CV vs Midi Exp Pedal to Control Filter Cutoff

[strytten]

Background to set up my question: My first synth was a Prophet 5, purchased in 1981 (I don’t 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. I’m 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 – 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 can’t 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 – here’s 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?

(I’ve posted this to both the Prophet 10 and OB-X8 boards.)

[Beaubourg]

I don't know what you mean by the filter pedal input being stepped? I really can't hear that. You sure you don't have the resonance cranked? And I don't get how any characteristic of the pedal would make it growl more? I have a pedal connected to the filter at all times, I agree, it is a really great way of expression. Did you try adjusting the range? There's a knob for that on my pedal that I have set about half way, ymmv.

[strytten]

Thank you Beaubourg for weighing in. Here is what I mean. If I begin with the pedal all the way back, and slowly move it forward to gradually increase the filter cutoff frequency, I can hear it increase in steps, instead of in a smooth manner. I notice this the most on my Prophet 10. I'll check my OB-X8 again this weekend to make sure my post was accurate as to the OB-X8.

I can see why my comment about "growl" might be confusing. A better way of describing it was that if I wanted to increase the filter cutoff in an aggressive way, the filter made a very aggressive sound (that's a good thing). Now, no matter how aggressive I am with the pedal, the filter just sounds toned down and smoothed out.

 I had assumed that the "steppi-ness" resulted from an internal conversion of the CV to MIDI before it reached the filter, but Chsn commented on the Prophet 10 chat group that Sequential adds a "slew" to the filter pedal input on all its products to help smooth out the sound and reduce the sensitivity to small foot movements. I don't know enough about electronics to understand this, but maybe this is what I'm hearing?

[Beaubourg]

I think the “slew” is there to smooth out shaky feet, it would rather smooth out the response. When you check, make sure to turn down any resonance in the filter. When turned up, the resonance will accentuate the overtones according to the harmonic series as you sweep past those frequencies of the fundamental. That could appear as steppiness.

[strytten]

Thank you Beaubourg for your suggestions. I had a chance to spend some time with this today. I am using a Moog EP-3 pedal* with the OB-X8 and I compared doing a slow sweep with the pedal to doing the slow sweep with the knob on the panel. As you suggested, I made sure that resonance was turned down to minimum setting. I confirmed my recollection - there is a difference. The sound of the pedal is "steppy" and the sound of the knob is smooth. On a scale of 1-10, the difference is about a 3 - it is enough to notice but not huge. The pedal is also less responsive: If I want to lean into it and make an aggressive, quick swell, there is some lag that is not an issue with the knob. (BTW these differences are more pronounced with the Prophet 10 Rev 4.)

I would love to find an alternative to the current pedal behavior that avoids these issues, either routing CV directly to the filter or maybe there are better ideas out there?

* The Moog EP-3 pedal has an adjustment knob on it that can be used to make the sweep range of the pedal narrower than the broadest setting. I experimented with different settings, but they do not seem to be causing these issues. I ultimately returned the pedal to its broadest setting.

[Beaubourg]

I have the same pedal. Perhaps your old pedal wasn’t linear?
I still can’t hear that stepping you’re talking about. Not trying to say it’s not there, but I can’t hear it.

[strytten]

Thanks, BeauBourg, I appreciate your reply and your experience. It is helpful to me to know that another user has the same gear and does not hear any problem. I have several other pedals around (some Moog, some other brands), and I'll spend more time trouble-shooting this with these other pedals.

[Analog Prophet]

I use Moog EP2 to control VCF to my OB-X8. Don’t experience as steppy, and CV has no steps.

Here a clip where I use the EP2 pedal to control the VCF and the filter movements sounds non-steppy in my ears. It’s not growling sounds but ambient pads:

https://youtu.be/TQrOctMS48Y?si=BfUyv5ELFGCdMKEy

[strytten]

Thank you Analog Prophet for sharing your experience and YouTube post. It is very helpful to me to hear the perspective of other musicians. First of all, I love the patches and performance on your YouTube post - bravo! I particularly like the ambience added by your assistant on the floor. The reverb is beautiful, and I realize that listening to your performance won't be apples to apples to what I've been doing without reverb. As best I can tell, when you increase the pedal to make the filter brighter, it seems to sound the same as mine. In other words, to my ear I hear a slight amount of "steppy-ness." It is like the pedal gets to a certain point and the brightness goes one stair up the staircase instead of using the ramp. In my case, this issue isn't a big deal as it still sounds very good - but doesn't have the magic I remember from my original OB-8 and its expression pedal. (However, I think this is more noticeable and more bothersome with my Prophet 10 Rev 4 compared to my original Prophet 10 and expression pedal.) Anyhow, I really appreciate your YouTube post and your taking the time to weigh in here on the forum. I just subscribed and will look forward to future YouTube posts!

[thomasfrank]

I use Moog EP2 to control VCF to my OB-X8. Don’t experience as steppy, and CV has no steps.

Here a clip where I use the EP2 pedal to control the VCF and the filter movements sounds non-steppy in my ears. It’s not growling sounds but ambient pads:

https://youtu.be/TQrOctMS48Y?si=BfUyv5ELFGCdMKEy fnf

I watched this clip. The filter movements are smooth and seamless.