Using 1K Tempcos with Electronotes designs

[Paul Schreiber]

Ian Fritz pointed out that I may have not been clear on the use
of these 1K tempcos.

The "correct" base-emitter voltage the log amp wants to see
is approx 18.02mv/octave. (26 x ln2) There are 2 ways to do this easily.

1) Use 100K input resistors and a 2K feedback resistor. This gives
1/50 = 20mv/octave. Then a trimmer divider on the output gives
the correct value.

This is what Electronotes uses.

2) Use 100K inputs and a 100K feedback. Then, use a 49.9K  1% resistor, in series with
a 10K trimpot and the 1K tempco (to ground)  as a divider. This gives an adjustible range.

This is the ASM-1 approach (sort of. I think a fixed 56K is used in some versions).

Note ARP used a 1.87K in the divider, which is even closer to "correct" (for filters,
no trimming is probably needed).

So, if you are building an Electronotes VCO that has a 2K feedback resistor in the
input summer, you will have to use 49.9K input resistors (a standard 1% value) instead
of 100Ks. The resistors that summ in the "Coarse" and "Fine" tuning pots will need
to be halved as well.

Paul Schreiber
Synthesis Technology