[sdiy] Roland 303 VCO questions??

[Dan Snazelle]

Thanks so much for the help!!

Sent from my iPhone

On Aug 15, 2011, at 4:37 AM, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:

>> i have been looking at this schematic all day and I would really
>> like to understand  how it works.
>> anyone want to take a shot at it?
>> the schematic I am working from is HERE:
>> 
>> http://www.ladyada.net/media/x0xb0x/mainboard_beta.png
>> 
>> I am interested in parts  in the VCO, basically IC11B to a little
>> bit past Q24
> 
> IC11B is the VCO CV buffer from the R-2R ladder DAC.  It also forms part of the
> portamento circuit with analogue switches IC12C/D and capacitor C35 to limit CV
> slew rate during programmed slides.
> 
> Matched transistor pair Q26 and it's servo amplifier IC11A convert the linear
> pitch CV from IC11B into an exponential current sink.  This achieves the
> approximate 1v/octave law for the oscillator.  The expo converter continuously
> drains current from capacitor C33 in order to make it's voltage ramp downwards
> linearly.  Q24, 25 and 27 form a thyristor.  This triggers when the voltage on
> C33 falls below a preset level determine by the 5.333V applied to Q24 emitter
> via R101.  When this thyristor implementation fires Q27 and Q25 both turn on,
> with each providing the base current to keep the other turned on.  This results
> in C33 being rapidly recharged up to approximately 12V via Q27 and Q25.  They
> stay in the conductive state like a thyristor would stay latched until the
> charging current into C33 falls below their holding current (typically a mA or
> so.)  Then both transistors drop out of conduction and the expo current sink
> causes the capacitor voltage to ramp downwards again at a rate determined by the
> pitch CV.
> 
> So in summary, you really have an exponential current sink centred around
> Q26/IC11A, and a crude voltage comparator/reset circuit based around Q24, Q25,
> and Q27.  This gives you a raw downwards ramping sawtooth with fixed amplitude
> and exponentially conforming frequency.
> 
> Q28 acts as a high input-impedance unity-gain buffer to buffer the voltage on
> the integrating capacitor before passing the sawtooth output on to the VCF or
> square wave shaper.  Q8 and it's associated circuitry form an over-driven
> common-emitter amplifier and act to shape the sawtooth waveform into something
> that is occasionally vaguely square in shape at some pitches!  Then it's onwards
> to the VCF...
> 
> I hope this helps,
> 
> -Richie,
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