sustain/gain control in op amp big muff

[thunderaxe]

hi guys,

i've been working on analyzing the circuit for the op amp big muff:



and i feel like i have a pretty good understanding of it, the first stage is a +7.7dB boost, second stage is a sallen-key filter, third is clipping, followed by standard big muff tone control and passive output volume.

what i'm curious about is why the sustain control is wired like a passive volume control between the filter and clipping stages, rather than a negative feedback resistor in the first stage. are there advantages to that approach, or is it maybe just because the transistor big muff does it like that and they were trying to keep it as similar as possible?

to further complicate matters, here is the keeley boutique version of it:



and in that one, lug 1 of the "buzz" pot isn't wired to ground, so it isn't even a typical voltage divider as passive volume control, but rather a variable resistor. again, what might be the reasoning behind that and advantages/disadvantages of each approach?

thanks for the help!

[drdn0]

hi guys,

i've been working on analyzing the circuit for the op amp big muff:



and i feel like i have a pretty good understanding of it, the first stage is a +7.7dB boost, second stage is a sallen-key filter, third is clipping, followed by standard big muff tone control and passive output volume.

what i'm curious about is why the sustain control is wired like a passive volume control between the filter and clipping stages, rather than a negative feedback resistor in the first stage. are there advantages to that approach, or is it maybe just because the transistor big muff does it like that and they were trying to keep it as similar as possible?

to further complicate matters, here is the keeley boutique version of it:



and in that one, lug 1 of the "buzz" pot isn't wired to ground, so it isn't even a typical voltage divider as passive volume control, but rather a variable resistor. again, what might be the reasoning behind that and advantages/disadvantages of each approach?

thanks for the help!

For the second part it's a bit easier - how do we calculate the gain of an inverting op-amp gain stage?

[thunderaxe]

For the second part it's a bit easier - how do we calculate the gain of an inverting op-amp gain stage?

ahhh, got it! so it does actually control the gain of an op amp rather than just being a passive volume control, only it's the gain of the clipping stage that is being controlled, not the boost stage like i imagined it would be. and presumably the big difference between varying the input resistor rather than the feedback resistor is that the former won't change the low pass corner frequency of the miller capacitor?

well now i'm curious to find out how it would affect the low pass if the gain pot was in the feedback loop instead. i've been playing around with a treble bleed on the gain pot in this circuit to try and retain a little more high end on lower gain settings, but wondering if there's a better way, akin to how the low pass in the RAT changes dynamically with gain...

[Andon]

If you were just looking to add more high end going into the gain stage I would think that changing the cap to ground after the 47K leading into the second op amp stage - C4 (10nF) or C8 (4.7nF) (same cap, different values depending on which schematic you're looking at) down to something like 1nF will allow more high frequency content to pass through. As they are, they low pass the signal at 339Hz-720Hz, respectively. To me that looks like the bottleneck for treble, since the LPF setups in the clipping op amp sections of both schematics are set for at least 2kHz.

[thunderaxe]

If you were just looking to add more high end going into the gain stage I would think that changing the cap to ground after the 47K leading into the second op amp stage - C4 (10nF) or C8 (4.7nF) (same cap, different values depending on which schematic you're looking at) down to something like 1nF will allow more high frequency content to pass through. As they are, they low pass the signal at 339Hz-720Hz, respectively. To me that looks like the bottleneck for treble, since the LPF setups in the clipping op amp sections of both schematics are set for at least 2kHz.

that low pass is part of a sallen-key filter so it's not just a straight single-pole low pass. i plugged the components values into this calculator:

http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

and got the following results:

op amp big muff/dream fuzz
Fc = 1.07 kHz
Q = 1.02
gain = 10

rotten apple/sour grape
Fc = 1.56 kHz
Q = 0.72
gain = 3.68

anyway, what i'm trying to achieve isn't just to have more treble pass into the clipping stage overall, if anything i think the output could use a little less treble. but i would like to have a little more consistency in the treble content between low and high gain settings, that's all.