Interesting low parts count VCF

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TedGettsterby
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Interesting low parts count VCF

Post by TedGettsterby » Tue Mar 30, 2021 2:31 pm

Just want to draw folks’ attention to this one opamp expo vcf:
https://www.elektormagazine.com/labs/an ... -one-opamp

the author mentions that it has potential polysynth applications and that they are working on a full synth voice design based on this filter

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Re: Interesting low parts count VCF

Post by guest » Tue Mar 30, 2021 2:46 pm

thanks for posting that! for anyone else looking to dig in, here is more info on it:
https://tudl1086.home.xs4all.nl/synthci ... tpvoks.htm
openmusiclabs.com

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MikeDB
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Re: Interesting low parts count VCF

Post by MikeDB » Tue Mar 30, 2021 5:33 pm

We used to do something similar with the LM346. You programmed frequency with the Set A/B/D pin, and Q with the set C pin. I expect the TS271 has a wider range though.

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Re: Interesting low parts count VCF

Post by jorg » Wed Mar 31, 2021 9:23 am

guest wrote:
Tue Mar 30, 2021 2:46 pm
thanks for posting that! for anyone else looking to dig in, here is more info on it:
https://tudl1086.home.xs4all.nl/synthci ... tpvoks.htm
Nice! I hadn't seen this chip - I thought the magic Polivoks chip was unobtainium. Mouser is stocking TS271, TS272 (dual), and TS274 (quad). All are about $0.62, quantity 1 (depending on the variant chosen).

Exciting stuff.

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devinw1
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Re: Interesting low parts count VCF

Post by devinw1 » Wed Mar 31, 2021 11:35 am

That's pretty cool. I remember reading about that old Russian op amp a long time ago. Pretty cool somebody found a modern chip that can be operated like this. That certainly is dead simple.

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Re: Interesting low parts count VCF

Post by lickspittle » Wed Mar 31, 2021 11:41 am

Paired with a simple AD envelope generator, it might make a nice, vactrol free, Low Pass Gate.

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deftinwulf
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Re: Interesting low parts count VCF

Post by deftinwulf » Wed Mar 31, 2021 11:44 am

Noob curiosity question: What is 1V/V? Does that read "1 volt per volt"? What is the significance of that specification?

Feel free to PM if this is derailing the thread.

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Re: Interesting low parts count VCF

Post by jorg » Wed Mar 31, 2021 12:07 pm

deftinwulf wrote:
Wed Mar 31, 2021 11:44 am
Noob curiosity question: What is 1V/V? Does that read "1 volt per volt"? What is the significance of that specification?

Feel free to PM if this is derailing the thread.
1 Volt out for 1 Volt in; i.e. a gain of 1.

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Re: Interesting low parts count VCF

Post by deftinwulf » Wed Mar 31, 2021 1:06 pm

jorg wrote:
Wed Mar 31, 2021 12:07 pm
1 Volt out for 1 Volt in; i.e. a gain of 1.
Cool, thank you. I'm intrigued to see what new VCF designs evolve from this discovery with the TS271!

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Re: Interesting low parts count VCF

Post by jorg » Wed Mar 31, 2021 1:48 pm

guest wrote:
Tue Mar 30, 2021 2:46 pm
thanks for posting that! for anyone else looking to dig in, here is more info on it:
https://tudl1086.home.xs4all.nl/synthci ... tpvoks.htm
Has anybody found a decent SPICE model for the TS271? The one from ST Microelectronics omits the bias input (the one thing that matters about the part!).

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Re: Interesting low parts count VCF

Post by guest » Wed Mar 31, 2021 4:09 pm

i was pretty happy to see a programmable opamp as well, i didnt know they were still a thing. unfortunately, the TS272 and TS274 are different opamps. i couldnt locate any other programmables in dual or quad. that LM346 looked like quite the chip.
openmusiclabs.com

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MikeDB
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Re: Interesting low parts count VCF

Post by MikeDB » Wed Mar 31, 2021 4:58 pm

guest wrote:
Wed Mar 31, 2021 4:09 pm
i couldnt locate any other programmables in dual or quad. that LM346 looked like quite the chip.
The LM346 has been around since the 70s and is still around on the surplus market. Problem was at low currents it distorted, and wasn't shall we say 100% stable at higher currents. The XR-095 BIFET version was a great improvement with the amps programmed in pairs, but Exar are long gone.

There's a few articles from people claiming to have 'discovered' it and are the first to use it in a synth. Sorry guys - 3 to 4 decades too late

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Re: Interesting low parts count VCF

Post by jorg » Thu Apr 01, 2021 7:46 am

guest wrote:
Wed Mar 31, 2021 4:09 pm
i was pretty happy to see a programmable opamp as well, i didnt know they were still a thing. unfortunately, the TS272 and TS274 are different opamps. i couldnt locate any other programmables in dual or quad. that LM346 looked like quite the chip.
Yeah, on closer inspection, the TS272 and TS274 omit the bias pin! They are sold in 3 different fixed-bias variants. :doh:

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Re: Interesting low parts count VCF

Post by SkyWriter » Thu Apr 01, 2021 7:54 am

There's always PAIA's old low parts count transistor designs. 1T LP and a 2T BP.

https://electro-music.com/wiki/pmwiki.p ... 0BPFAndLPF

I built these guys with TV parts when I was a kid.
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Re: Interesting low parts count VCF

Post by jorg » Thu Apr 01, 2021 8:14 am

SkyWriter wrote:
Thu Apr 01, 2021 7:54 am
There's always PAIA's old low parts count transistor designs. 1T LP and a 2T BP.

https://electro-music.com/wiki/pmwiki.p ... 0BPFAndLPF

I built these guys with TV parts when I was a kid.
Those are super cute! :love:

sublimiter
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Re: Interesting low parts count VCF

Post by sublimiter » Thu Apr 01, 2021 8:20 am

very interesting. will absolutely be adding a handful of TS271s to my next mouser order.

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KSS
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Re: Interesting low parts count VCF

Post by KSS » Sun Apr 04, 2021 7:29 pm

Surprised not see anyone mentionthe 4250 programmable OPA. Those have been doing cloning duty in polivoks filters for a long time.

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Re: Interesting low parts count VCF

Post by KSS » Sun Apr 04, 2021 7:39 pm

jorg wrote:
Thu Apr 01, 2021 8:14 am
SkyWriter wrote:
Thu Apr 01, 2021 7:54 am
There's always PAIA's old low parts count transistor designs. 1T LP and a 2T BP.

https://electro-music.com/wiki/pmwiki.p ... 0BPFAndLPF

I built these guys with TV parts when I was a kid.
Those are super cute! :love:
The BPF is worth making. Add the range switches seen in a mod article in Polyphony. <--They're now mostly all online.
But the LPF is pretty dismal. Even its designer called it more of a tone control than proper synth LPF. Try a 'similar' Steiner VCF instead. Or even better, a moog ladder. The mmg's -/+10V is close enough to PAIAs -/+9V that it just works. Once you hear PAIA's unijunction based VCOs -like a 901- through moogs 24dB LPF you'll wonder -or finally understand- why PAIA gets little love. But everyone shold read their Docs. Those are a great and under-utilized synth learning resource!
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Re: Interesting low parts count VCF

Post by guest » Sun Apr 04, 2021 7:54 pm

i tried searching for other PGAs out there, but the modern definition is a digitally switched opamp, and maybe variable gain amplifier (VGA) is the new term? the 4250 doesnt seem to be an active part, but you can get the LM359, i wonder if that would work?
openmusiclabs.com

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Re: Interesting low parts count VCF

Post by KSS » Sun Apr 04, 2021 8:48 pm

guest wrote:
Sun Apr 04, 2021 7:54 pm
but you can get the LM359, i wonder if that would work?
That's an interesting part. It's a Norton, so CDA. But it's a dual with Iset going to both amps. Certainly worth a try in a filter app.

But also for any old 3900 apps, based on the DS improvenent claims over 3900. Quad v dual but might be worth getting a few and re-visiting some old designs.

--------------------------
The 4250 is still out there if you know where to look, but i was surprised it's as far off the beaten path as it appears to be now.
Last edited by KSS on Sun Apr 04, 2021 9:10 pm, edited 1 time in total.

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Re: Interesting low parts count VCF

Post by guest » Sun Apr 04, 2021 9:08 pm

and in case anyone is keeping a list of these, the OP-32 is still available.
openmusiclabs.com

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Re: Interesting low parts count VCF

Post by nigel » Sun Apr 04, 2021 11:04 pm

guest wrote:
Sun Apr 04, 2021 7:54 pm
i tried searching for other PGAs out there, but the modern definition is a digitally switched opamp, and maybe variable gain amplifier (VGA) is the new term? the 4250 doesnt seem to be an active part, but you can get the LM359, i wonder if that would work?
I got several 4250s from Tayda a while ago to build a Polivox-ish VCF. Checking now, they are still listed, but out of stock.

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Re: Interesting low parts count VCF

Post by vladosh » Mon Apr 05, 2021 5:48 am

i got a bunch of 4250's from ali and built this vcf ,second time

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