[sdiy] Additive Synthesis questions

[David G. Dixon]

> If I was going to make an analogue additive synthesizer, is there a simple
> way of building a sine wave oscillator with just a few parts that aren't
> some expensive special function chips?
> 
> What would be a good number of sine waves for analogue additive synthesis?
> 4, 7, as many as can be crammed on a panel?

First, bear in mind that I know NOTHING about this, but I'm also interested
in it (for later).  (Care to read on...?)

For generating lots of low-cost sine waves, why not simply filter square
waves?  The square waves can be generated in harmonic ratios off of a single
VCO with dividers, and then a dedicated filter (BP with some Q, perhaps?)
could be stuck on each square output.  As long as each filter tracks the
frequency of each input square wave, you should get a decent sine wave.

Otherwise, a single expo current source divided into many different triangle
cores which are then shaped into sines.  You could do each tricore/sine
shaper with an LM13700 (one for the integrator, one for the sine shaper), a
TL072 (ditto), one capacitor (for the integrator), two 2N3904s (for the
schmitt trigger), two trimmers (for the sine shaper), and about 14
resistors.  If you're clever, the whole thing could be mounted on a PCB
about 2" x 1", a collection of which could then be plugged into a master
control board with the expo converter and the routing transistors/resistors
on it.  Each sine board would cost less than $5.  However, getting the exact
frequency ratios might be a challenge (but perhaps this would be part of the
charm?).

I think that anything else (excluding digital) would be prohibitive in terms
of parts volume and board complexity, if not actual cost.