2 VCO's - 1 Log convertor (polysynth design)

[terry michaels]

Message text written by Tony Clark
>>   OK, crazy idea... Im trying to build my polysynth, it has 2 VCO's per 
> voice, but what Im wondering is , does anyone know a way of driving 2
VCO's 
> (usual ramp/square) from one log convertor, essentially they will play
the 
> same note, but one may be detuned from the other... I had though of
messing 
> with the reset level on the ramp generator, but this will affect the
level 
> of the 2nd VCO.

   I assume that you can achieve what you want by using a three 
transistor current mirror, where you have your typical one transistor 
front end, but then two outputs (two transistors in parallel but with the 
outputs individual, one into each VCO comparator).  I imagine that for 
this to work, the two output transistors should be _extremely_ well 
matched.  :)
   However, how could you then separate the tuning of your VCO's?  You'd 
lose the ability to detune (except by changing the reset level of one 
VCO which would also affect that VCO's output level) or have fifths/octave 
separations.  Seems like a bigger negative than dealing with the added 
parts count, IMO.

   Tony
<

Say you have the expo converter design where the base of the second
transistor (expo transistor) is grounded, the emitter is driven by the
first transistor and an opamp to generate a Vbe offset, and the collector
goes to a current controlled oscillator.  You can add a second transistor
by connecting the emitter of another, matched transistor to the emitter of
the other expo transistor (the three transistor current mirror mentioned
above) driving the second oscillator.  If the base of the second expo
transistor is connected to a small variable voltage source, such as the
wiper of a pot fed through a resistive voltage divider to the base, you can
offset the frequency of one oscillator relative to the other.  The offset
will alter the standing current of the second expo transistor without
changing the scale factor, the oscillators should still track each other. 
Plus or minus 18 millivolts will give a plus or minus  one octave tuning
range.

Terry Michaels