[sdiy] Unstability of oscillators and psychoacoustic qualities

[Tony Allgood]

> It is known that new style VCO boards at least have a design-flaw
which makes them less stable than there electronics need them to be...

The minimoog VCO card came in three distinctly different flavours. It
was the second version that had the temp co resistors in the daft place.
The final revision used the uA726 heated NPN pair which is now
troublesome to find a replacement for. But really, there are so many
other sources of error in the mini that cause it to drift. Scaling drift
seems to be fairly good on the later series, but absolute drift is still
pretty awful.  The op-amps on the octave buffer card are just one source
of error here.

Whatever, I do find the later minimoog VCOs to be very pleasant sounding
and I don't think this can be attributed to any long term drift of the
VCO. Cloning the core of the later VCO design but with different expo
and better board layout still produces a VCO with the same sound as the
mini but with inherently better freq stability in the long term. I have
never been able to measure residual FM accurately enough.

Running three of these VCOs in a state variable filter built with a
linearised 3280 also produces a sound not unlike a mini... which was
quite unexpected.

Also interesting is increasing the level of the fundamental frequency to
the VCO output. Say by adding a touch of sine or triangle to the main
sawtooth signal appears to increase 'warmth'.

Regards,

Tony Allgood                       Penrith, Cumbria, England

Oakley Sound Systems                     www.oakleysound.com
Modular projects            www.oakleysound.com/projects.htm