[sdiy] Expo-Con

[Ian Fritz]

At 09:59 AM 2/4/2015, Tony Kalomiris wrote:
>Can you please enlighten me on what you mean by '0' and delta Vbe - do you 
>mean the initial frequency (with no CV input), and the difference between 
>the exponential pairs  Base-Emitter voltages?

Right, the frequency with zero differential base voltage on the pair.  At 
this point the temperature-dependent expression in the converter's 
exponential factor is zero, so smallest drift is around that point.  To go 
with that frequency, it's typical to use a 2 nF integrating cap and a 20 uA 
standing current  (for a 0 - 5 V Saw core).


>Incidentally I would be curious to know how others are 'tuning' 
>synthesizers these days (my favourite is reference beating and 'listening' 
>(have strobe tuner and counter...)

You mean as far as setting up the tracking? I don't think any one method is 
better than another.  I have a little box with a push-button switch that 
puts out 0 V or 1 V.  I use that with a counter to get a correct octave 
around 100 Hz, then I adjust the HFT trimmer for a good octave at the high 
end.  Maybe 5 - 10 kHz. (Depends on how good the core and converter 
transistor are.)

For everyday tuning I usually use a counter, but using an inexpensive 
guitar tuner seems popular nowadays.

Ian