[sdiy] heated OTA as expo , was Re: (no subject)
uwe.schueler at uni-tuebingen.de
Wed Mar 27 11:35:55 CET 2019
Tim Ressel <timr at circuitabbey.com>: schreibt...
> I still prefer the heater method of temperature stabilization, a la
> AN299. Makes it all just go away.
Tim, did you read my posts on muffs about using the LM13700 as heated
expo tri core ?
for discussion about the principle and
were I publish schematics & layout of the final VCO
it is also discussed by "Guest" in his paper on the last page. He (and
others) is not a fan of heated
expo designs. Most complaints are:
1) needs extra 40mA heater current
compared to the >100W amplifier behind many synths I can't really see
the problem. Battery operation is also not a hot topic for analog synths
2) warm up time
True. Using an LM13700 in SMD (PDIP is obsolete anyways) improves both
power requirements and warm up time.
Personally, I don't mind a 1..2min. warm up time
Another thing I want to mention: Guest shows the error graph of the
heated OTA (Fig.54) obviously without
HF compensation. It's clear, that if one misuses analog chips with
small transistor geometries like LM13700
or MC1496 as heated expo pairs, it can't be done without proper Rbb
I've done this with 1 additional opamp as Rossum style compensation or
recently with a NIC generating -Rbb for compensation. With proper HF
compensation 10 octaves expo conformity are possible (measured) with
this low cost circuit.
Meanwhile me and my friends have build 8 Expo-OTA VCOs and we are very
satisfied with them. I wonder why this
low cost principle doesn't find it's way into the DIY scene. Maybe
part count is too high
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