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 Forum index » DIY Hardware and Software » Thomas Henry designs
An XR-2206 Non-Hint
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Thomas Henry



Joined: Mar 25, 2007
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PostPosted: Fri Jul 13, 2007 5:48 pm    Post subject: An XR-2206 Non-Hint
Subject description: ...or what not to do...
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In my VCO Chip Cookbook, I mention that the four quadrant multiplier in the XR-2206 would be lousy as a VCA. In fact, I had a chance to find out just how bad it is this past week.

I've developed a rather unusual electronic percussion synthesizer with no commercial equivalent, and among the chips in it is the XR-2206. Somewhere along the way in the design cycle I thought I might use the internal multiplier as a VCA and save a chip or two. I figured, even as bad as I knew it to be, it would be okay for percussive sounds.

In a nutshell, no way! Even for simple VCA applications, this chip stinks. There are several problems. First, "off" means down -55dB which is still very audible. Second, it's hard to find "off" in the first place---it corresponds to a control voltage around 1/2V+, meaning that a trimmer will be needed. Third, even in the "off" position, the amount of background hiss is quite objectionable.

Like I say, I thought I might get away with it in this situation, but it's clear the unit is abysmal even as a simple VCA for percussion.

However, the story has a good ending. I wound up using the multiplier as a ring modulator, and in this context it's great! I probably wouldn't have considered that option before, but didn't want to waste part of the chip.

So again, in the XR-2206: VCA no, ring modulator yes. I hope this saves someone else a wasted evening pursuing that which doesn't exist! By the way, the new circuit is the most difficult one I've ever pulled off. It took a solid 14 twelve-hour days to get it right. See what I do for summer vacations!

Finally, I figured out another way to put the XR-2206 under exponential control. (There's one version in the VCO Chip Cookbook that is particularly powerful, but this new method is ideal for non-critical applications like tuning the drums via a keyboard). When I get a chance, I'll draw it up and share it here.

Thomas Henry
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StephenGiles



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PostPosted: Fri Jul 13, 2007 11:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

That sounds terrific Thomas, I've had a 2206 stashed away since the early 80s which I bought for an Elektor circuit. Do you see anything of Craig Anderton these days?
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Scott Stites
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PostPosted: Sat Jul 14, 2007 9:06 am    Post subject: Reply with quote  Mark this post and the followings unread

I'll drop an XR-2206 hint: that VCO Chip Cookbook is great - I've even found myself actually using the easy math in it <gasp>. It's a real lifesaver to have around when working with these chips.

Exar, Philips and Intersil all missed the boat when they were writing their apps for these ICs - if they had Thomas writing them, I think you'd find NE566s in the checkout counter impulse buy section, along with the chewing gum and candy bars.

Sweet!

Cheers,
Scott

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State Machine
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PostPosted: Sat Jul 14, 2007 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I find the following chips to be the best for building a VCA:

In order of my preference

1) SSM2164 (VCA)
2) THAT2180 (VCA)
3) LM13700 (OTA)
4) CA/LM3080 (OTA)

The first two amplifiers are inherently LOG responsive but can be easily be made to respond LINEARLY. I know for sure about the SSM2164 because I have hand rolled my own VCA modules using these chips and they have +20 to -100 dB dynamic control! The second two are, and you all know this, are linear within a small operating range but can make excellent VCA's.

I think the SSM2164 is by far the best, IMO, VCA chip out there right now and it has four sections or "gain cells" built in. With one SSM2164 IC, you can build 4 LOG VCA's or 2 LIN VCA's since you have to use two gain cells in the feedback path of the other. By putting the gain cell in the feedback path, you essentially get a amplifier with a output response of (10^(Log x)) with essentially distills down to "X" (example 10^Log100=100) thus the function becomes linear by eliminating the Log function. I can supply the derivation if anyone wants it ... just PM me.

I agree though, the XR2206 really is cool as a ring modulator !! Very Happy
I may be a bit biased in my opinion but the VCA Chip Handbook really does have a very good section on the XR2206 IC Very Happy

Bill
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Thomas Henry



Joined: Mar 25, 2007
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Location: Southern Minnesota
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PostPosted: Thu Jul 26, 2007 7:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

StephenGiles wrote:
That sounds terrific Thomas, I've had a 2206 stashed away since the early 80s which I bought for an Elektor circuit. Do you see anything of Craig Anderton these days?


Craig is the person who gave me my start in electronic music writing. All told, he probably edited some forty articles of mine, and always very generously gave me hints and encouragement. And something probably no one knows, he's also the reason I ended up as a college professor, which radically changed my life. For in fact, he wrote a glowing letter of recommendation for me that turned the tide when first getting into academia.

And now the surprising answer to your question, Stephen. No I don't see much of Craig these days, for the very simple reason: we've never met! I've always considered him a great friend and colleague, but our communications have been exclusively through the US mail!

In fact, I don't believe I've met any synth people in person. But that'll change in a couple weeks when I undertake a trip to the hotbed of creationism in the US.

Thomas Henry
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para



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PostPosted: Fri Jul 27, 2007 4:33 am    Post subject: Reply with quote  Mark this post and the followings unread

"the hotbed of creationism" please be careful we don't want to lose you.


steven
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Scott Stites
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PostPosted: Sat Jul 28, 2007 3:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

The welcoming committee has fancied up the scaffolding with some purty red, white and blue bunting, the Ladies Temperance Union has generously donated 123 torches, and the Farmer's Grange contributed not only two and a half bushel of chicken feathers, but also a gallon and a half bucket of tar. Old man Jenkins even pulled a rail out of his lower pasture fence for the occasion. Col. Brownback should be on time to represent the side that is all righteous and true, and Praise-The-Lord Unified Brethern has agreed to loan their King James for all the evidence required.

This is going to be the biggest thing to hit these here parts since Thomas A. Edison dropped by to toast that elephant back in ought 3 (do that with AC, Tesla!).

This is gonna to be a hootenanny!

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Dan Lavin



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PostPosted: Mon Nov 05, 2007 5:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thomas (or anyone else who has tried sync with the XR2206),

Can you explain the difference between sync with the XR VCO (page 9 of the 21st Century Synth book) and figure 89 in the VCO Chip Cookbook? Obviously the XR circuit is easier to implement (less parts) and I know figure 89 is the traditional way to do hard sync. I guess more importantly, does one sound better (more intense) than the other?
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Thomas Henry



Joined: Mar 25, 2007
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PostPosted: Mon Nov 05, 2007 6:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Aha! Someone finally noticed. I think that the XR sync is one of my more clever ideas, if I may be so immodest.

The traditional method is self-explanatory. The switch shorts out the cap.

The XR method uses the internal mechanism that switches between timing resistors at pins 7 and 8. Pin 7 is a virtual resistor, being the main current control input. But pin 8 is locked at 1k (the smallest legal value); it guarantees the effective period of oscillations is way shorter than anything pin 7 can come up with. The upshot is that the timing cap is also shorted out lickety split (perhaps more than once, but you won't hear it).

Truthfully, due to the timing constant imposed by the 1k, the waveform probably isn't perfect at sync time. However, your ears will not detect the difference, I guarantee it. For oscilloscope art the circuit would create noticeable visual distortion, but for electronic music you'll never know you aren't hearing true hard sync.

It sounds good, so why open a can of beans with a stick of dynamite? As if you didn't know that was coming...

Thomas Henry
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Dan Lavin



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PostPosted: Mon Nov 05, 2007 8:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interestingly enough, I was going to ask if the XR-VCO approach was an "aha" moment! Fortunately, I didn't steal your thunder! Wink
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