Expo conv. heater (tomg et al.)

[tomg]

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> From: Mark Smart <smart at nn.com>
> To: synth-diy at mailhost.bpa.nl
> Subject: Re: Expo conv. heater (tomg et al.)
> Date: Monday, February 01, 1999 8:03 PM
> 
> I've been messing with a circuit using a heated exponential converter
> lately, as well, as part of my Roland GR-300 guitar synth hacking
project.
> I need a log(1/x) converter to turn the GR-300's pitch control voltages
> (which are proportional to the cycle period of the guitar string) to a 1
> volt/octave CV to use with everything else in the world. My analog
divider is
> based on the one in Electronotes AN-114 (which is in turn a lot like the
> one in National AN-30) and uses the heater from Rick
> Jansen's Formant VCF circuit. This heater is just like the one in another

> National Application Note whose number I can't remember, but with a temp 
> adjustment pot. I tried to improve the analog divider by following advice
> from the Op-Amp Cookbook, etc., i.e. use really low drift op-amps
> (currently it's a PMI OP-400) and polystyrene capacitors in the feedback
> loops.
> 
> The circuit works great, and I can play my ARP Axxe from the hi e string
of
> the guitar at incredible speed, but the thing that bugs me about it is
how
> long it takes to warm up. I did a test this weekend where I tuned the
> circuit while it was warm, then turned it off and went home. I came back
> the next day and turned it on, and the pitch was a half step sharp. I
> waited about 10 minutes, and it came back to pitch just fine. The problem
> is that I had to wait 10 minutes. It would be great to get rid of that
time
> lag. I thought of a couple of possible solutions.
> 
Your heater is not warm enough the current should be in the 40ma range.
My prototype is ready in less than 60secs more like 45.

Get a few resistors and a 3046. Then try this experiment.

http://www.mindspring.net/~vco/misc/pics/htr-3046.gif

-tg