Big Muff Pi Op-Amp Sustain and Tonestack topology questions

[bushidov]

So, another circuit that I am trying to get a good understanding of is the Big Muff Pi Op-Amp schematic. Here is the schematic I am working off of:



Here are a couple things I am a little confused on:
1. I can see on op amp U1B, where the sustain portion of the pedal is coming from a capacitor going from before the low pass filter of R5 and C2 to the output of the op-amp. This being capacitor C3. What is it doing there? Is it like a miller cap or something?

2. The whole RC network on the feedback of R6, R7, and R8 along with C4 and the 10K Sustain pot. I think I've seen this topology before but cannot remember where. What is this again? (Vaguely looks like part of a baxandall, but I could be way off)

3. In the tonestack, it looks a lot like the original BMP tonestack, which I pretty much get. The only part that is a little off is what C7 is doing there. I know normally the tone knob either rolls off an HP filter on one side or a LP filter on the other, but the C7 going into the LP Filter of R11 and C9 isn't making sense to me. Any help?

Again, I greatly appreciate the knowledge you folks toss back at me and I thank you all for your time in answering my questions.

[Mark Hammer]

2. The whole RC network on the feedback of R6, R7, and R8 along with C4 and the 10K Sustain pot. I think I've seen this topology before but cannot remember where. What is this again? (Vaguely looks like part of a baxandall, but I could be way off)

Imagine R8 wasn't there.  U1B would then look like a pretty standard non-inverting op-amp stage, with some gain.  Just why R8 needs to connect to C8, instead of directly to ground, I couldn't tell you.

3. In the tonestack, it looks a lot like the original BMP tonestack, which I pretty much get. The only part that is a little off is what C7 is doing there. I know normally the tone knob either rolls off an HP filter on one side or a LP filter on the other, but the C7 going into the LP Filter of R11 and C9 isn't making sense to me. Any help?

Again, I greatly appreciate the knowledge you folks toss back at me and I thank you all for your time in answering my questions.

U2 is biased to +4.5V.  While the more familiar transistor BMP provides a means to remove/block any DC from being part of the final output, the op-amp version needs to attend to that.  The more familiar tonestack, incorporating C8 and C9 would take care of the DC on the treble side, via C8, but not on the bass side.  So C7 keeps the DC out while providing enough bandwidth for the bass part of the signal.

[duck_arse]

feedback and feedforward loops usually have me befuddled too, so I'll leave those two points. but it is often argued that a standard big muff tone control needs a DC blocking cap to the low pass section, as there is a DC path through all the resistors to ground.

or, wait for Mark to press Post.

[Mark Hammer]

I won't say that there is NO benefit to inclusion of a DC-blocking cap in the bass/lowpass part of a standard BMP tonestack.  But where the standard BMP structure includes a gain-recovery stage after the tonestack, there IS a DC-blocking cap just ahead of the Volume pot.  So, whatever is unattended to before that stage, does not impair the circuit from making sure than no DC leaves the pedal at the output.

[Scruffie]

1. U1B is a Sallen Key Filter, has a cut off of something like 1.2KHz and a Q of 1, IIRC.

2. Again, part of the Sallen Key, adding gain, adjusts the Q factor and the Electro is a shunting cap (and some very, very tiny bass cut, think of it as a standard non-inverting gain stage like a tube screamer input with a cap/resistor and it might be more obvious), if those resistors were just grounded, the bias would be way off and the pot is just sharing the cap because it can (and acts as a filter).

3. C7,  DC blocking, without it, the output of the op amp would be going through R11, the tone control and volume pot and seeing the output directly, along with all the other resistors to ground.

[bloxstompboxes]

I think I still have a couple of these fabbed PCBs if you would like one. I think the last I charged was like $5 or something.

[bushidov]

Ah, that makes a lot of sense guys, and you are correct Scruffie, now that I look at it, it is a Sallen-Key Low Pass filter.

It's got a frequency cut of about 1071 Hz with a Q of 0.26074921057529, when doing the calculations from here:
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

And Mark and Duck_arse are correct as of the DC blocking cap. Now that you explained the path of "uh-oh", I can see why that might be necessary (Scruffie called it out too).

I am still confused about the R6, R7, and R8 along with C4 and the 10K Sustain pot part though.

Imagine R8 wasn't there.  U1B would then look like a pretty standard non-inverting op-amp stage, with some gain.

1. I am looking at the Sallen-Key part and removing R8 and it still isn't making sense. I would figure a non-inverting op amp would make the Sustain take the place of R6 as a variable resistor or inline with R6 to make a minimal resistance across the negative feedback to vary the gain on that op amp. Or maybe that's kind of what it is doing? R6 parallel with R7 + R8 + VR1, but then VR1 never "varies" because VR1, pins 1&2 and VR1 pins 2&3 are still both into that negative feedback loop. The only part that varies is where the voltage divider is at between VR1, pins 1&2 and VR1 pins 2&3 going out. So I am still confused as to how that varies the "sustain".

2. And then there is that C4 value in there. It looks like an RC network of R7 to C4, but if that's the case, that's a low pass filter with the R coming first and the C going to ground. If that is the case, it's a low pass filter with a frequency cut off of 0.26 Hz. That doesn't sound correct. All signals under 0.26 Hz passes? For it to make sense, shouldn't it be a high pass filter? But a high pass filter is cap to resistor to ground?

Sorry, I am missing some fundamentals, I am sure. Thanks again for all the help!

[Scruffie]

If you go about half way down that calculator page you linked, you'll see there's a version with two extra resistors in the feedback loop (just pretend the electro isn't there), that's why your Q is only 0.2, it's not acting as just a buffer.

Break it in to its core components, a sallen key filter and a non-inverting gain stage. Ignore R8 and the sustain pot as I think that's just confusing you.

[Scruffie]

Just to cover R8 & the sustain pot (even though I've told you to ignore them  ) don't think of it as part of that sallen/gain stage, just think of it as a volume pot (which it is).

R8 stops it going to zero (you don't want zero gain, do you?) and if it wasn't connected to C4, you'd have a variable resistor hanging off an op amp output with 4.5V on one side of it (from the op amp) and ground on the other end of it, so ignoring the fact the opamp wouldn't like that much, what would happen when you turned it? Crackle.

It might be easier to just think of it as having its own electro hanging off the bottom, but it's just saving a part and sharing that C4, another option would be for it to connect to a V.Ref. Otherwise you'd need an additional cap from the opamp output before the sustain pot to block DC.

[bushidov]

Ah, so it is. Probably help if I scrolled down to the bottom of that page. I appreciate the technical breakdown of what is going on there with that Sallen-Key Filter.