[sdiy] Polymoog Resonator - no-pots equivalent

Tom Wiltshire tom at electricdruid.net
Sun Aug 27 16:09:29 CEST 2017


I’d build a few SVF filters to play with. You can adjust what frequency range they cover easily enough with the CV mixers and by experimenting you might find ranges which you think are reasonable. You’d also find out how much of a problem the overlapping ranges are - how bad *is* the mayhem exactly?! It might be fairly tame.
I think you’ll get closer to “perfect” with an iterative approach (build a prototype, use it, experiment with it, modify it/build another prototype, etc) than you will by thinking about it more. For me, prototyping always throws up stuff I hadn’t thought of, or hadn’t considered important, and often things I thought were a big deal turn out not to be that important after all.

T.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 27 Aug 2017, at 14:07, sleepy_dog at gmx.de wrote:
> 
> > "Why worry so much about “mayhem..."
> 
> Well. I do remember the Yusynth guy write about the uncontrolled resonance of the original Steiner VCF and how it blew tweeters (and maybe ears).
> That kinda stuck in my mind :-D
> Generally speaking, perhaps it would be smart to have some sort of limiting *somewhere* in the signal chain of a synth, but so far, I have not examined a whole "synth as a product" schematic in that regard. (I only looked at whole synth schems which were commercial ones from the 1980's - oh boy are those drawings a MESS, as the gold standard apparently. Probably needs wallpaper to print them on.)
> 
> Thanks for the advice. Perhaps also I should already build one of those damn things & play around with it, and worry about making the perfect device later.
> 
> 
> Tom Wiltshire wrote:
>> Why worry so much about “mayhem will ensue”? What’s the worst that can happen? You get a resonant howl and some distortion, until you turn it down. No-ones head or circuit is going to blow up from that. I think we’ve all had something run away into a howl of feedback many times.
>> 
>> If I were you, I’d make the ranges fairly wide and hence more versatile, and I wouldn’t worry about them overlapping. E.g. Allow the user access to some settings that produce “non-recommended” results.
>> 
>> It’s also worth pointing out that the example you’ve chosen is a unusual case. Although there might be five formants specified for those vowel sounds, as you’ve pointed out the Moog Resonator did a reasonable job with only three, and anyone who’s heard a well-played wah pedal knows you can get an identifiably vocal character with only a single peak. So I wouldn’t worry too much about the detail of that part either, at least for voices. Perhaps if you can, find some information about the main resonances of some other instruments like trumpets, clarinets, and so on.  Those might give you other ideas about what ranges you want to cover.
>> 
>> HTH,
>> Tom
>> 
>> ==================
>>        Electric Druid
>> Synth & Stompbox DIY
>> ==================
>> 
>>> On 27 Aug 2017, at 11:35, sleepy_dog at gmx.de wrote:
>>> 
>>> 
>>> Ok, now I'm wondering about one thing, if I were to do a resonator with VCFs:
>>> 
>>> Which ranges to use for the bands?
>>> Initially, I was going to go by the polymoog's 60..300, 300..1500, 1500..7500 Hz bands.
>>> And thought I'd make the control ranges such that they have some small gaps in between, as I thought if you can make them overlap and resonance is high up, mayhem will ensue.
>>> 
>>> Now, I've looked at e.g. formant tables such as this one:
>>> https://www.cs.sfu.ca/~tamaras/project468_rdone/Formanttable.png
>>> 
>>> If I was to use the first 3 formants to produce vowels, look at "A", that's already ouf of what the polymoog ranges can do.
>>> Well in the demo it did manage to do that, so I guess they were just all shifted down, and the ratios roughly kept or something like that?
>>> But then look at other vowels in the table.
>>> I don't really or primarily want to do speech synthesis with this analog thing, but stumbling upon the table made me question my idea that I should make the ranges non-overlapping.
>>> 
>>> Now if I was to make this more versatile, what would be a good idea to do?
>>> Allow all bands to have wider, overlapping, ranges and add some sort of circuitry preventing anyone's head (or circuitry) from exploding?
>>> Or perhaps... keep it non-overlapping, but add  range switches which shift *all* band's ranges somewhat up or down?
>>> But that's less versatile again? Look at how close together are even the 4th and 5th formant to the rest, but then there's probably a reason why the Polymoog's 3 bands are space more apart with their centers.
>>> 
>>> Now, of course, I could dynamically prevent overlap in the digital control voltage generating part.
>>> But should the adjustment anywhere ever be off for whatever reason, that's not going to ensure that...
>>> 
>>> - Steve
>>> 
>>> 
>>> Am 21.08.2017 um 19:46 schrieb Tom Wiltshire:
>>>> Hi Steve,
>>>> 
>>>> Yes, three multimode filters would be the same as the PolyMoog resonator. That’s what it is:
>>>> 
>>>> http://www.synthfool.com/docs/Moog/polymoogresonator.jpg
>>>> 
>>>> Note that the schematic shows us that the filters are state-variable filters (which we’d guessed anyway because of the HP, BP, and LP outputs) and that they’re *not* VCFs. They’re just based on op-amps. You also mention the slopes. A 12dB SVF has 6dB slopes for the BP output. That’s normal, since that’s a 2-pole bandpass filter, and it’s a 2-pole filter. The way I think about it is that you need to filter off both the low and the high frequencies, so you have to share the poles out to do the job!
>>>> 
>>>> The Moog schematic is about as simple as such a thing could get, since it is pretty much just the three SVFs joined together. Personally I don’t like the way you have to change the response of all the filters together instead of individually, but that would be easy to change. Compare with this schematic:
>>>> 
>>>> http://sound.whsites.net/articles/st-var-f1.gif
>>>> 
>>>> 
>>>> Obviously you could go wild with this. Some years ago I built SVF+VCA modules around the 2164. This would be perfect for this, since you get a 2-pole VCF with VC resonance and a VCA all on the one chip.
>>>> 
>>>> http://electricdruid.net/images/svf+vca.jpg
>>>> 
>>>> I don’t seem to have the schematic online, but ask me if you’re interested.
>>>> 
>>>> HTH,
>>>> Tom
>>>> 
>>>> ==================
>>>>        Electric Druid
>>>> Synth & Stompbox DIY
>>>> ==================
>>>> 
>>>>> On 21 Aug 2017, at 17:16, Steve <sleepy_dog at gmx.de> wrote:
>>>>> 
>>>>> Hey experienced SDIYers,
>>>>> 
>>>>> say, would slapping together 3 {VCA, multi-mode VCF 12 dB/oct with Q control} combos be the functional equivalent of the Moog Resonator? (with modified VCF control ranges, limited to 1 of those 3 bands each)
>>>>> Although I faintly remember some of those multi mode VCFs having only 6 dB/oct slope on BP. Maybe not those. Are there designs with the same slope for all, or is this a general feature of such VCFs?
>>>>>  My goal is to make something that can shape sounds similarly to the Moog thing, as demo'ed on YT by some guy enhancing vocal and brass sounds.
>>>>> https://www.youtube.com/watch?v=_XUiJi5153Y
>>>>> 
>>>>> But in a much smaller box, without the battalion of pots.
>>>>> 
>>>>> I thought about just taking J.Haible's schematic and replacing all the pots with MCP41HV digipots, but that'd be 14 (some are stereo), a bit expensive *and* wiper noise, that doesn't taste well together ;)
>>>>> 
>>>>> 
>>>>> - Steve
>>>>> _______________________________________________
>>>>> Synth-diy mailing list
>>>>> Synth-diy at synth-diy.org
>>>>> http://synth-diy.org/mailman/listinfo/synth-diy
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at synth-diy.org
>>> http://synth-diy.org/mailman/listinfo/synth-diy
>> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy





More information about the Synth-diy mailing list