An XR-2206 Non-Hint

via Thomas Henry on electromusic-com:

"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"