Which paraphonic synths have self-oscillating filters

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Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Thu Aug 06, 2015 12:21 am

...besides the Mono/Poly? How well do they track the kbd and how or if at all do you use it?

I'm thinking of modding my Juno 106 using the last remaining filter chip so I'd like to know this.

There's still only one VCA in the Mono/Poly for all 4 voices in "polyphonic" mode, so it still goes after the filter. The tracking then follows last note priority I believe.

I recall the EX-800 not being able to self-oscillate. How does it behave with the Moogslayer mod?

What about some of the Crumar stringers?

Can the Polymoog filter be set to self-oscillate? Does every synth voice having its own VCA (thus they must be placed before the filter) affect the tracking? Resonance is just positive feedback, right? Sp it might be unstable when the level of the input signal is not constant. However when set into self-oscillation, the filter should be on its own without issues.
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Re: Which paraphonic synths have self-oscillating filters

Post by commodorejohn » Thu Aug 06, 2015 2:52 am

The Moog Opus-3 does, though there's no key-tracking. The Crumar Orchestrator/Multiman S does as well (but again, no key-tracking.)
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Re: Which paraphonic synths have self-oscillating filters

Post by HideawayStudio » Thu Aug 06, 2015 10:16 am

Mooger5 wrote:Can the Polymoog filter be set to self-oscillate? Does every synth voice having its own VCA (thus they must be placed before the filter) affect the tracking? Resonance is just positive feedback, right? Sp it might be unstable when the level of the input signal is not constant. However when set into self-oscillation, the filter should be on its own without issues.
The Polymoog is unusual in that it has a basic polyphonic low pass filter and VCA on every key without resonance and a paraphonic Moog ladder resonant filter bolted on the back end of the synth. The paraphonic filter can be setup to self-oscillate and is fed with a tracking signal derived from the monophonic synth CV sections for external synth control but generally its not used this way and I'm not sure it would translate well to a polyphonic synth.

Resonance (or regeneration) is indeed positive feedback but in the case of filters its very dependent on the phase characteristics of the filter about the cutoff. Some filters work well with regen about this point and others don't. In most cases some sort of taming is involved to avoid wild self-oscillation often using clipping diodes. I mention this because you can't create a regen path on any old filter and sound like you'd expect a resonant filter to sound. Some filters, eg the Curtis filters in the SCI Six-Trak or the later filters in the APR2600 are so nicely setup in this department that they can consistently be used across several octaves as a sinewave oscillator.

One synth I can think of with a paraphonic filter that can be tweaked into self oscillation is the little SIEL EX-80.

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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Thu Aug 06, 2015 10:37 am

commodorejohn wrote:The Moog Opus-3 does, though there's no key-tracking. The Crumar Orchestrator/Multiman S does as well (but again, no key-tracking.)
Thanks. It looks like when there's key-tracking available on a paraphonic filter there will be no self-oscillation allowed. And when it is, it won't track the keyboard. It has to do voice assignment, I think. If every voice would send Fc to the filter based on which note on the kbd, the voltage buildup when playing chords would result in a mess, I believe.
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Re: Which paraphonic synths have self-oscillating filters

Post by adamstan » Thu Aug 06, 2015 11:06 am

It rather depends on VCA arrangement. Mono/Poly is completely paraphonic - all voices share common VCF and VCA, so the filter can be before VCA. OTOH - in Polymoog each voice has its own VCA, and then the sum is fed into master VCF. It cannot be allowed to self-oscillate, because then it would drone/whistle all the time, even with no keys pressed, as there is no VCA after it.
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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Thu Aug 06, 2015 11:10 am

HideawayStudio wrote:
Mooger5 wrote:Can the Polymoog filter be set to self-oscillate? Does every synth voice having its own VCA (thus they must be placed before the filter) affect the tracking? Resonance is just positive feedback, right? Sp it might be unstable when the level of the input signal is not constant. However when set into self-oscillation, the filter should be on its own without issues.
The Polymoog is unusual in that it has a basic polyphonic low pass filter and VCA on every key without resonance and a paraphonic Moog ladder resonant filter bolted on the back end of the synth. The paraphonic filter can be setup to self-oscillate and is fed with a tracking signal derived from the monophonic synth CV sections for external synth control but generally its not used this way and I'm not sure it would translate well to a polyphonic synth.

Resonance (or regeneration) is indeed positive feedback but in the case of filters its very dependent on the phase characteristics of the filter about the cutoff. Some filters work well with regen about this point and others don't. In most cases some sort of taming is involved to avoid wild self-oscillation often using clipping diodes. I mention this because you can't create a regen path on any old filter and sound like you'd expect a resonant filter to sound. Some filters, eg the Curtis filters in the SCI Six-Trak or the later filters in the APR2600 are so nicely setup in this department that they can consistently be used across several octaves as a sinewave oscillator.

One synth I can think of with a paraphonic filter that can be tweaked into self oscillation is the little SIEL EX-80.
Thanks. There might be some confusion. The filter already exists: it's the last good 80017a chip I own, that I have recently found. I'm not gonna buy 5 more refurbished chips or clones for the Juno. Previously I had used SSM2044-based cards but those chips went for a battered Polysix. With only one 80017a left I'm thinking about the following:

In Poly 2 mode the first voice played is always voice nr 1. If all the voices including that one go through their own VCAs, then mixed into the single filter, and with the filter assigned to voice nr1 only, the result will be single-triggering paraphony. Only voice nr1 will send Fc voltage (key tracking plus EG and LFO). The problem happens when the first key pressed is suddenly lifted while playing a chord: if there's no long release all the other voices will shut. Not much of an issue with a suitable playing technique. The problem can be overcome by having all voices send Fc to the VCF, but I suspect the summed voltages won't result in a musical way particularly when the filter resonance is turned up high.
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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Thu Aug 06, 2015 11:12 am

adamstan wrote:It cannot be allowed to self-oscillate, because then it would drone/whistle all the time, even with no keys pressed, as there is no VCA after it.
Hadn't thought of this one. Many thanks.

I could use the 80017a internal VCA for that, if a voice ->vca->vcf->vca arrangement is still sane.

Well, by turning a trimmer on the voiceboard I can turn the resonance full up without the self-oscillation. Key tracking is still useful.

Still interested to know how other synths deal with the problem specially the new paraphonics.
Last edited by Mooger5 on Thu Aug 06, 2015 11:29 am, edited 1 time in total.
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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Thu Aug 06, 2015 11:43 am

zoomtheline wrote:You know what, I have never tried my stratus but I imagine it will as the res screams. Just never used it to any degree like that. Which also means the trilogy will too.

But don't get me started into whether it's paraphonic or not, it's it's own beast. It has more than one VCF and VCA but they are clustered around certain keys so it's a weird setup. Others more tech than I could maybe explain it.

Mine has one Filter env chip down so all, say, A and E keys do not trigger the filter env the same as the rest of the keys.
Interesting. So got they got around it by distributing several filters across the keyboard? Spaced by a fifth or something? Probably they assigned a filter and EG to a group of dividers so every A and E and their octaves share the same filter. That's clever. Thanks!
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Re: Which paraphonic synths have self-oscillating filters

Post by Push-Pull » Fri Aug 07, 2015 8:17 am

Which paraphonic synths have self-oscillating filters
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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Fri Aug 07, 2015 10:08 am

Push-Pull wrote:
Which paraphonic synths have self-oscillating filters
The Korg Delta...
Thanks. But isn't that only after tweaking, if I recall? How well does it track the keyboard?
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Re: Which paraphonic synths have self-oscillating filters

Post by sam » Fri Aug 07, 2015 6:17 pm

The roland rs505....sure it self oscillates.
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Re: Which paraphonic synths have self-oscillating filters

Post by pricklyrobot » Fri Aug 07, 2015 9:51 pm

HideawayStudio wrote:Some filters, eg the Curtis filters in the SCI Six-Trak or the later filters in the APR2600 are so nicely setup in this department that they can consistently be used across several octaves as a sinewave oscillator.
I remember my old Prophet 600 working pretty well in this capacity.
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Re: Which paraphonic synths have self-oscillating filters

Post by celebutante » Sun Aug 09, 2015 7:48 am

My Delta doesn't self-oscillate. I seem to recall that you can tweak them to, but that it immensely shortens the life of the filter (I can't verify that though).
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Re: Which paraphonic synths have self-oscillating filters

Post by Push-Pull » Sun Aug 09, 2015 11:04 am

Mooger5 wrote:
Push-Pull wrote:
Which paraphonic synths have self-oscillating filters
The Korg Delta...
Thanks. But isn't that only after tweaking, if I recall? How well does it track the keyboard?
I don't remember if the tracking was really good, but you have the possibility to track:

Image

Celebutante: yes, there is a simple trimmer inside. And I can't see why it shortens the life of the filter... The only "problem" I remember is that the resonnance was not easy to adjust, because it can be suddenly hard and loud !
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Re: Which paraphonic synths have self-oscillating filters

Post by Mooger5 » Tue Aug 11, 2015 3:48 pm

Well my initial concerns regarding vcas placed before the filter were obviously unfounded as we can see in synths featuring source mixers: altering the level of the oscillators won´t affect the resonance response. Guess as long as the filter is high impedance there won´t be any trouble.

Been putting a lot of thought on this project. There´s a few limits to it, like single triggering only and first note priority. Multi-trig and last note prio would be possible by rewriting the firmware. As it is, I think it will be a worthy exercise.

Thanks all for your anwers.
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