Fast analog sw. as VC resistor

Mark Smart smart at
Wed Jan 8 17:47:10 CET 1997

> More importantly, 'tho, why do you need all this for an env gen? Does
> the circuit use all three pot leads? I mean, most ADSR gens I've seen
> use one divider for the sustain (which in this context is really just
> a "reference voltage" that could come from anything - an opamp or a
> DAC, no pot needed), but the other three are just two terminal
> resistors.

The ARP envelope generator circuit I was planning on using has
floating three-terminal pots on both the sustain and release function, as I
mentioned in an earlier post.  Perhaps the sustain could be replaced with a
reference voltage; I'll have to study the circuit some more to see if this can
be done.

> That should only take one switch, one resistor, and a
> comparator. And, since these aren't exactly "linear" anyway, do the
> nonlinearities hurt anything?

They might not. In fact, they might be helpful in simulating a
log-taper pot, which this circuit calls for, as I also mentioned.
> I'm not trying to be argumentetive here, but just pointing out some
> design issues I'm unclear on. Is the idea to build a VC ADSR gen - or
> to build the perfect VC pot? ADSR is pretty easy - VC pot might be a
> bit harder.

I'd like to do both, if possible. If this pot idea will work on audio
signals, I'd like to use it for filter resonance as well. Plus I just 
want to figure out how to do it because it's cool.

> The LM339 has a response time of about 1uS. At 100KHz clock, this
> amounts to 1% of your total "t." Thus, across the middle you might be
> able to make this appear as an offset - but at the endpoints, it's
> going to cause a noticeable curve. Reduce the clock to 10KHz and this
> becomes less prominent.
> That 1uS is optomistic. Since the output is OC, risetime is entirely
> determined by R and the input C of the CMOS part attached. If you want
> to make this thing really slam home, use a 470 Ohm resistor instead of
> the 3.3K you mentioned.

Interesting. I'll try that.
> >> It also might help to have a little positive feedback (like a resistor in
> >> the 560K to 1M range between the output and +in) on the comparator to speed
> >> it up.
> This'll help with hysteresis, but it won't really help risetime.
> Change the pullup resistor and you'll improve risetime. Probably a
> bigger limitation, 'tho, is the CMOS switch. Something better, like a
> DG series, would help much here.

I saw those in the Analog Devices data book I just got. How much do 
they cost? I haven't seen any prices yet and would be interested
to try them out. I assume they aren't $80 apiece like some of the analog
multiplier chips!

> Switching time is important, but so
> is glitch energy - and CMOS switches like the 4066 tend to have a LOT
> of glitch energy. This also adds nonlinearities. 

I'm interested to know more about that. What's glitch energy?

> Another question, just for the heck of it: does this HAVE to be VC? I
> mean, if the idea is a small preset synth and you need multiple
> controls, you can get a Maxim chip with 8 8bit Iout DACs in a package.
> You CAN use a micro to set it, but all you really need is a shift
> register. To control many, use the same shift reg with an extra
> control bit to enable another device. One of these chips costs 5
> bucks: that's less than a buck a control. Five bucks to control two
> ADSR gens. Not too shabby, and it's easy to lay out one 20 pin chip. 

Hmm. Can this setup be used to control multiple three-terminal pots? 

*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at                      *

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