Thomas Henry XR-2206 VCO Page is Up

Scott Stites

Finished the basic page - only have one sound sample, so more will have to be added.

http://mypeoplepc.com/members/scottnoanh/birthofasynth/id20.html

Schematics, board layout, parts list, waveform pics included.

Cheers,
Scott "Now where was I on that MultiPhase?" Stites

ian-s · Posted: Thu Jun 29, 2006 2:15 am Post subject:

Nice VCO.
The VC skew looks like cross fading with an inverted copy of the output wave but only on half cycles. An interesting idea. Smile

dnny · Posted: Thu Jun 29, 2006 5:35 am Post subject:

nice sounding VCO indeed and not too complicated - so just perfect for newbies like me

thanks Scott & Thomas

mosc · Posted: Thu Jun 29, 2006 6:36 am Post subject:

Scott, great web page... Great project...

Years ago I played around with those XRs and never had any luck with them. This control circuit apparently makes this into a good oscillator chip. Congrats to to Thomas Henry and yourself. Very Happy

Scott Stites · Posted: Thu Jun 29, 2006 9:36 am Post subject:

Thanks guys! Thomas really did all of the work, I was just along for the ride. The most I contributed were some hair-brained solutions to a few things we ran up against (solutions of which Tom would provide a far more eloquent means of implementing).

For example, in the first draft, the skew control was just a manual control. We thought it would be really great to put it under voltage control, but the means Tom had thought of first was using an OTA. He had a method devised that would use a CA3080, but he wanted to avoid discontinued parts, and a dual OTA wouldn't be efficient, leaving a 'spare' unused section. Of course, I blathered on about using that for something like dynamic depth modulation or some such rot, but that would be adding more circuitry to a design that was intended to be a very efficient, bang for the buck sort of thing. Then, out of the blue, Tom sent the draft for voltage control of the skew using a method that I would never have dreamed up.

Yep, Howard, I think he found the right balance of current through the expo pair to get the best possible V/Oct response out of the XR-2206.

It's been a real gas doing this again, and it's nice to see Thomas hasn't just walked out of the DIY Synth world - the wheels are still turning. In fact, he's been dropping some clues about his latest module foray, which sounds pretty cool.....

Take care,
Scott

bigtex · Posted: Thu Jun 29, 2006 2:33 pm Post subject:

Wow. I really like the sound of the modulated skew. I've had a 2206 sitting in my parts bin for like 6 years now. Time to make something!

Thanks!

Scott Stites · Posted: Thu Jun 29, 2006 2:36 pm Post subject:

The skew is really the best part of the VCO, I think. Thomas really liked controlling it with the CV out of a pitch bender (I don't have anything like that right now), but I can imagine it would be pretty cool.

I like using it with an EG - that can create some pretty unusual effects.

Cheers,
Scott

piedwagtail · Posted: Thu Jun 29, 2006 8:58 pm Post subject:

I can't keep up with the Stites Foundry!

It's interesting that like a great composer,one can discern a consistent continuity in Thomas's designs.Same failsafe blocks put together into a rock solid working design.Which when you hear it will not fail to bring a smile to your face.
That consistency is remarkable considering the sheer amount of published circuits one tries that never quite work.Long Live Thomas Henry!

Robert

Uncle Krunkus · Posted: Fri Jun 30, 2006 6:52 am Post subject:

Very nice work Scott (as usual)

"must... stop myself....being distracted....by Scott's....projects....ahhhh!!"

"you're a charmer Scott Teneman! Yeess!" - Cartman

Well if I finish my SoundLab, I won't feel so bad about tracking down some XR2206s, SN27466s, MN3011s etc.

Scott Stites · Posted: Fri Jun 30, 2006 9:53 am Post subject:

piedwagtail · Posted: Fri Jun 30, 2006 11:27 pm Post subject:

bugbrand · Posted: Sat Jul 01, 2006 2:33 am Post subject:

OooohOooooohOoooooh this does look good, very good!
Available in UK from Farnell for about £2.50...

D'ya reckon you can change cap C7 to something else to give VC-LFO operation? Would seem like it??!!

Super Work!

Scott Stites · Posted: Sat Jul 01, 2006 4:03 am Post subject:

Robert,

You have an uncanny knack for hitting the head of the nail each and every time (about once every three times I end up with a sore thumb).

The irony of it is that Thomas instructs on some pretty high order mathematics up in Minnesota. I often wonder how many, if any, of his students realize his history. Can you imagine a synth class from him? Even a history of synth's class from him would be something else. Stories from the pre-internet heydey antics with Craig Anderton, Bernie Hutchins, John Simonton. I think a book about that would be a very interesting read. Something only synth geeks like us would enjoy Very Happy

A TH blog would be great on Magic Smoke. I'm not sure if he'd have the time to do something like that (once his school term fires up, he pretty well gets snowed under). He's on a break between semesters now, and man, is he cooking up all kinds of stuff.

A couple of summers ago, before he folded up MAP, he was in a similar mode. The Mankato filter (I really need to post some samples of that) was only one of the things that came along at that time. It is a hell of a nice filter, way more bang for the buck than even the XR VCO. Easy to build, yet with specs and a sound that just bowls you over (think cross between a Moog ladder sound and an SSM2040). I can't wait for Magic Smoke to get moving on that.

But that wasn't the only thing. Hopefully the other stuff will surface in one way or the other.

Cheers,
Scott

Scott Stites · Posted: Sat Jul 01, 2006 4:05 am Post subject:

Hey Bugbrand,

I dunno - I suppose! I'll have to try a very large cap and see how low it will go. I'll let you know what I find out.

Cheers,
Scott

zipzap · Posted: Wed Jul 05, 2006 12:38 am Post subject:

Wow, this is cool!
Been away for some 10 Days, just returned to find all those great new threads. I Spent days with that chip, ending up controlling it with a vactrol. I love vactrols, but for vcos they´re not the best choice.
5 Octaves! Man, i didn´t imagine this was possible with that chip.
What i really liked while experimenting was using a pot for sine/tri control, rather than a switch. Could be a stereo pot, find some values to get amplitude stable. Maybe it could be voltage controlled...
The vc skew is too cool for words.
One idea could be to add some variable resistor to pin 8.
The effect would be a variable release time of the generated wave. this could add frequencies other than the octave.
I think it´s more of an effect thing. If you set it to give a 5th at one freq, that interval will not stay the same when the vco pitch is changed. When Skew is carefully set/modded at propably rather low level this might give some interesting overtones.

zipzap · Posted: Wed Jul 05, 2006 12:49 am Post subject:

oh, about the cab: i was using a 470uf cab an measured (after setting my scope to dc which took me a day to find out) some 15 minutes of clean sinewave with no loss in amplitude.

bugbrand · Posted: Wed Jul 05, 2006 1:12 am Post subject:

zipzap · Posted: Wed Jul 05, 2006 2:57 am Post subject:

Have fun!
BTW, i understand that a simple circuit is what you want. If you can live with another pot and jack than consider using pin 1 for am. I had so much fun with that. Just don´t know if it can be implemented alogside the vc skew circuit. Don´t really yet understand how that´s working...

Scott Stites · Posted: Wed Jul 05, 2006 8:06 am Post subject:

Hey ZipZap,

It really is a spiffy design. Thomas has been just going to town lately - I can hardly keep up with him.

Have you ever heard of his SuperController? It was in his book "Build a Better Music Synthesizer" and was a module based off of the SN76477. He's developed an improved version of it - and it doesn't use an SN76477 at all! Just the digital noise clock section of it is amazing - less than 1Hz through 350 kHz clock rate, expo CV, very common parts. Absolutely no discernible psuedo-random repeat even at the highest clock rate (no chugging). And that's just the tip of the iceberg with this thing.

The past couple of days things have been arriving in my inbox for what to do with that SN76477 that's been sitting in its parts drawer waiting for an application. It'll rock your world!

What he ends up doing with these two designs is still up in the air.

As for the XR VCO, the resistor at pin 8 is for the sync function Tom devised for this circuit. If you look closely enough, you'll see that it probably is the only VCO sync that's based on FSK! It's certainly one of the most unusual syncs I ever heard - neither hard nor soft. Anyway, the 1K resistor is there to throw the frequency, ever so briefly, up to some great number.

Tom is using the mulitplier section to derive the VC Skew. I'm not sure what altering the function at pin 1 will do. FWIW, audio rate VC skew sounds to me just like AM.

Take care,
Scott

zipzap · Posted: Wed Jul 05, 2006 11:20 am Post subject:

Well, giving it a closer look i see that i missunderstood something there. While experimenting with the chip i was getting a ramp-wave by connecting the pulse out to the fsk in, just like in the datasheet. than the resistor value at pin 8 had the effect i just described.
Now in this case the ramp must be made somehow different. Can´t use my right hand right now. As soon as i can i´ll check this out. Good way to learn by building a design, than experimenting with values, see what happenes.
About the AM, I was writing about it in the other thread, I like that it is bidirectional with negative cv inverting the wave, like a rigmod. On the scope it looked way different than what´s going on with that vc-skew. But very often things like that sound very much the same, although the effect is created differently. So there is no benefit in adding too much, just because it´s possible. Have to force myself not to want a pot for everything sometimes.
But --- maybe fast AM and slowly modded skew will sound great, unheard? I don´t know...

Scott Stites · Posted: Wed Jul 05, 2006 12:40 pm Post subject:

Hey ZipZap,

I'm all for trying new things out with any design - if you've got a light on over your head, don't ever let anyone extinguish it! That's where a good bulk of the interesting and new circuits come from, IMO.

Take care,

Fellow pot(entiometer) freak,
Scott

zipzap · Posted: Thu Jul 06, 2006 1:01 am Post subject:

"Those happy accidents - let them happen" - Bob Ross
Just had to think about when i was cutting some wire while my state variable filter was running. Ever since it´s my favorite sine/cosine vco.

Now i´m still trying to figure out how that skew controll is working. The way i understand it the pulse out is controlling the AM-input. When the voltage at the cathode of d4 gets higher than the pulse less of that pulsewave can affect pin 1. But why does that lead to that double freq. ramp coming in? As i´ve said, i´ll have to check it out myself, play with the values and learn. Truly fascinating design. Useing otas, fets or vactrols is always the obvious way for me. If you go another way with just opamps and diodes that´s quite ellegant to me!

Uncle Krunkus · Posted: Thu Jul 06, 2006 5:17 am Post subject:

I just had another look at the schem, and it started talking to me!
It said, "I have ellegance written all over me, I am comprised of esoteric wonderment and you will not understand how I do what I do today. Or tomorrow! Maybe one day when you can snatch the stones from my hand, then you will understand!" Smile

bugbrand · Posted: Thu Jul 27, 2006 2:10 am Post subject:

I finally made a one of these yesterday (still gotta do the fine tuning) but DAMN its great! Mighty thanks to Scott & Thomas!

Maybe I get around to doing a new PCB layout too... The one online seems quite big and I'm a fan of having pots mounted on the board (reckoning three pots - tune, fine and initial skew (all with +V, -V connections) on the board)

I also added a switch for range - 47n cap for audio, 1000n cap for lfo -- DOPEST!

Looking forward to trying more today and then to making about 4 more of them!

WHoopWhoop!

Scott Stites · Posted: Thu Jul 27, 2006 9:43 am Post subject:

Hi Bugbrand,

I'm glad you tried the different cap sizes - I'd moved on before I could try that. How's it work for LFO ranges?

Sheez, Tom's just cranking stuff out like you wouldn't believe. He sent a project to me the other days that covers 23 pages of schematics and parts lists.

Then, bam, he sends me yet another thing last night - something totally different, but cool as hell. Not that I'm complaining - this is just amazing. I think he had a year or two of pent-up ideas that needed released....

I just wonder what he's up to now.

Take care,
Scott