Modding Moogerfooger Envelope Filter

[ethniccheese]

I am thinking about modding my Moogerfooger envelope pedal to include a "downard" envelope control.  If that is impractical, I would at least like to be able to reverse the control voltage output coming out of the pedal.  

Does anyone know how I would go about doing that?  Could I use an opamp to reverse the control voltage output?  Would I lose the full range of the control voltage?

Thanks,
Randall Osborne

[Mark Hammer]

The MF-101 4-pole Lowpass Filter has an envelope out and a control-voltage input jack.

In principle, that ought to permit you to take the envelope voltage to an off-board invertor, and feed it back to the CV input, for an inverted envelope and downward drive, without having to modify or otherwise jeopardize the existing unit.

Normally, an upward sweep consists of extracting an envelope and superimposing that on top of an existing DC voltage (usually 0v but it could be something else, too, if you have tunable initial frequency).  What the downward sweep consists of is essentially providing a fixed DC voltage for a "starting point" and subtracting the envelope from that.

If it were me, I'd:

a) check the coumentation very thoroughly to see what's mentioned in there, and if it doesn't say anything...

b) contact Moog service/support, and ask if it is possible to send the envelope signal out for further modification and feed it back to the CV input.  In view of Moog's legacy as the benchmark of modular processing for decades, I would be VERY surprised if you could NOT do this, but it is always best to check.  Hell, they may even be able to send you a schem for a simple invertor module.

[swt]

this looks like a really good project to have. Anybody has a schem of this?. Or maybe wants to trace it or send me some pictures from the pcb and layout, so i can do it??. Thanks a lot!

[Mark Hammer]

Many of the DIY Synth sites have schematics of various 4-pole lowpass filters, such as diode ladder filters.  Electronic Musician even had a 4-pole "Minimoog" filter project years ago.  Stick an envelope follower on there and you're in solid Moogerfooger territory.

Here's a good starting point: http://www.geocities.com/SoHo/Museum/4459/grey_pages/diyers.html

Scott Bernardi's redesign of his site is nice and has PCB layouts: http://home.comcast.net/~sbernardi/elec/og2/sitemap.html

If you want, you can go here and build a precise replica of a Minimoog filter using their board design: http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-moog-minimoog.html#3a

[swt]

well mark!! That's more than a starting point. Nice info in there. Thanks a lot!