My Envelope filter sounds like Darth Vader

[cjlectronics]

Just built a Seamoon Funk machine and modified the circuit with a JFET front end using Jack Orman's Dr. Quack.  The pedal works pretty good, but the filter sounds like Darth Vader when I'm not playing.  Has anybody else experienced this? 

I read through Mark Hammer's Technology of envelope controlled filters at GEOFEX, (thank you for the priceless information) and Mark touched on the filter operation. From this I tend to want to focus on a DC component in the filter? Could there possibly be an offset issue or CMRR issue with the 1458 op amp?

Thanks,
CJ

[cjlectronics]

BUMP, maybe someone has experienced this b4?

[Mark Hammer]

Hi, guy.  Hope the family's all well.  Thanks for the nod.

"sounds like Darth Vader" is, um, not an especially helpful description.  And when you say "when I'm not playing", do you mean that there is no guitar signal at all, or are you referring to that period as the string vibration is dying out?

[cjlectronics]

Hi Mark, thanks for the reply.  I hope you folks in the attic of America aren't getting to frozen over with the climate events we've been seeing lately. 

I hear a continuous wind blowing through a tube (the best description i can think of) when I hold the strings on my guitar and not playing.  It makes me think that the high gain on the peak detector is biasing the filter op amp to a point where the noise floor can be heard anytime the effect is enabled.  Is this normal?  I get the feeling that this circuit was designed around a particular mfg's IC?  This is why I'm curious if anyone else has experienced the same results.  I've tried different dual op amps with varying results.  The NE5532 doesn't work at all... no output.  The 4558 has the same results.  A TL072 is not a responsive as the 1458, but Darth Vader still lives in the filter circuit. 

CJ

[Mark Hammer]

Th FM needs a 1458 or a pair of 741s, just as its kissin' cousin the Doctor Q does, although I suspect that is really a need on the part of the envelope follower and not the filter section itself.  That being said, the Dr. Quack modification, which includes not only the JFET front end, but some additional biasing tricks, makes it possible to use virtually any dual op-amp.

One of the quirks of the FM, though I imagine this would depend on which schematic you are working from, is that it includes variable gain on the filter section, via that 5M pot.  If you've got the pot dimed, that could be your issue right there.  Turning the feedback resistance to max drops the center frequency of the filter downwards, so it could conceivably take some noise source, and turn it into resonant noise via the action of the filter.

I made myself one, from RG's drawing, and just opted for a 330k fixed resistor and 1M pot instead of the much wider range (47k + 5M) shown.