Big Muff pi Mod for more drive?

[g.e.o]

hello again! i have built the big muff pi (stock version) from Tonepad http://www.tonepad.com/project.asp?id=3 and its great but i need more gain. i looked at the shematic and i guess i have to change some of the parts (resistors and caps) before the sustain pot. i was wondering which should i change to get more drive? should i change them all?

thanx a lot!

George

[brett]

Hi
Huh?.  the BMP has lots of gain.
Check that your transistors are inserted corrently.  Swapping Collectors for Emitters will reduce the gain in each stage from from 150 to about 10.

But, it everything is ok and you really do want more gain, you can install high hFE transistors such as 2N5089s, and replace the 100 ohm emitter resistors in the distortion sections with 500 ohm or 1k trimpots.  Trim them below 100 ohms for extra gain and above 100 ohms for less gain. 

The Yun uses this mod in his BMPs.
cheers

[runmikeyrun]

add another gain stage in front of the first one.  You can start small, try the gain recovery stage (the one after the tone control) or a simple booster like the bazz fuss.  It won't take much.  If that's not enough you can try building another one of the first gain stages.  Remember- too much gain and it'll turn to mush.  You can also mess with your emitter resistor values (try decreasing them one at a time), clipping diodes, and coupling cap values.  To make the most drastic changes, use the first or second gain stages, for more subtle ones, use the third.  Or change all 3 if you dare    I added a bazz fuss with germanium diode in front of mine and it added a good deal of fuzz.  It does color the circuit significantly though so you can try a clean boost with lots of gain.  Might be a good idea to put a trim pot on the emitter resistor so you can play with it until you get it right amount of boost.

[g.e.o]

thanx a lot guys. i will try those mods and see what happens. also, Brett it does have a lot of gain but when i use it with the tone at zero and sustain(drive) at 10 it has less than i want. this is the best setting for my taste and i suggest it to you as well if u wanna get a sound like the intro at "Sorrow" song from Pink Floyd's "Momentary lapse of reason". its the same sound with my BM but it cant be that massive coz the original one was recorded from the PA system of LA sports arena. i do need more gain though for a solo sound more close to the "What do you want from me" song of Pink Floyd.

thats all. will let u know about the mods and if i find something good to suggest.

Take care.

George

[sfr]

When I have the sound that I like, but just want more gain out of the pedal, I tack an LPB on the output of the effect, either separately or inside the box.  (In front is cool too sometimes, but sometimes it just destroys the tone of the effect I'm feeding with it.  Other times it's cool.  Some pedals (my Dist+ comes to mind) get a tonal change with a boost in front, but no volume increase, so I actually have boxed up some distortion pedals with switchable boosts before *and* after the effect)

[g.e.o]

hey sfr. i will try one coz i need it for my dist+ i guess. i get a sound i dont like very much so it might help.

thanx a lot!

George

[Urso]

Change the 390K resistor of the output gain stage to 470K.
Or try the Creamer Dreamer mod: http://aronnelson.com/gallery/album03/Creammy_Dreamer?full=1

[g.e.o]

thanx a lot for the reply urso. i'll check out this schematic to see what i can do about it. 

[petemoore]

  I like my Muff kinda mushy the way it is.
  For harder clip and cruchier attacks I have a Rangemaster type thing in front, of course I have that RM in front of the DIST+ and FF too, that or the OS Comp makes nice crunch boosting distorter distortion increase.

[g.e.o]

nice one pete. haven't thought of trying a comp for that. will try it. am about to built the dynacomp anyway. cheers!

[Eb7+9]

... also, check out Aron's mods page, there I mention tweaking the first 8k2 (or 10k green) series resistance after the Sustain control ... replace it by a 10k trimmer and lower to taste ... IMO that's a dominant parameter

~jc

[g.e.o]

thanx Eb7+9! cool mod i think. also, nice nickname. my favorite it E7+9 but wont argue for just a b!!!

[brett]

Hi

Change the 390K resistor of the output gain stage to 470K.

Does that make much difference?  I already did that, expecting that it would change the bias just a smidgen, but not make any real difference.

Also,

there I mention tweaking the first 8k2 (or 10k green) series resistance after the Sustain control ... replace it by a 10k trimmer and lower to taste ... IMO that's a dominant parameter

I was lazy about the 8k2s (there are two, I think) and used 10ks.  Any suggestions what that does?  I thought it was just an addition to the 100k input impedance of the next section. ??

My BMP has so much gain that I bever take the sustain control past 10 o'clock.  I also have my hand on the strings whenever I'm not playing (and half the time I am) coz I get feedback in about 0.1 seconds if the strings are free.  I do play a bit loud, but it's through a couple of low-gain amps (18W and JTM60).
cheers

[Eb7+9]

Any suggestions what that does?  I thought it was just an addition to the 100k input impedance of the next section. ??

I'm talking about the first of the two - for some reason the second doesn't seem to affect things as much ... this resistor in part works like a grid blocker resistor does in tube stages (reducing voltage drive as current developes) but also by affecting the clipping point of the next stage by starving off Base current going into that next stage ... reducing its value allows more current to flow into the Base, widening the Base cutoff limits and increasing the ability of that next stage to drive its diodes harder ...

I suspect this situation is partly similar to the series resistor Joe Davisson uses in his compression stages - he also uses a 10k resistor in that position if I recall ... the way I see it there's two ways you can make a Bipolar gain stage limit or shut-off the current passing through it, (i) by attaining saturation or cutoff voltage limits in the collector circuit and (ii) by preventing (recombination) current from entering the Base ... I'm thinking there's some of the second mode happening in that section of the BMP if the resistor is made large enough - conversely, lowering that resistor would reduce that constraining component somewhat ...

my old BMP was forever lame, that mod woke it up - personally I'm not a fan at all of changing other component values ... also, no need to remove that resistor entirely, going too low is no good either - seems there's a sweet spot ...

[R.G.]

Each of the clipping stages of the BMP is effectively a feedback amplifier. The feedback resistor is the collector-to-base, the series input resistor is, well, the input resistor. As long as the diodes are not conducting, below their clipping thresholds, the gain is as close to Rf/Ri as the open loop gain will support. That obviously gets changed when the diodes conduct, or when the feedback capacitance starts reducing the gain at higher frequencies.

By raising the input resistor, you lower the closed loop gain. By lowering the input resistor, you raise the closed loop gain. There's no more mystery to it than that.

The imperfections of the limited gain, symmetry and biasing of a bipolar instead of an opamp doing the amplification add plenty of special cases around this, but the change in gain with input and feedback resistors is the key to understanding it.

[Eb7+9]

By raising the input resistor, you lower the closed loop gain. By lowering the input resistor, you raise the closed loop gain. There's no more mystery to it than that ... The imperfections of the limited gain, symmetry and biasing of a bipolar instead of an opamp doing the amplification add plenty of special cases around this, but the change in gain with input and feedback resistors is the key to understanding it.

... sounds like snowjob talk - I can recommend some good beginner texts if you want to learn something on the subject of feedback loop analysis - then again that would serve no purpose in explaining what's going on here ... first, the resistor "we're" talking about is NOT part of any feedback loop, it's in series between two cascaded gain stages ... second, you don't use feedback analysis (small signal) to describe clipping behavior (large signal) - this is something properly trained design engineers understand ...

[g.e.o]

interesting turn to this conversation but would really appreciate, at least for this post, to just reply for the certain question please than argue whats wrong-right. if anyone had tried something on his BMP (tonepad's layout) for increasing the gain or is sure that was he recommends is right, just say it. no need for argument though.

thank u all very much!

George

[NoFi]

... also, check out Aron's mods page, there I mention tweaking the first 8k2 (or 10k green) series resistance after the Sustain control ... replace it by a 10k trimmer and lower to taste ... IMO that's a dominant parameter

I'll second that and it also works with the op amp big muffs. They have an 8k2 resistor before the last op amp stage.
Loads of gain available.

[petemoore]

Change the 390K resistor of the output gain stage to 470K.
Does that make much difference?  I already did that, expecting that it would
 ..it'd be a difference After all the tone and clipping..slight volume increase?..and a base bias change on a stage simply used to boost output    
  or...possibly slightly clipping one side of signal swing when pushed hard enough in the last Q..hard to say..I don't know how close to the rail/distorting as a result .. that recovery stage's signal swingwould be

[R.G.]

... sounds like snowjob talk - I can recommend some good beginner texts if you want to learn something on the subject of feedback loop analysis - then again that would serve no purpose in explaining what's going on here ... first, the resistor "we're" talking about is NOT part of any feedback loop, it's in series between two cascaded gain stages ... second, you don't use feedback analysis (small signal) to describe clipping behavior (large signal) - this is something properly trained design engineers understand ...

Thanks, JC. I read beginner, intermediate and advanced texts on a number of things for fun. I can also recommend some ones I've enjoyed for you if you'd like.
And congrats on finding those hypens.

First: yes, the resistor under question is not inside a feedback loop. But it helps determine the gain of a feedback loop. For small signals, below clipping and nonlinearities (and we'll get to clipping in a moment), the resulting closed loop gain of any inverting gain block is determined by two things: the feedback and the input resistor. The resistor in question acts like a series input resistor with the 470K feedback resistor until other things start modifying the response - like clipping diodes.

Second: the clipping diodes. Until the clipping diodes conduct, they are effectively not there. This is true on an instant-by-instant basis, so for signals small enough not to exceed the clipping thresholds, the diodes are effectively not there. In fact it's true for signals which ARE large enough to exceed the clipping thresholds for those portions of the waveform less than the clipping thresholds. The instantaneous gain of the circuit is whatever it is below clipping. When the diodes start conducting, the instantaneous gain gets reduced by the diode conduction to something smaller. The gain is time varying. It's linear(-ish; there are lots of other effects going on in a transtor) up until the clipping parts start having an effect. Then the clipping parts start predominating.

Properly trained design engineers use whatever analyses produce the best description of the circuit at hand under the circumstances that apply - even if you have to shift gears nimbly from one description to another as waveforms change.

would really appreciate, at least for this post, to just reply for the certain question please

Sorry.

You can increase the gain several ways. One is to reduce the 8.2K input resistor. Another is to increase the 470K feedback resistor, but you can't do that without also affecting the bias of the transistor. You can split the 470K resistor into two resistances of about half the total, and put a cap to ground from the middle of the two. You can bypass the emitter resistor. You can increase the collector resistor. You'll probably have to dink with the biasing if you mess with the collector resistor too.

The simplest thing to do is to put in another transistor gain stage like the two clipping stages that are already there.

How much gain are we talking about?

In the simulator, the existing stage shows a gain of about 35db (56x) open loop, and about 28db (25x) closed loop. Messing with the circuit components as I described can get you another 2x to 4x. That may or may not be enough for you.