I got to thinking about a really simple stand alone sequencer/synth voice...
I wanted to approximate the sounds of a synth for use with a simple sequencer and do it with the absolute minimum of parts, run it off a single sided power supply. (ie. 9 volt battery or a cleanly regulated Wall Wart.)
After a little breadboard experimenting I went with a voltage starved 40106 oscillator for the VCO's, a VCF based on Tim Escobedo's Q&D VCF, a single transistor VCA and simple EG. I use each of those terms loosly.
It will make sounds with just about anything that will generate a couple volts for the pitch Control and provide a short Gate of a few volts to generate an envelope for the VCA and VCF.
Parts count is quite low with about 33 caps and resistors, 2 IC's, a transistor & 5 pots.
I did a quick on-line search - and even if you don't try hard - you can source all the parts for under $15 for everything but knobs and a box to put it in)
There is no part used that a poorly stocked Radio Shack won't have (though that might bring the cost up a bit)
Controls are Filter resonance, Filter Cutoff, EG Attack, EG Decay, and VCO#2 frequency adjust, and a switch to choose VCO 1, VCO 2 or both.
This is an unruly beast. This device is not, nor could it be scaled to any known standard :) and it's not particularly frequency stable.
You might be able to set your sequencer for a musically useable loop using a single voice - but if you change one step of the sequence on the fly, you are likely to experience unpredictable results.
When the two voices are combined the resulting cocophony is wonderful, but really hard to manage.
Sound Lab Ultimate Keyboard Synth