BIGGER MUFF  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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dylan159
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Post by dylan159 »

A few months ago, I got the idea of reworking the Big Muff in a modern fashion, using op amps, while remaining true to the logic behind the original. The op amp muff is a cool circuit but deviates substantially from the original (I don't think anyone would say they are the same) and for one thing, the tone and volume control attached passively at then end don't thrill me. The logic behind the original muff is simple: an amplifier, two clipping stages, a tone control and a recovery stage. How can we improve upon that now that we can make use of op amps, while keeping the clipping stages the same? Here's how I'd do it:
biggermuff1v1.png
So, the first change is in the input stage. It's a simple low gain noninverting op amp with a gain of 5.7x, high input impedance, pretty flat frequency response. Why do I think this makes a good change? For one thing, a jfet input op amp like this should be quieter than a simple bjt input stage like the original with a guitar source, and surely has better psrr. For the gain, I simulated the original input stage with a simulated guitar input source, and all things considered, like gain loss due to the low input impedance of the input stage (which accounting for the biasing, the emitter resistance, and the miller effect turns out to be an unexpectedly low 50K), the source impedance of the guitar, and the 10k output impedance of this stage reducing the gain of the first clipping stage, it turns out that it's pretty close, and it surely sounds close, with just a bit more treble let on by C3, which is either inconsequential or beneficial but can be easily changed by a bigger value.
The only real judgement call is the input impedance. I don't like losing signal before I can do anything with it, and I never found muffs to excel in guitar volume interaction, so I went with my go-to input stage, with the high input impedance, some amount of gain but without risking any clipping, low resistor values. But I know some people won't like if i just take that from them, so I left the option of a more faithful input stage, inverting, with a lower input impedance (which can be made lower if you really say so) and the same amount of gain. The variable lowpass should make for something interesting too. This one has the advantage of keeping the pedal overall noninverting too.
Here lies the only update from v1.0: I had this stage set as variable gain, which is optimal for headroom, and also has good noise performance, but with the disadvantage that it can't go below 2x gain, and the effect was kinda subtle. I changed the control to a variable attenuator because my only concern was noise here, and this performs just as well in that regard but can go down to 0. The headroom is still good because the input stage won't clip easily at all, and if you really want to torture it, it all gets mixed up with the rest honestly.
Enough about the input stage. The clipping stages haven't been touched much at all, except for being a kind of middle of the road mashup of different versions, with a focus on using common values. Feel free to copy your favorite one here.
And finally the tone control, which is something I'm proud of. I didn't like the idea of a lossy tone control followed by a recovery stage when we could have an active one by means of op amps and feedback that doesn't need that. A friend mentioned a trick to make any tone control active which was right what I wanted, so I took the muff tone control and made it active! That's the kind of thing that has been made easy by op amps and we can do to take the design into the future. I let myself use some less standard values than I like to copy the values in the NY muff tone control, and the only other addition is an "AMZ presence" mid control to have continuously adjustable control over the midrange overlap. Here's the simulation plots comparing this active version to the passive one using the same values:
tone.png
and with the mids pot at full:
tonemids.png
As you can see they're very similar, but without the losses and with a stronger effect. I'm not saying it's an improvement because that would be subjective, but I can say that the result is quite pleasant, faithful to the original, and works very well. Also in one go we solved the gain recovery problem and also got a very good output impedance with it.
About the power supply, the only thing of note is the capacitance multiplier for the bjt supply to really crank down the supply noise on those stages with a lot of gain and poor psrr.
Here's a compact vero layout for it
biggermuff1.1.png
Some sound samples:


And if you're curious, a clip of v1.1 at minimum gain:
cleanmuff.wav
(1.69 MiB) Downloaded 185 times
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Post by DaveLTX »

:worship:

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Post by dylan159 »

DaveLTX wrote: 09 Apr 2021, 04:51:worship:
:lol:
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Post by MGaburriJazzDude »

Nice design! I'll have it in mind!

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Post by Nocentelli »

Nice work, thanks for sharing the design. I periodically think I am bored with the BMP, have seen and tried every tweak, mod/extra feature and have exhausted the potential of the circuit, but someone always comes along soon enough with something else I just have to try out...... :shred:

Weirdly, I don't think I had seen that type of implementation of an active bmp tonestack before (sort of like a one pot baxandall?) when I read your post yesterday. Then, literally this morning, I noticed bugg had traced the ehx ripped speaker fuzz and put it up at pedal-pcbs and it is eerily similar to your design! It has two LPB-1 stages with a divider pot gain control between then instead of the two BMP clipping stages, it has an opamp input (buffer rather than boost) and a Q3 with variable bias for the "ripped speaker" control, but at the output there is the same (to my eye) type of modified tilt control.

Just about to try both circuits out on the breadboard, fun saturday ahead.
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Post by dylan159 »

Nocentelli wrote: 10 Apr 2021, 09:30 Nice work, thanks for sharing the design. I periodically think I am bored with the BMP, have seen and tried every tweak, mod/extra feature and have exhausted the potential of the circuit, but someone always comes along soon enough with something else I just have to try out...... :shred:

Weirdly, I don't think I had seen that type of implementation of an active bmp tonestack before (sort of like a one pot baxandall?) when I read your post yesterday. Then, literally this morning, I noticed bugg had traced the ehx ripped speaker fuzz and put it up at pedal-pcbs and it is eerily similar to your design! It has two LPB-1 stages with a divider pot gain control between then instead of the two BMP clipping stages, it has an opamp input (buffer rather than boost) and a Q3 with variable bias for the "ripped speaker" control, but at the output there is the same (to my eye) type of modified tilt control.

Just about to try both circuits out on the breadboard, fun saturday ahead.
Thanks, I'm glad you're giving it a try. While I'm not the biggest fuzz enthusiast and I think there is such a thing as too much fuzz, the muff is always fun to work with I agree.
About the ripped speaker, it's a different kind of fuzz and I'll refrain from a complete analysis, but you're right that it's kind of the same arrangement. Their tone control is more like a classic "active tilt" with slightly different values, while mine comes from an idea by Teemu Kyttala to make the TMB active mentioned by a friend. They end up being pretty much the same, save from two series resistors, which is a fun example of convergence. The similarity with a baxandall is that both apply negative feedback to an inverting amplifier and that's it I think by the way.
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Post by ppluis0 »

Hi folks,

dylan159: thank you for bring us this improved version of a BMP

Some time ago I read a post (perhaps made by Pinkjimiphoton ?) indicating that one of the 1N4148 used as clippers can be replaced by an 1N4000 in order to reduce overall noise.

Anybody here remember that post ? Which one of those four diodes must be replaced ? :hmmm:

Cheers,
Jose

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Post by dylan159 »

ppluis0 wrote: 10 Apr 2021, 20:31 Hi folks,

dylan159: thank you for bring us this improved version of a BMP

Some time ago I read a post (perhaps made by Pinkjimiphoton ?) indicating that one of the 1N4148 used as clippers can be replaced by an 1N4000 in order to reduce overall noise.

Anybody here remember that post ? Which one of those four diodes must be replaced ? :hmmm:

Cheers,
Jose
thank you for your attention. improved? I don't claim that, I'll let you try it and decide.
i guess you mean the 1N400x rectifier diodes. Why would it make a difference, for noise? No, I don't think it would. The main differences of note are junction capacitance a bit and reverse current a bit more, which might justify their use as clipping diodes instead of 4148. But for noise I don't think it makes a difference, not even the high frequency effect of capacitance on perceived noise.
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Post by Intripped »

ppluis0 wrote: 10 Apr 2021, 20:31 Hi folks,

dylan159: thank you for bring us this improved version of a BMP

Some time ago I read a post (perhaps made by Pinkjimiphoton ?) indicating that one of the 1N4148 used as clippers can be replaced by an 1N4000 in order to reduce overall noise.

Anybody here remember that post ? Which one of those four diodes must be replaced ? :hmmm:

Cheers,
Jose
I remember that post, and yes it was from Pinkjimiphoton.
He suggested to put a Ge diode in parallel with D4 (just D4 and not D5, referring to dylan159's schematic here in this page), in order to achieve a "fluid tone" and less background noise

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Post by ppluis0 »

Intripped wrote: 10 Apr 2021, 23:01 I remember that post, and yes it was from Pinkjimiphoton.
He suggested to put a Ge diode in parallel with D4 (just D4 and not D5, referring to dylan159's schematic here in this page), in order to achieve a "fluid tone" and less background noise
OK, I'm not remembering exactly that Jimi wrote about one Ge diode in one exact position.

I will try that in my next breadboard season... 8)

Cheers,
Jose

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Post by bmxguitarsbmx »

Very cool tone control section Dylan! That's clever man!

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Post by Cub »

Here's the thread: big muff pi noise reduction mod, but I still don't know which diode is the one, though. "D4 on the board that Jim was using" but what board is that? A lot of confusion and frustration could have been prevented if it was written along the lines of "the diode in the first/second clipping stage that has the Anode/Cathode on the collector"

I figured that for somebody with my limited set of skills, the best way to find out (without stepping on any toes) was to build one without snipping off the legs of the four stock diodes, put that germanium diode on a couple of gator leads to try it on all four positions. But alas, I haven't gotten round to build a muff in a long, long time.
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Post by dylan159 »

Cub wrote: 11 Apr 2021, 08:43 Here's the thread: big muff pi noise reduction mod, but I still don't know which diode is the one, though. "D4 on the board that Jim was using" but what board is that? A lot of confusion and frustration could have been prevented if it was written along the lines of "the diode in the first/second clipping stage that has the Anode/Cathode on the collector"

I figured that for somebody with my limited set of skills, the best way to find out (without stepping on any toes) was to build one without snipping off the legs of the four stock diodes, put that germanium diode on a couple of gator leads to try it on all four positions. But alas, I haven't gotten round to build a muff in a long, long time.
Yeah, the thread could be more informative and wanders off with terms like "the sea" or "magma hum" or suggestions like increasing the sustain stopper because otherwise it's too low :hmmm:
I'd like to know more about the mechanism, just as you, and the why: if it has to be germanium, and in that orientation, what does it tell us? the one thing peculiar about germanium is leakage, but the diodes are AC coupled. Is it still capacitance? I don't think so. They still have a peculiar VI curve, mostly because of the internal resistance, but is that why? Is it because it's in parallel with a BAT41 with an almost comparable Vf, and the two diodes in parallel are what makes the deal? Is it just because the slightly lower Vf means lower threshold so lower gain to get the same effect? I don't want to go too far from the topic here, but if you want to test and make another thread about it let me know.
Or you know, build mine, no magma sea here :D
Interesting that nyc reissues use BAT41 by the way.
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Post by 287m »

Nice work!
Why power conditioning trannies only? Its there problem if share with opamp?

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Post by dylan159 »

287m wrote: 12 Apr 2021, 12:08 Nice work!
Why power conditioning trannies only? Its there problem if share with opamp?
The cap multiplier drops some voltage, and the op amps have better psrr and less gain overall, but slightly less headroom in this case. I think this is the best compromise of filtering and headroom.
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Post by 287m »

Thank you. Learn something again.

One question (again). Any possibility to replace 2 clipping stage with opamp circuit? Just wondering.
I know, different opamp build never sound same, but not as big difference like trannies circuit.

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Post by dylan159 »

287m wrote: 13 Apr 2021, 21:03 Thank you. Learn something again.

One question (again). Any possibility to replace 2 clipping stage with opamp circuit? Just wondering.
I know, different opamp build never sound same, but not as big difference like trannies circuit.
Probably yeah, it won't be exactly the same, but at that point you can swap both input and clipping stages for a single stage with the same gain and frequency response, and you'll have a "muff" with just two op amp stages.
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Post by 287m »

read that word probably make me excited, unfortunately don't know how to do at this moment.

W-wait, just two opamp stage? Somehow make me remember Cream Puff.

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Post by JiM »

A bit off-topic, but for a single-chip BMP work-alike you may look at the 22/7 from RunoffGroove : http://www.runoffgroove.com/22-7.html
I only give negative feedback.

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Post by dylan159 »

JiM wrote: 14 Apr 2021, 18:24 A bit off-topic, but for a single-chip BMP work-alike you may look at the 22/7 from RunoffGroove : http://www.runoffgroove.com/22-7.html
Yeah i remember that one, pretty close to the original in concept. I'd be uneasy using inverters because they're noisy though.
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