[sdiy] Cheapest, simplest but still reliable VCO & VCA for polyphonic project

[George Hearn]

Paul Maddox and Tom Wiltshire IMO have the right idea in suggesting that you
don't really need that many VCA's provided you have switched source/dest.
controls.  I would strongly argue that for that 'analogue' sound in a poly
you really need analogue vcadsr's (and analogue portamento).  Believe me I
would love to cut corners in components and cost but sound is paramount to
me, LFO's can be digital, but not env's.  
	My currently active poly voicecard design uses just four VCA's, one
for key-follow amount, one for filter envelope amount, one for vco2
modulation amount with switchable destinations and one for filter envelope
modulation amount with switchable destinations and the range of achievable
sounds is surprising.  In the absence of CEMs one is left with a real
problem when it comes to efficient analogue polysynth voicecard design that
is 100% voltage controlled and efficient in the use of components.
'Cheapest' is often a misnomer for easiest and simplest, as electronic
components and placement are cheap relative to other costs when developing a
product.  I have developed a range of tiny 'black boxes' including an 0.8"
DIL14 VCO, 0.7" DIL16 VCADSR & VCF and and 0.5" DIL10 Dual Linear
temperature independent DC/AC VCA currently in the BETA testing phase and
soon to be available to all.  These will bridge the gulf left when CEMs &
SSM chips disappeared.  I'll keep anyone interested posted and freebies are
to be had for anyone serious about designing with them in the meant time
(contact me). Regards, George 

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Paul Maddox
Sent: 08 January 2009 20:22
To: elektrodwarf at yahoo.se
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Cheapest, simplest but still reliable VCO & VCA for
polyphonic project

Karl,

> Hi list, so i wanna make a poly synth and as usual i'm looking for  
> the cheapest possible VCOs and VCAs that'll still cut it. Note; i'm  
> not after absolutely perfect pitch or anything, in fact i'm going  
> analog to avoid this.

You need to speak to George Hearn ;-
http://diy.synth.net/gallery/main.php?g2_itemId=1662

That board, IIRC, has two VCOs, two VCADSRs, VCF and dual VCA.

> Next question is in regards to the fact that since i'm doing a poly  
> synth where one set of knobs controls all the voices, i'll have to  
> make *every* parameter voltage-controlled, so i'll need a hell of a  
> lot of VCAs. Any suggestions on good, cheap and very low parts-count  
> VCAs? I'm guessing a LM13700-design.. ?

SSM2164 gives you 4 vca's in a single 16 pin package, BUT, they're AC  
only (IIRC).

The other thing, if you're planning on controlling them from a micro  
(front panel with memory/rotary encoders perhaps) you could use a  
multiplying DAC. Analogue devices do some octal DACs each with their  
own Vref input and output. I think they're a 20pin package and use SPI  
to control them.

Also, consider if you really need lots of VCAs, look at stuff like the  
prophet 5 and memory moog, the modulation sections of both are pretty  
complete, but also quite simple and require very few VCAs.

I mean, for the output of the LFO do you need loads of VCAs? one for  
every routing?
I'm planning on using a single VCA for the output of my LFO and FETs  
to control destination on/off. Simple, lower cost, and a simpler  
panel :-)

Just some ideas
Paul

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