Mutant Vactrol Filter

[Scott Stites]

Note: This is one of those threads that kind of grew out of it's britches pretty quickly. It was originally named "Mutant Vactrol Filter", but went beyond that after a few posts. So if any of the following interest you:

Vactrols
Vactrol Filters
Vactrol Pedigrees
Phase Shifters
LEDs VS Lamps
Buchla Synthesizers
Buchla Music Easel
Don Buchla
Links to Howard's Buchla Adventures
The Amazing Charles Cohen
Tubes
Tube Phasers
Toilet Paper Tube Phasers (with optional rotating alimentary canal attachment)
LED's as Opto Control Sources
Cool Demos from the Seventies and Beyond
Nord Emulation of the Buchla Music Easel
Potted Parts
Potted Posters
Fear of High Voltage
Or Any Other manner of Man, Beast or Machine

.......then read on!
========

The past few weeks I've been noodling around with a Vactrol Filter design.

The heart of the thing is based on the topology of the Buchla Low Pass Gate, but it moved out of that timbral vicinity pretty quickly

It's been quite a fun project. I put up a page about it here:

http://mypeoplepc.com/members/scottnoanh/birthofasynth/id17.html

There are a bunch of samples, and two compositions made using it as the sole filter, "Birth of the Mutant" and "Jungle Mutant".

I put up a PDF with notes and schemos as well. Kinda big pdf, sorry about that....

Cheers,
Scott

[v-un-v]

Cheers for this Scott

As for those mp3's, I really like 'jungle'- top stuff!!

[v-un-v]

Scott, where does one get hold of the VTL5C3/2 ? I've only been able to get hold of the single version (number escapes me)

Did you do a board for this circuit, or was it another breadboard project?

[mosc]

Birth of the Mutant - a very nice track. I like it.

[Scott Stites]

Thanks Howard and V-un-V!

I've never made a board for anything - I usually use strip or protoboard for final projects (I'm way too lazy, I think). This filter is still on breadboard.

I get my VTL5C3/2's from Allied Electronics here in the states.

http://www.alliedelec.com

IIRC, Newark Electronics, which is part of Farnell, has them here in the states. They are almost as expensive as Small Bear, which has them, too. Allied is about half the price of Small Bear, and one may be better off having a number of them. I could procure some for you from Allied if you'd like.

I've seen two VTL5C3's substitued for a single VTL5C3/2, but (a) I think that's a waste of precious VTL5C3's and (b) the specs aren't quite identical - IIRC, the VTL5C3 goes lower in resistance.

The reason why having a number of them may not be a bad idea is that the VTL5C3/2 is very nice for:

Buchla Low Pass Gate (Each LPG takes a single VTL5C3/2)

Korg PS3001 Resonator - also known as a triple resonant filter - an *exceedingly* wonderful filter (takes three VTL5C3/2's). If you're not familiar with it, check out this bit about the MAM RS3, which is based upon it:

http://www.midiwall.com/gear/rs3.html

First time I heard that thing, I thought it was some really nice, exotic phase shifter. A friend of mine has one, and a Prophet 5 through it is just gorgeous.....I've built the original Korg version of it - simple mono, but, man! - that's really nice (I can find schemos for you - very easy circuit, considering what it does).

Phase Shifters - two VTL5C3/2's and a few extra components can get you a four stage shifter very easily. I've got one now on breadboard that has six VTL5C3/2's for 12 stages - I hope to make it a 24 stage mono, 12 stage stereo with rotary switch selectable number of stages. Really sweet and super easy.

There are other things, too, I'm sure.

Cheers,
Scott

[Scott Stites]

Oh, yeah, Grant Richter's #3 Standard Wogglebug uses one VTL5C3 and one VTL5C3/2:

http://www.musicsynthesizer.com/WoggleBug/WoggleBug.html

Hmm- and I'm thinking the Buchla Fluctuating Random Voltage uses the /2 as well - built it and can't remember if it's a single or a dual - dual I think.

Take care,
Scott

[Scott Stites]

Found a copy of the 12 stage VTL5C3/2 phase shifter here - it's divided into two six stage channels (right and left) and modulated with (IIRC) a triangle and pulse wave mixed together on one channel and the inverted copy on the other channel. So, it's a bit atypical of your normal phase shifter sample (which is usually modulated by sine or triangle). Kind of a crappy quality recording, and it's just a noodle, but shows how nice a VTL5C3/2 works for phase shifting. Not multi-tracked - just the DW6000 through the phase shifter.

OK, I'll shut up

Cheers,
Scott

[Scott Stites]

Ah, dang, here's another one that's a little more typical of phase shifting (though it's still modulated in the same manner). IIRC, I recorded the 10th tap in one channel and the 12th tap in the other on this recording for a stereo effect. This sample demonstrates some of the neat acoustical curves of the VTL5C3 series. Same setup (and crappy record quality - pops in there from my stoopit computer when transferring from the D8. My computer is not....modern).

Cheers,
Scott

[v-un-v]

Oh I do love a good phaser!

My favourite phaser must be the Shulte Compact Phasing A unit. Again it's an ageing vactrol design. I've also got a couple of the original Guyatone phaser pedals. They have a really searing Autobahn-esq phasing sound a bit like the Compact A (but of course the Compact A was the device used on that LP and other artists too).

Here's the original schematic of the Shulte design for anyone who fancies building one Do report back and give us all a shout with your experiences if any one out there puts one together!

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[Scott Stites]

!!!!! Whoa!!!! Thank you!!!! I've been looking for that for months now!!!!

Tangerine Dream phaser, I believe, for one. I've read about this one, and I was wanting to incorporate a particular trick they did into my phaser- (quoting Sean Costello here) "8 phase shift stages, but with negative feedback around the first 2 stages only with a simple R/C lowpass filter in the feedback loop, causing a pronounced single resonant peak in the output signal."

I've gotten some really great results putting the regen negative around the first couple of stages, but I've never known what the time constant was for the low pass filter.

I believe Juergen Haible cloned one of these.....

Cheers to the max!
Scott

[Scott Stites]

(Sigh) This got me to thinking about that phase shifter project, which is still on breadboard. Pulled out a back-up of samples, and realized I sure would like to finish it up.

It started out with playing with the LFO circuit of the STD-1m and the MN3007, then I decided to add theta processing, then got into the phase shifter, which became its own project, then something else came along. Can't get back to it before I finish this sequencer project, though, so it'll be a while.

Still, think I'll add a page about it. One very interesting thing about this phase shifter was an idea Joe Kramer had - what would it sound like in stereo with a multi-stage phase shifter if one tapped from the different outputs in the same chain? One of those two samples above touches on that, but I've found one I did where it really was a very interesting effect (swept like a standard phase shifter). The stereo image swirls in an interesting manner.

Towards the end of it, I also got into selecting an odd and even insertion stage for the resonance, which was quite interesting. I got so far with the project that I had the rotary switches (four of them) for selecting output and regeneration taps, and had all of the other parts. Then whatever it was came along and.....

Guess I've hijacked my own thread here.

Cheers,
Scott

[toppobrillo]

hey scott, you ever used or thought about using digipots? i think they are relatively new thing, but when i first noticed them in a new mouser catalog i got in the mail [i always look almost all the way thru them for new things] i was really buzzing with ideas..

the 'controllable spread' i was talking about in the thread about the synare circuit could easily be implemented this way. now they come in 1's, 2's and even 6x packages of different values ,they are made with as little as like 32 taps, or as much as 1024, volatile or non-volatile, in low or 'high' voltage varieties from a few different manufacturers. ..whew..

anyway, sorry if i am gettting off the topic, but the phaser talk made me think of it. maxim i think has a DIP [not that SOIC is impossible, just a bitch to breadboard] 6x digipot in 10, 50 or 100k with the 'low' terminals of 3 of the pots at a common pin. since they are all going to ground anyways, this would be an ideal app forit, and, in fact, i think they show it as an app of it!

so yeah, and obviously, many other different [exotic even] filter configurations could be made easily controllable in this way troo. exciting stuff. i ordered some of these from maxim and AD too.

[bugbrand]

Yes Phasing is indeed DOPEST.!.

Hey, Tom, I may well be interested in combining an order with you to get some of these Vactrols. Still haven't found anywhere in the UK with them..

'cos they're always quite expensive, I wondered if anyone has tried DIY ones? You know, glue an LED and LDR together, paint it black... I know that their response would be different from the proper ones, but if you made a batch of, say, 100 with exactly the same components then you'd have a stable reference to start with and I reckon the cost would be a fraction of the proper ones. Guessing there's a LOT of scope for variations using different LDRs..

Ooh, yes, those digi pots do sound interesting.. Hey, something I've been doing recently is requesting Analog Devices free samples - you can def. get some of their digipots to try and I'm awaiting some matched transistors, analogue switches etc from them. Previously got an SSM2164 quad VCA from them and am just in the process of trying to make a module from it using the Log/Lin converters from a MFOS VCA.

Anyone know any other good places for free chip samples?

(shame you can't get AD633s for ringmods for free from Analog Devices..!..)

[Macaba]

[bugbrand]

Not meant to be sarcasm, no.!. I'm sure the AD633 wasn't available for sample when I checked recently, but I'd be happy if I'm wrong.!.


EDIT::: YES, I've checked & I was wrong.!. ACE.

[Uncle Krunkus]

Very nice samples you posted here Scott.
What schem would you recommend as a first phaser project?

[v-un-v]

[blue hell]

[v-un-v]

[Scott Stites]

AD633 samples. Yes. I've also got them. Another interesting IC I've obtained (but have yet put it to the test), is the MLT04. Four multipliers on one IC. The divisor on them is 2.5 instead of 10 on the AD633, may not be as accurate, not sure if they work well in audio domain (I think they would), but basically it's four AD ring modulators on one IC. I wonder how well they would work for a more ambitious project such as a Bode style frequency shifter?

Digipots sound like a fun thing to try out. My experience with them has been at my work (never extended into my DIY domain). I'm not familiar with the newer products, though I think I may have some flyers around here from Maxim. The one thing that may concern me about them (and I have no idea if this is a concern, not really trying them out in this app) is that they may have a bit of quantization or steppiness to their sweep?

There are several reasons why I use Vactrols, particularly the 5C3 series - one is that they are quite easy to work with. Another reason is that they are extremely quiet. Yet another, however, is the main reason:

The VTL5C3 (and VTL5C3/2) is what I refer to as the 'golden' Vactrol because of its response characteristics - I.E., the curve of its response when transitioning from dark to light and light to dark resistance. This curve, I believe through sheer serendipity, closely mimics the percussive envelope of certain natural objects. This response molds sharp transients into very 'organic' type responses. The response can range from a malleting sound, a 'thwipping' sound, or a plucking sound, depending on what's going on at the time. Interestingly, this response adapts to how *hard* or *soft* the transient is (in terms of amplitude) just as a natural object such as a string or drumhead would react when being plucked or struck softly or with more intensity.

Other Vactrols do not possess this particular response, though their response may be good for other jobs, such as compressors. limiters, AGC's. I've got a number of VTL5C2's (for the longest time, BGMicro was selling them for 50 cents apiece - apparently they're gone) and these are a much slower Vactrol. I tried them in a 10 stage phase shifter long ago, they worked, but the response was very lazy.

A very large part of the charm of Buchla modules is that Don Buchla often used Vactrols as the control element in them. The 292 Low Pass Gate and 291 Band Pass filter each possess these 'acoustic' qualities as a result. I think Don used something other than the 5C3 in the early days (probably didn't exist), but all of the modern equivalents available from other module suppliers use them. I imagine they're used in the Buchla 200e, judging by the sound of things. I also believe Don's control circuits enhanced the acoustic response to some degree. Someone who would know a lot more about that kind of thing would probably be someone who used to work at Buchla. Hmmm.....wonder if there's anybody like that around here.....

I've found there are some caveats with working with Vactrols. First is, they *are* slow compared to semiconductor devices, and they, by their very nature, are not as accurate as any semiconductor device would be. The Mutant Filter has a V/Oct input - tracking from a low note to a high note will always have a bit of filter slew to it. It can approximate V/Oct response well enough, but will never be 'spot on' across any wide range. You will never get an instantaneous sharp attack from a 5C3, it will always impose that bit of grooviness to it. As I understand it, Hammamatsu (sp?) used to make extremely fast optocouplers that were 'invisible' to the ear. Perhaps some of the current Silonex parts are as well.

5C3's have that great organic sound, which I love, but you are, to a certain degree, locked into that organic sound. Every time I fire up the LPG or 291 adaptations I've built, I end up doing something more in the ball park of Morton Subotnick (not that I could even approximate his great work) than, say, Tangerine Dream (again, I'm not saying I come close to that either, but I think you understand what I mean). I, by all means, recommend Vactrol devices, but I also recommend having non-Vactrol devices if one wants a modular that can cover both the "East Coast" and "West Coast" sounds.

Phase shifters are a bit less critical - usually they're swept and the Vactrol's response serves to limit excessive sweep at higher frequencies - a great little feature of them, actually. And they're quiet, very quiet. And easy. A lot of manufacturer's used LDRs in phase shifters - Moog, Roland, Schulte (Thanks again Tom!!!), and Mutron, to name just a few. One area where a FET, OTA, Diode or other device (digipot?) based phaser will work better is if you're controlling it with sharp transients, and you want instantaneous response - for example, when using it with a S&H and you want that crisp steppiness in the response. While on the subject of phase shifters, as a side note, I think a phase shifter is every bit as valid as a VCF as Low Pass, Band Pass, or High Pass - it's simply an All Pass filter. If you use one in that capacity, quite often people will ask what the hell filter did you use? I have the same feelings about flangers, too, BTW.

Rolling your own is certainly something you can do - you can hand select the response you want. There are a number of methods for putting them together.

Motohiko Takeda (great guy, BTW) has this info:

http://www.netsurfer.aleph.co.jp/~takeda/radio/phaser/indexE.html

Grant Richter, prince of Vactrol control, wrote this great piece:

http://www.musicsynthesizer.com/Hairball/hairball.html

Some clown by the name of Scott Stites put this on the Experimentalists Anonymous board:

http://experimentalistsanonymous.com/board/post-874.html&sid=ccd5584d2431a8fe20873ca52a376625

Uncle K - I'll dig up what I have on the Vactrol Phase Shifter. Really, I just took the normal op amp All Pass Filter configuration and put in a Vactrol for R to ground. I used (IIRC ) 27n caps (I'll check). Just chain the stages together for more notches. I mixed the output back into the input quite simply for regeneration.

For what sounds to me to be a great OTA phase shifter, check out the Bergfotron VCP here:

http://hem.bredband.net/bersyn/

But, then again, I think Jan's solution is brilliant!!

Cheers,
Scott

[mosc]

[Scott Stites]

Thanks for that insight, Howard! From what I've read, Don has very specific ideas of what electronic music is, and his instruments are structured specifically based on those ideas. From what I gather, he certainly is not interested in synthesizing existing instruments (IIRC, he even has a bit of disdain for the term 'synthesizer'). Everything from the control surface to what the modules do is out of phase with just about every other manufacturer. Serge sort of evolved from that whole scene, didn't it? Serge is a bit different philosophically, kind of a bridge between the Moog and Buchla paradigms. Serge is probably still closer to the Buchla school of thought, though the control and sound structure is not separated as it is with Buchla. Grant Richter's Wiard approach pays particular homage to Buchla/Serge methodology.

I took a look at some of the Buchla schematics I have here. The 200 series LPG rev B used VTL2C2/2, the rev C schematic I have doesn't name the Vactrol. I can't find a datasheet for the 2C2's. Seems to me the 100 series used either lamps and LDR's or devices that had lamps rather than diodes in them (LED's were probably a bit hard to come by in the mid to late sixties ).

Noodles is a pretty good descriptor. I've seen 'Bug Music' used to describe Subotnick's style. I don't know if he coined it, but I think I first saw that term used by Paul Schreiber of MOTM.

Cheers,
Scott

[v-un-v]

[v-un-v]

[v-un-v]