mandag 22. april 2013

Prepare to be sawed to pieces!

I've been working on something lately. It involves seven saw oscillators, each to be tuned slightly apart and mixed together, all controlled from the same 1V/oct CV. Can you guess what it is? Does it sound like something you have heard before?

Oh yes! It's the amazing Super Saw! This time in glorious analog :-D

I am fairly certain I will be able to pull this off, but I have no idea of its stability or real sound. I am implementing the circuit part by part. So far I've worked out how to create the detune control slope, the mix curves and the saw oscillators.

The detune pot is not linear, instead it follows a "trumpet" shaped function, somewhat like an exponential function but much sharper than you would be able to create with a normal exponential converter, so I've approximated it using four linear sections in the way described here: http://www.oldcrows.net/~patchell/archives/idealdiode.html (I got the link from Scott Stites on the electro-music.com forum). I've done the math and breadboarded this part of the circuits, and it works as expected although it requires some more work to get a stable +/- 15V.

The mix curves for oscillator 1-3 and 5-7 are essentially exponential functions, and I indend to use the AD633 for that. I have done the math but not yet tried the circuit in practice. The curve for oscillator 4 is just a decending linear function which will be easy.

The oscillators are basically the cores from Yves Usson's VCO: http://yusynth.net/Modular/index_en.html
I've ripped out the main parts - the CV mixer, exponential converter and saw wave generator, leaving out the sine, square/pulse wave, sync possibilities and exponential fm. My version is based on a surface mounted SSM2212 (replacement for the SSM2210) and I've managed to fit all seven oscillators on a 160 * 100 mm PCB. I have also duplicated the power line parts, not sure if this is necessary but I wanted to have the oscillators as separate as possible.

Here is the schematic:
The main thought here is that one can connect one (pre-mixed) 1V/oct CV that goes to all the oscillators, and one detune voltage per oscillator. One could of course connect a separate 1V/oct CV to any of the detune voltage inputs too, to get separate control of each oscillator. One could also attach the missing parts of the oscillators to a separate board to get seven full blown oscillators, but a few modifications will be necessary as there is nowhere to connect the sync and pwm inputs.

Here is the board, almost finished. Ground planes are missing though:


My idea is currently to fit the additional parts - the detune slope and mix curve generators, three VCA/mixers to mix the 7 oscillators, possibly some noise generators and a VCF to a second board of the same size, that can be stacked together with this one for a complete one-voice Super saw oscillator (only leaving out one or more envelope generators.

This is really turning out to be a fun project, I hope I will complete it. Also, I expect it to be drifting as hell,  but that remains to be seen. Perhaps it would be possible to make a digital circuit, compu-tune style for compensating this, that remains to be seen. I am already fantasizing about a six voice analog super saw - imagine the pain of having to keep 42 oscillators in tune...

PS: The board uses 5mm pitch resistors in order to fit everyting in such a small space.


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