Arp 4072 VCF Bleedthrough Fix

negativspace · Joined: Dec 25, 2010 Posts: 15 Location: Manhattan, Kansas

Yves,

I built one of these for a fellow over on Muffwiggler's and we're trying to fix the bleedthrough issue. He forwarded me some instructions you gave him, quoted here:

"The cure is to replace the resonance potentiometer by a dual gang model. Connecting the CW lugs in parallel, the CCW lugs in parallel, and one wiper lug connected to the normal resonance circuit while the other wiper lug connected to the R43 resistor."

I am unclear on one thing - when you say 'connect to R43' do you mean in place of R43, or to its input leg, or to its output leg? Thank you!

dstrength · Posted: Sat May 21, 2011 12:48 pm Post subject:

I think you're just replacing the T-connection on the schematic..so looking at the schematic, remove the T and tie the left side of R43 to the other wiper.

yusynth · Joined: Nov 24, 2005 Posts: 1156 Location: France

Ok I am back home.

I mean "connect to" not "replace", this means unsolder the left leg of R43, solder a wire this leg and connect this wire to the wiper lug of the second potentiometer of the dual-gang pot.

negativspace · Joined: Dec 25, 2010 Posts: 15 Location: Manhattan, Kansas

Perfect. Thanks for the clarification! Thanks to dstrength for the help too. I'm out of town on business right now but I will assemble the module next week and report back. Very Happy

negativspace · Joined: Dec 25, 2010 Posts: 15 Location: Manhattan, Kansas

This seems to do the trick. It absolutely kills the bleedthrough and seems to leave the low end intact at high resonance. The resonance control response is altered slightly - self-osc seems to come in differently - but it's neither more- or less-usable... just different. It still sings beautifully in self-oscillation and tracks my entire 5-octave keyboard. Very Happy

Thanks again.

yusynth · Joined: Nov 24, 2005 Posts: 1156 Location: France

Sebo · Joined: Apr 27, 2007 Posts: 517 Location: Argentina

Hi:
I did the fix and now the bleedthrough is gone.
Thank you Yves!!!

yusynth · Joined: Nov 24, 2005 Posts: 1156 Location: France

Thanks for the feedback !

I will publish this mod then Cool

djfill · Joined: Jan 10, 2012 Posts: 23 Location: Cardiff, Wales, UK

Hi Guys...

I think I have the 'Bleedthrough' issue and just wanted to confim the symptoms with someone before I try this fix.

When I set the frequency to 0 and increase the reasonance, i can still hear the waveform slightly. Is this bleedthrough? (The answer seems obvious when i put it like this but I just want clarification before i start making a mess of my board.)

Thanks.

yusynth · Joined: Nov 24, 2005 Posts: 1156 Location: France

djfill · Joined: Jan 10, 2012 Posts: 23 Location: Cardiff, Wales, UK