critique my circuit?

[thunderaxe]

hi all,

i'm working on my first unique(ish) design -- it's an adaptation of the keeley rotten apple (pedalPCB sour grape), which is itself an adaptation of the EHX op amp big muff (pedalPCB dream fuzz). the input, sallen-key and clipping sections are largely the same, then i've added a tone section based on the neve 1073, a clean blend, and as a bonus i used the clean blend path as an optional buffered bypass. it works and sounds good on the breadboard but can you give it a once-over and let me know if i've made any mistakes or if you have any suggestions for how i could do this better or smarter?

just to preempt questions about this: having the clean level control after the distortion volume is deliberate. from a usage standpoint i would've preferred it to be before the volume but doing it this way prevents me from having to boost the clean signal ~22x and clipping the op amp to get it to a level where it can compete with the distorted signal.

any and all advice is appreciated.

thank you!

 

[thunderaxe]

based on some very helpful replies and advice i got over at diystompboxes, i've made the following changes:

-VREF filtering cap is increased to 100uF
-GAIN pot lug 3 is tied to the wiper
-VOLUME and CLEAN pots are increased to A100K
-summing amp input and feedback resistors are increased to 1M

sounds great on the breadboard, unless anybody has any other suggestions, the next step is to build the circuit additions on vero, wire it all up and try to fit it all in a box!

 

[thunderaxe]

okay, so before i get my PCBs printed i wanted to build a prototype to test the circuit in an actual stompbox, so i made a hack job of the original circuit on its PCB and the tone section, clean buffer, and output stage on vero. it's ugly, but it works. except for one thing -- with the VOLUME and CLEAN pots turned to zero, where there should be silence, instead there is a fizzy, extremely high-passed ghost of the distorted signal still in there. turning CLEAN all the way up, which should sound identical to the bypassed sound, instead sounds like the bypassed signal with a superimposed fizzy version of the signal.

at first i thought this might be because using VREF as the signal ground was polluting VREF, but it doesn't matter where i ground lugs 1 of VOLUME & CLEAN to, including giving them their own dedicated cap directly to power supply ground, that fizzy ghost signal is still in there. lifting them from ground entirely makes the signal jump to full volume, which is to be suspected. with no audio signal running through the pedal, lifting the grounds brings in a lot of white noise.

at one point i suspected that maybe all of the wires criss-crossing my circuit as a result of my hack job meant that some of them had become microphonic, but lifting them up and away from the circuit board and placing a piece of steel between them and the circuit board made no difference whatsoever. i would have expected at least a little fading in and out or some kind of audible change as i moved the wires if it was because of that.

removing the wire from lug 2 of VOLUME to the output stage (how the distorted signal enters the summing amp), however, made the fizzy ghost signal almost entirely go away. it's still in there in the tiniest amount if i really listen for it but about a hundred times quieter. good enough for me. obviously that isn't a fix though, just an attempt to isolate the problem.

any ideas on what might be happening here?

 

[thunderaxe]

i've been thinking about how i can get better headroom out of this pedal. the clipping cap is allowing a lot of bass to pass through the clipping stage at full volume and if it isn't clipping the op amp at that stage, it definitely will if it's boosted in the tone section. and as much as i do love that kind of blown-out sound, my primary goal with this pedal is increased clarity.

my first thought was to increase the voltage supply to 18v, and while i still think that that would work, i can't help but think there must be an easier way, namely attenuating the volume of the whole signal after the clipping stage. the output volume is already much higher than it needs to be, so there shouldn't be any problem with knocking it down a bit earlier in the signal chain and getting some better headroom that way. so i figured i'd put a simple voltage divider between the clipping and tone stages and hope it doesn't interact with either stage:



but of course it will. the series resistance of R27 gets added to the values of R14 and R16 and completely throws off the curves of the tone section.

so then i thought, since the tone section already has series resistances in R14 and R16, maybe i can just eliminate R27 and let R14 & R16 form a voltage divider with R26 (of any value). but according to my simulation in falstad, there is no value for R26, even going as low as 1 nano-ohm, that will cause the volume to attenuate.

so my question to you is, short of an inverting op-amp with fractional gain, is there a good way to attenuate the volume after the clipping stage to gain some more internal headroom in the pedal?