Input Impedance ?'s

[GibsonGM]

I just threw together the Phuncgnosis & boxed it in a PVC end cap.  Sounds really great, but in bypass it's totally sucking tone.  I used a SPDT not-true-bypass swich: I know, I should've known better!!  Thought an OA-based ckt like this had lots of impedance.   Before I get a DPDT, I was wondering if anyone knows how to increase the input impedance to a circuit like this...more resistance to ground at the input of U1b, maybe??  I could add a BJT buffer fairly easily, but don't want to have to completely tear it down.  I was going to use it on the comind Sat. at a gig, and time is a little critical...maybe I'll try putting it in the FX loop of my amp or something temporarily.

schematic here:

http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html

[slacker]

I don't think you can do anything to improve the input impedance. U1b is an inverting op amp stage so the impedance is set by the 22k input resistor, although this might also be affected by the connection to the envelope follower. If you raise the 22k significantly you'll probably mess up the filter.
You could wire the bypass so the output is always connected and the input is switched, the output has 100k impedance, set by the 100k resister to ground. This might not load your signal down or you could increase this resistor to 200k which would lessen the effect.
You could also try putting the effect after a pedal with an output buffer that might solve the problem.

[R.G.]

Put a buffer in front of it. A FET buffer or NPN emitter follower would probably work. An opamp buffer is easy as well.

Buffers are not magic. They're in fact so simple that they're kind of boring.

[GibsonGM]

Thanks for the suggestions, guys.  I know that switching it this way is nutty, but it was all I had lying around, LOL.   It's doing OK thru my effects loop, and that'll get me thru the wedding I have to play.   Like some wahs, the phunc probably really calls for true bypass - I hadn't considered just how the filter works! 

[Paul Marossy]

I know that switching it this way is nutty, but it was all I had lying around, LOL.   It's doing OK thru my effects loop, and that'll get me thru the wedding I have to play.   Like some wahs, the phunc probably really calls for true bypass - I hadn't considered just how the filter works! 

If you use the buffer like RG suggests, you can probably leave the SPDT in there. I don't think you'll really notice any tone sucking in bypass mode with buffer in place.