Tempco resistors

Stopp,Gene gene.stopp at telematics.com
Thu Dec 11 18:04:00 CET 1997


I've tried the two kinds of resistors that Chris speaks of a while back,
when he provided me with samples, and sure enough - one of them just
doesn't work in the feedback loop of an op-amp! It goes non-linear in
some strange way, while measuring fine on an ohmmeter. I never tried to
go into the cause  - I just used the passive ladder approach (op-amp CV
summer at unity gain into a passive 100K/2K ladder, V/Oct trim in the
summer's feedback loop). Since this worked fine I stopped messing with
it. Anybody have an educated description of this problem?

I should probably mention that although I claim in the ASM-1
documentation that a tempco is not *really* needed, they sure are nice
to have. If you don't have them, your volts-per-octave response will
drift with temperature (*not* tuning!) so if your trimpots are buried
you are in trouble unless you don't need tracking (like analog drum
sounds). Many times, in my setup, I use an external MIDI-CV converter
with a front-panel scale trimpot so it just takes a few seconds after
everything is warmed up to tweak the scale. The problem is, if I shut
down and come back in a couple days, I have to do it again since the
ambient temperature will probably be different. I recently added another
ASM-1 into my setup, using tempcos and Analog Devices MAT-01 matched
tranny pairs in the VCO's, and *those* VCO's will keep perfect tuning
(in tune, beat-free to each other) for weeks. Power up, power down,
mornings, evenings, anytime, they will be perfect. So this is what you
get if you use matched trannys and tempcos - keep this in mind.

If you want to build a machine for live use, and you want to use the
VCO's for melodic parts, then I would definitely advise using tempcos
and matched pairs.

 - Gene



Yup, that "someone" be me.

KRL Bantry Components, Manchester NH. If you look through the Synth DIY
archives, you'll find all the info - phone number, address, who to ask
for on the phone, etc. I don't have that info handy anymore. It is VERY
important to get the right kind of resistor - some of the ones they sell
have a very high inductance and won't work correctly in some situations
(like when placed in the feedback loop of an inverting op-amp, as many
expo-converter tempcos are)! Again, searching through the archives you
will find at least 3 posts I've made over the course of the past year
giving the KRL model numbers for the right and the wrong kind of
tempcos... The "right" kind are much cheaper than $20 each - more like
$3.75 each for the 25 I bought...

 - CList

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