[sdiy] Matched pair expo converter shootout

[Aaron Lanterman]

Mark,

I'm going to cc the SDIY list on this in case they have any thoughts.

Your mission is to compare the performance of all those transistor pairs I 
gave you when they're used in a linear-to-expo voltage-to-current 
conversion role.

I had thought there was a schematic on the LM394 datasheet but I can't 
seem to find it now.

We need a canonical circuit that you can drop the various transistors into 
and measure things. What you want to do is plot the voltage in vs. the 
current out, and see over what range you get a good exponential 
characteristic. How high can you go before it slides off the exponential 
curve? That determines how high you can play in tune. Try all the 
transistor pairs I gave you... the ones that stay exponential over the 
widest range "win."

Did I give you CA3046s and CA3083s too? If not I should have. Remind me 
tomorrow.

The curve will vary with temperature - but it will still be exponential.

Hmmm - let's check out Ian's website. Ian is a master synth designer in 
general and an expert on VCOs in particular, you probably can't go wrong 
there.

Ah, here we go. Sawtooth VCOs typically have tweaks in their expo 
converters for the non-instantaneous reset time of the sawtooth that would 
confuse us. So let's check out his triangle designs. Ah, here we go:

http://home.earthlink.net/~ijfritz/sy_cir4.htm

Scroll down to where you see "expo converter," there's a circuit using an 
NPN pair. That's a quite advanced and complicated circuit... let's maybe 
start with something simpler...

Check out Rene's VCO 3:

http://www.uni-bonn.de/~uzs159/

The circuitry to the upper left is the expo converter - the current "out" 
is flowing into the collector of the right half of the 2SC1583. When 
building, you'd ignore the linear FM inputs and you really only need one 
CV input. That might be a good place to start.

If you come to my 5:30-7:30 office hours tomorrow, I'll bring my copy of 
Hal Chamberlin's book and we'll see what's in there.

Maybe someone on the list has a "canonical" expo converter they'd suggest?

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
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