Muroidea (Rat) Volume Drop

[byrnej]

I have two Rat circuits I'm working on and they both suffer from the same issue. The volume will drop when I move the filter knob clockwise. If I keep the filter between around 7 o clock (minimum) and 11-noon-ish, it works fine but after around noon, the volume with drop significantly as the tone becomes darker. The distortion and volume work great and the sound out of it in the range where the filer works is fantastic.

Build docs with schematic: https://docs.pedalpcb.com/project/Muroidea.pdf

I'm pretty new to circuits but looking for things that would impact the filter I was thinking C9, C8, or the diodes may be an issue. Am I heading the right direction here or am I way off? Any suggestions about what would make it drop that like?

 

[Nostradoomus]

Is the volume doing a sort of note ducking thing? Like it’s bottoming out? Or is it a straight across the board drop?

If it’s bottoming out lower C2 to 2.2uf or lower....I usually put a 1uf there.

 

[byrnej]

I've had a few issues with the rat PCBs from bottoming out if I am playing fast on a lower string--the attack gets off and it can't really track my playing. This occurred when I cranked the volume and distortion. When I ran an OD or boost in front of it, this issue disappeared so what I did was put in a little boost built on a veroboard inside the pedal. So this solved my problem with that, but may try that different value for C2 as well.

Anyways, with or without that boost present the filter always causes a volume drop. Filter pot works from 7 to noon-ish, after that the volume is significantly drops off. This occurs on two rat boards, which I put together at the same time. Maybe a bad batch of some parts? If it's all dependent on the filter, which part do you think could be the culperate?

 

[Nostradoomus]

Lowering C2 cures the bottoming out on low notes for me, and I play in caveman tuning haha. Can you take a video of the sound you’re describing? It’s hard to tell, kinda sounds like what normally happens when you cut off the treble...perceived volume and such!

 

[benny_profane]

If you change C2, you’re altering the frequency within the gain stage. I’d suggest an audio probe before and after tone section before you start swapping components out.

Also, have there been any substitutions? Particularly with the op amp, you may be experiencing some instability in the feedback network. Is this at all affected by the sweep of the gain potentiometer? The C2 fix would be for something like that. But, if you’re not seeing odd behavior with the gain pot sweep, I don’t think that’s your issue.

 

[byrnej]

I going to mark this as solved. The volume drop was just mostly perceived when I cut out the high end, once I ran my amp at a decent level and not night-project-while-the-family-sleeps-level it was more obvious. Still seems to be a slight drop but not much.

I will say that the all my rat circuits (2 Muroidea and 1 Parentheses) bottom out with the volume cranked and playing fast. I did switch out C2 on one of the Muroidea PCBs to a 1uF and still had the issue. I don't play in caveman tuning like Nostradoomus, just standard E and drop D (not currently setup for standard C and D). Anyways, I fixed it on the two Muroideas by throwing in a small veroboard boost--maybe I'll revisit them at later time but they seem to work well. I'll probably post them in the build reports in the next few days.

Thanks for the help and feedback everyone.

 

[benny_profane]

It’s a passive tone control, so by its very nature it cuts volume. Sounds like things are behaving normally. If it does bottom out, though, the op amp is being overloaded in some way, and the fix would involve finding out why the op amp is not stable.

 

[byrnej]

Ya know, I'll remove that solved response since I want to figure out the bottoming out issue.

Any reason why a boost or overdrive in front would solve the bottoming out issue? Maybe cutting out the bottom end out of the signal?

I remember getting the LM308 from some place I thought was legit, though I can't name it off-the-top of my head. I have switched them out for OP07s I got from mouser incase and had the same issue. I'll have to try it again and mess with some settings and see what happens.

 

[Nostradoomus]

The boost you’re using may be cutting the bass frequencies enough that it doesn’t happen, yeah. The LM308 is not a stable opamp in the first place and the designers of the rat definitely exploited it’s instability in the design.

https://www.electrosmash.com/proco-rat is a good read.

I have really hot humbuckers in my caveman guitar, that had an effect on it as well. Doesn’t bottom out or oscillate with lower impedance pickups. I’ve always lowered C2 to 1uf or lower (470n works too) for my friends that play in doom bands haha.

 

[byrnej]

I've been playing with other some people who are in mid-life crises and we were playing more stoner and doom-type stuff (at least before COVID), but, for now, keeping tuning around standard E or drop D, so not low at all. My passive bridge pickup, Duncan Custom, is moderate to hot--overwound PAF hot--but not modern bareknuckle kind of hot so it shouldn't cause too much of a headache on that end. Oh well, just going to take more testing or I can just leave it the way it is with the boost.

 

[Nostradoomus]

Yeah I’ve got BKP Warpigs and my guitar is tuned to F# haha