VCF with voltage controlled resonance?

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

I'm working on a poly synth based on Yusynth modules and want to have one VCF per voice. Problem is I want to have one resonance pot to control all the VCF's, but the resonance isn't voltage controlled.

I'm looking at the minimoog VCF at the moment, but all the VCF's I've looked at seem to have the same issue with resonance.

I was thinking about replacing the resonance pot in the circuit with an OTA based VCA. I was wondering if anyone has tried this and whether or not it was successful, or if there are any other options that I should consider?

The LM13700 data sheet has an example of using it as a floating voltage controlled variable resistor, would that be a better option?

minisystem · Joined: Nov 16, 2012 Posts: 25 Location: Toronto

I pm'ed Yves about this a couple of months ago as I wanted to do the same for his 4072 Arp VCF. He more or less suggested what you are proposing: put the resonance under VCA control. Yves' diode filter has VC resonance. I also had a look at the MFOS 24db/oct VCF which has VC resonance: http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=VCFJAN2006/VCFJAN2006.html&VPW=1339&VPH=695

Someone who knows what they're doing (ie. not me!) could gain some insight from those designs I suspect. Wink

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

Cool, thanks! I got a pretty good idea what's going on in the MFOS design, but I don't know why there's a gain of 4 before the divider on the OTA input. You would think you could just adjust the divider ratio scratch

The Yusynth ARP (edit: meant diode ladder) filter it looks like he's using a transistor to shunt the feedback to ground, that looks much simpler.

kkissinger · Posted: Thu Mar 07, 2013 9:00 am Post subject:

You might take a look at the schematic for the Yusynth Diode Ladder Filter:

http://yusynth.net/Modular/index_en.html

He uses an FET to implement voltage-controlled resonance.

Another approach would be to use vactrols in the feedback/resonance loop (although I've never tried it with vactrols but no reason why it wouldn't work).

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

Oops, I said ARP filter in my previous post but meant the diode filter. The question is, do I do a diode filter instead of the Moog filter, or try to adapt the FET method to the Moog filter? I'm leaning towards the the Moog filter, but don't know if my analog circuit skills are quite up to the task of adapting the resonance control.

Vactrols aren't a bad idea either, that would be pretty simple to implement. Only problem is I prefer to avoid them as they're not as easy to source as I'd like and a touch expensive. Would much rather keep everything on Mouser if I can.

Thanks for the input!

kkissinger · Posted: Thu Mar 07, 2013 1:40 pm Post subject:

I have an idea that might work for you -- but I want to patch it up to test it out. I will get back to you later this evening.

minisystem · Joined: Nov 16, 2012 Posts: 25 Location: Toronto

yusynth · Joined: Nov 24, 2005 Posts: 1165 Location: France

Hi , yes the EMS resonance control can be directly adapted to the Moog filter.

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

Well, I took a stab at this with a JFET (2n5484) and 22uF cap I had handy. I removed the resonance pot and shorted the two leads together, attached the cap to that and the other end of the cap to the drain of the JFET. Source of JFET to ground.

I was able to get some semblance of control but not entirely satisfactory. With the gate at 0 volts (JFET on, should be no resonance) I still got some resonance. With the gate at -15V (actually should be off around -3V so -15V should be more than enough) I got more resonance but couldn't get to self oscillation (even with the trimmer maxed out).

So I dunno if the 2n5484 is not appropriate or if there's some other circuitry needed to address these issues or what. There is some other stuff in the EMS circuit that I couldn't really follow.

Something else that occurred to me, when the JFET is on (0 resonance), wouldn't it have a negative side effect of bleeding off some of the output?

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

I played around with it a little more and can get self oscillation without a problem now , so the off state of the JFET seems to be working.

I checked the on resistance of the 2n5484 and it was nearly 300 ohms which explains why I couldn't get the resonance all the way "off". I swapped it out for a j112 which has an on resistance of 30 ohms and works quite a bit better. I still feel like there's a little more resonance than I'd like though.

gdavis · Joined: Feb 27, 2013 Posts: 27 Location: San Diego

Well I downloaded LTSpice to experiment some more (haven't used spice in quite a while).

Looks like the 20uF cap is too small and makes a good amount of low end roll off, a 220uF seems to work better (500uF is even better but might be getting a little out of hand practically speaking). Adjusting R29 from 180 to 140 and adding a 1k resistor where the emphasis pot would go seems to help, though it looks like it adds a little overall gain which can probably be compensated by the output buffer if needed.

Looks pretty close in the simulation. I didn't test all these changes on my protoype but as close as I'm getting, I'm not sure I'd notice the difference if I wasn't doing a side by side comparison.

However, I also simulated a solution using an OTA as a VCA to control the feedback. This really flattens out the response below the cuttoff when resonance is off, and provides a nice peak as it's turned on.

I'll have to try a prototype, but at this point I think I'm going to go with the OTA solution, though I'm a little irked at the prospect of only using one half of a LM13700.

If you're not too picky, the JFET solution works well and is pretty simple.

minisystem · Joined: Nov 16, 2012 Posts: 25 Location: Toronto

Sounds promising. Looking forward to hearing how you sort it out. I eventually want to implement CV resonance for Yves' 4072 filter, but it has been pushed onto the back burner for now.

minisystem · Joined: Nov 16, 2012 Posts: 25 Location: Toronto

I took a cue from gdavis and started doing some LTSpice simulation of the 4072 filter to try to work out voltage controlled resonance. With almost no analog design skills to speak of, I worked from the examples of others: Starting with gdavis' LM13700 solution, I went through several iterations and finally arrived at the MFOS LM13700-based VCA (just the VCA cell, no CV scaling has been implemented yet). This seems to do the trick (at least in simulation). Next step, I guess, is to breadboard it and see how it pans out in reality...