CV vs Midi Exp Pedal to Control Filter Cutoff

CV vs Midi Exp Pedal to Control Filter Cutoff
« on: October 21, 2023, 11:18:11 AM »
Background to set up my question: My first synth was a Prophet 5, purchased in 1981 (I donít even know which Rev it was). I discovered that for my playing style, a filter expression pedal was a game-changer, and I used the pedal all the time. Iím pretty sure the pedal I used at the time was CV. When I leaned into that pedal the synth would growl and was so expressive, just as if I was turning the knob on the panel. In 1983 or so I bought an OB-8 and I used a pedal in the same way and I had the same experience, where leaning into the pedal was just as if I was turning the knob on the panel. However, there was one problem with the OB-8/pedal Ė just plugging the pedal into the OB-8 overrode the filter cutoff setting for each patch. Fortunately, I found someone to perform a mod to the OB-8 by putting a little switch on top that would engage or bypass the pedal, so I could switch modes easily.

Over the years my life took me in other directions and I sold off my gear. Over the last few years, I am getting back into synthesizers and I now own a Prophet 10 Rev 4 and OB-X8. I use an expression pedal to drive the filter cutoff on each but it is disappointing, it is just not the same as my original experience. I canít get that ballsy growl, just a clinical sound that is a little steppy. It seems as if the pedal input is getting converted to MIDI or something inside the synth.

Thanks for reading this far Ė hereís my question: Is it possible to accomplish a true CV connection to the filter cutoff like my original synths? Or has anyone found another solution?

(Iíve posted this to both the Prophet 10 and OB-X8 boards.)
Prophet 10 Rev4, OB-X8, Moog One, Rhodes Mk8, Osmose, OB-6 desktop, Trigon 6 Desktop, Kawai VPC-1, Steinway D

chysn

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Re: CV vs Midi Exp Pedal to Control Filter Cutoff
« Reply #1 on: October 21, 2023, 03:10:21 PM »
DSI/Sequential pedal CV inputs are slewed to stabilize response for people playing with (notoriously unsteady) feet. So if you're hooking up other CV sources, the response will be slower than you probably expect.
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Re: CV vs Midi Exp Pedal to Control Filter Cutoff
« Reply #2 on: October 23, 2023, 07:28:27 AM »
DSI/Sequential pedal CV inputs are slewed to stabilize response for people playing with (notoriously unsteady) feet. So if you're hooking up other CV sources, the response will be slower than you probably expect.

Thank you, Chysn - this is interesting. I had assumed that the reason my filter pedal exp doesn't sound right is that the voltage is being converted to 127 step MIDI-like signal, with less resolution than continuous CV, and that the solution would be to bypass this conversion somehow and get CV directly to the filter. Now I'm wondering if the "slew" you describe is contributing to that problem, or possibly the sole cause? I guess my end-question is whether I have any options to get the sound I want. (Full disclosure, I know very little about how electronics work.)
Prophet 10 Rev4, OB-X8, Moog One, Rhodes Mk8, Osmose, OB-6 desktop, Trigon 6 Desktop, Kawai VPC-1, Steinway D