Problems adding a low-pass filter to Decadence

[Reptoid]

I'm working on the Decadence project and there is some crazy high end boost on the output so I tried adding a simple low-pass filter to the output, a series resistor with a cap to ground. Seems simple enough. Cool it works BUT the output pot now controls how much it works. More output, more filter. So I added a buffer from the amzfx site (which is basically lifted from the tub screamer schematic) after the output pot and THEN the low-pass filter after the buffer. The volume pot STILL affects how the filter works, why? I even tried inserting the buffer BEFORE the output pot with the filter at the input of the buffer but I get the same results. Pretty confusing. What am I missing here?

 

[chris.knudson]

Got any photos?

 

[Reptoid]

Got any photos?

I could send some photos but the pedal works perfectly fine on it's own, I assume the high boost is part of the design. I have a Ten Years pedal based on the same Peavey amp design that I am comparing it to and I see the same high freq boost from the output pot on both pedals. I am using Room Eq Wizard to run frequency sweeps to check the frequency response. The Ten Years pedal has an output boost stage and I assume a low-pass somewhere because there is something between the output pot and final output that boosts the signal and takes care of the high freq problem. It's just super hard to trace the Ten Years because of the weird aesthetic layout it uses and super thin traces.

Right now the buffer and LPF are breadboarded and they work, it's just super confusing why the output pot should control the cutoff of the filter when there is a buffer between them.

 

[chris.knudson]

Yeah, I am having a very similar problem with pedal I am troubleshooting myself. I'm with you -- I don't understand it.