Octave quantizer (was: thermal tips re expo converters)

[jorgen.bergfors at idg.se]

>SO I have egg on my face.... Regarding octave switching, while drawing a copy
>of the Sequential Circuits Pro-One schematic, I realized that it gives 1R, 2R,
>3R... and not 1R, 2R, 4R. This schematic will only work for a volt/octave
>system. The idea of putting two resistors in parallel for 1/2R, one resistor
>for R, two resistors in series for 2R, etc would still be easier than choosing
>different values. If you use clever switching and use the same matched
>resistors, there might still be some benefits here. Not obvious at first is
>that putting 1% resistors in series or parallel actually (statisticlly) gives
>you a good chance of matching to less than 1% (instead of the 2% you would
>expect). Choosing matched resistors from all like values is certainly an
>advantage. I'm posting the schematic anyway, maybe someone will like it.

I just made a voltage controlled octave quantizer. It uses 5 equal resistors for 8 octaves (1 volt steps). They are connected in series, two pairs and one single. Then a trimmer is connected to a constant voltage. The  trimmer is adjusted so that the voltage after the single resistor is exactly 4 volts. Then the voltages between the resistors are switched in by a 4066 and summed. You have 1V, 2V and 4 volts, which need to be buffered before they are summed. I addition to this I have a separate divider that gives the voltage for a 5th. It is 0,5something volts. It is connected thorugh the fourth 4066 switch. I also have a fine tune input that gets summed also.
The 4066 is controlled by John Simonton's 4 bit quantizer. The five resistors for the divider and the three resistors for the summer need to be matched. The result is that I can select 8 octaves and the 5ths between, using an ordinary potentiometer or control voltage. The fine tune pot only need to cover a little over a 5th, which  makes it easier to adjust the tuning.
The complete circuit uses two TL074, one 4066 and a sizable heap of resistors. Maybe you can do this with fewer parts, but my circuit works really well.

/Jorgen