Schematics / Soundlab Mark IIVCO By Ray Wilson
The Sound Lab Mini-Synth Mark II has two identical VCOs. I will describe the operation of VCO1. VCO2 operates in an identical manner.
Again we see a control voltage summer followed by a voltage to exponential current convertor that looks very similar to the one used for the VCF. In this case we do not invert the sense of the current from the expo current generator. Instead the current that flows into the collector of Q8 is used to cause the integrator which functions as the heart of the oscillator to ramp up at a rate which is directly porportional to the amount of current flowing into the collector of Q8. Again we have the high frequency compensation circuit that helps to mitigate high end oscillator flatness. In this case two factors are compensated for at once. Both the bulk emitter resistance issue of Q8 and the ramp core reset time. (For an excellent article on Log and Exponential (Antilog) Circuits see Bernie Hutchins' ELECTRONOTES S-019) Both of those factors tend to cause the oscillator to become flat at high frequencies (high in this case being above about 2KHz). After the synthesizer if completed you must calibrate the VCO scale factor (using trimmer R49) and high frequency compensation (using trimmer R43). Again it is very important to use a well matched pair of discrete NPN transistors (2N3904, 2N2222, etc) or a chip containing matched NPN transistors (SSM2210 for example). For temperature compensation R59 should be a 2K +3300 ppm temperature compensator. To get things working you can use an unmatched pair of NPN transistors for Q7 and Q8 and a regular 2K resistor for R59. I sell both matched discrete NPN (and PNP) transistors and 2K 2% 1/4 watt +3300 ppm temperature compensators if you need them.