First synth design brainstorm..

[cloudscapes]

Over the last several weeks I've been thinking about how to make my first non-modular synth and I figure now's a good time to start writing up a design document. Here's a test I built on the weekend:

http://nearworlds.org/stuff/diy/diy_synth_01.mp3
http://nearworlds.org/stuff/diy/diy_synth_02.mp3

It's an AVR controlling a 12bit DAC control-voltaging an XR2206 func generator. I think this is how I will make the oscillators. I'm quite pleased with how it sounds, and the 12bit DAC resolution is good for 30hz-8000hz with a bit of drift (but I don't mind drift). I'd use 16bit but I haven't found many affordable ones in DIP (that weren't parallel). I've already got a DAC word data table for each note! I'll be using an SSM2044 as a filter and I'll be generating the LFOs and ADSRs on the AVR. Or on another if it gets resource-heavy. There are other things I want to do like programable 2399 delay and stuff, but they can wait. Patches are a must.

Then it dawned on me how many DACs I'd need.

- 4 DACs for the two VCOs (two 12bit for freq, two 8bit for mix)
- 1 DAC for noise mix (8bit)
- 2 DACs for the LPF (8bit for resonance and freq)
- 4/8 DACs for the 2/4 LFOs (8bit for the wave, 8bit for the depth)
- 2/4 DACs for the 2/4 ADSRs (12bit for the shape, don't need depth)
- others for the extra stuff I want, 2399 control, chorus, etc)

...then I made a face.

Am I doing this right? Do I have the right idea? There are a few octal DACs (8bit) in small-ish DIP packages on mouser (TLC5628), and I'll be using the MCP4822 double 12bit DACs where I need the resolution, but it still adds up to so much! I can share the SPI line but I'll still need many pins for chip select! There are also the ADCs I'll need for pots, though I suppose I can use a 16->1 channel multiplexer for that.

I haven't looked much into multiplexers in the past but I think I'll be needing some for switching between waveforms, routing LFOs, stuff like that also.

Am I crazy or am I sort of on the right track? I'm amazed at what a lot of you come up with here and so I will seriously consider to any tips you may have. Building a modular isn't really an option for me. I need it to be ultra-portable (throw it i na backpack with other stuff) and I'll be doing much of the flexibility in code.

[DrJustice]

You could use a single DAC and a multiplexer with a sample and hold on each output. That's how it's usually done.

DJ
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[cloudscapes]

[magman]

I've seen quite a few synth designs that use SSM chips, like the SSM2300 and the SSM5510. These are Octal S&H designs - so 8 S&H circuits in one chip.

You can get the SSM2300 from Vintage Planet for less than 18 euro's, though the SSM5510 is more like $60.

I keep an eye on this sort of chip, as I would like to work on some computer controlled designs in the not too distant future.

Regards

Magman

[DrJustice]

[cloudscapes]

[cloudscapes]

Just tried the S&H opamp trick, works perfectly! Didn't have a multiplexer on me but I could still see. Used a 0.1uf simply because I have a billion of those lying around.

It'll amount to a lot of parts. More chips than using many DACs, but I'll get more resolution out of it and it'll be cheaper!

[cloudscapes]

made myself a little daughterboard to help tidy the eventual breadboard cluster that will arise!

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[KammutierSpule]

There is some of my ideas to help in your brainstorming:

http://electro-music.com/forum/topic-30501.html
http://electro-music.com/forum/topic-23758.html

for LFO and other stuff you dont need so many bits in DAC at all...

KS

[cloudscapes]

[volting]

Hi cloudscapes,

I just got a bunch of XR2206's, wondering how you have wired them up to be voltage controlled. I cant seem find any clear information on how to do that in the datasheet (or anywhere else for that matter).

Thanks,
v