Expo conv. heater (tomg et al.)

[Mark Smart]

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.

1. Reduce the current-limiting resistor on the heater transistor to below
   33 ohms (with a 15V supply). Maybe this would blow up the transistor. It
   says in the databook that the limit on the collector current for the
   transistors in a 3046 is 50 mA...15 volts/33 ohms is 455 mA! ???

2. I've always been amazed by the stability of ARP synthesizers. Now that
   I'm actually trying to build something like it, I have even more respect
   for them, because, while I'm tweaking like crazy trying to get this darn
   heater to work, that ARP sits there solid as I rock as soon as you turn
   it on (I measured it, and hi E is always the same voltage within a few
   millivolts). The stability of the ARPs is due to their "linear voltage
   to exponential current converter" circuit, which consists of a closely
   matched pair of transistors (one NPN and the other PNP) stuck together.
   The heat drift of the two transistors cancels out and the output stays
   very stable. I think this circuit has been discussed on here before;
   sorry if I am repeating stuff people already know. (If there are
   searchable archives for this list anymore, someone please tell me where
   they are.) I know this circuit can be used to make rock-solid expo 
   converters for VCF's and VCO's, but I was wondering if it was possible to 
   adapt it to make an analog divider like I need. The problem is I don't
   really understand how the ARP circuit works, and I'm just starting to
   figure out the analog divider. It would be great to have the analog
   divider be as stable as an ARP!

Until just recently I had put a heat sink on my 3046, thinking that it
would get heated up quickly and not be able to cool down fast enough. Boy,
was I wrong about that. It heats up very slowly, but cools down if you even
fan it slightly with a piece of paper. All the heat sink does is make it
more sensitive. Mr. Klein has definitely got the right idea with the
styrofoam! I need to get his book at some point.

In case anyone is curious, I'm not running the heater on a separate power
supply. So far everything I have built is running parasitically off of the
GR-300's +/- 15V supply. So far I only have one expo converter built, but
the finished project will need at least six, so the current capacity might
become a problem.

Historical trivia: Moog copied the exp conv circuit from ARP, and ARP was 
going to sue them, but then Moog brought up the fact that ARP had stolen their 
ladder filter. So both companies just decided to let it go. But ARP did
stop making ladder filters after that.

I would appreciate the advice of you analog gurus. Thanks!

************************************************
*     Mark Smart                               *
*     Electronics Engineer                     *
*     And Musician                             *
*     smart at medusa.nn.com                      *
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