it's range. Which begs a question... Can you use it balance signal levels? If yes, I don't see what's the issue (why would you have to set volume control to certain level?).
If excessive volume is an issue and the control only provides too little range to work with then you can, say, substitute the 25K pot with a 10K pot and wire a 15K series resistor to it. Now the series resistor will permanently cut signal voltage by more than half.
If the attached schematic is accurate the signal is buffered with an inverting voltage amplifier with gain of 330K/68K. That's around x 5. You get unity gain if you replace the 330K feedback resistor with 68K.
Agree and add:
1) varying gain at the active stages will only change *gain* , not *volume* , because it ends in a "fixed volume" clipper.
2) yes, the after clipping volume is 4X what most others would put out, because they use strings of 4 Silicon diodes for clipping instead of the traditional single one.
Try using just 2 diodes in antiparallel, you might find it what you want.
Even better, use one pointing "right" , 2 pointing "left" (as seen on the schematic) for a more interesting sound (just try it
) and still reasonable volume.
3) having 4X buffer gain (12 dB) sounds somewhat crazy to me, and unusable in some cases.
*OR* make it true bypass with the proper switch, and use only the distortion path.
I also suggest you try to get an Audio/Log taper pot as output volume control, it will have a way smoother range ... that's what they were invented for
You might fixed pad it down for unity gain on the clean setting only (not affecting distortion level or gain) by replacing the 47k resistor with, say, 33k+12k and taking signal from the junction.
You might even add a small 1000 or 2200 pF ceramic cap in parallel with 100k to get some extra "shimmering/transparent" sound, which would make clean sound subjectively cleaner.
At least it would compensate somewhat the slight brightness loss caused by the somewhat low 68k input impedance, which is there all the time, even when nominally "off" .
1) varying gain at the active stages will only change *gain* , not *volume* , because it ends in a "fixed volume" clipper.
2) yes, the after clipping volume is 4X what most others would put out, because they use strings of 4 Silicon diodes for clipping instead of the traditional single one.
Try using just 2 diodes in antiparallel, you might find it what you want.
Even better, use one pointing "right" , 2 pointing "left" (as seen on the schematic) for a more interesting sound (just try it
) and still reasonable volume.3) having 4X buffer gain (12 dB) sounds somewhat crazy to me, and unusable in some cases.
*OR* make it true bypass with the proper switch, and use only the distortion path.
I also suggest you try to get an Audio/Log taper pot as output volume control, it will have a way smoother range ... that's what they were invented for
You might fixed pad it down for unity gain on the clean setting only (not affecting distortion level or gain) by replacing the 47k resistor with, say, 33k+12k and taking signal from the junction.
You might even add a small 1000 or 2200 pF ceramic cap in parallel with 100k to get some extra "shimmering/transparent" sound, which would make clean sound subjectively cleaner.
At least it would compensate somewhat the slight brightness loss caused by the somewhat low 68k input impedance, which is there all the time, even when nominally "off" .
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I could do that, although it would be a lot of work to take the pedal apart to adjust it. I'll probably go with the 68k feedback resistor.
I'll probably do this. Would I leave the 100k where it is and add one between the pot and the ground? Or by "series" do you mean connected to the slider of the pot? As in, replace the 100k.
This sounds like a good solution, although wouldn't that distort the signal more? I kind of want to take the sound I have and make it less loud. I guess the volume pot solution would be the way around that.2) yes, the after clipping volume is 4X what most others would put out, because they use strings of 4 Silicon diodes for clipping instead of the traditional single one.
Try using just 2 diodes in antiparallel, you might find it what you want.
Even better, use one pointing "right" , 2 pointing "left" (as seen on the schematic) for a more interesting sound (just try it
Yes, it should absolutely have an audio taper pot. Not sure how I overlooked that.I also suggest you try to get an Audio/Log taper pot as output volume control, it will have a way smoother range ... that's what they were invented for
So this would be basically a voltage divider (still gain of 5, but only taking a portion of the output)?
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