[sdiy] DC vs AC in Audio/CV paths for hardwired synth.

[Harry Bissell Jr]

You may have missed a few points about all DC
CV switching.

An all DC system has to have very low offset voltages
or you will get progressive errors in the system. For
example, a VCA will change the overall AC level of a
signal but will also change the DC level.

Removing DC offsets inherent in some designs will be
a challenge as well...  Some waveforms from VCOs are
centered around ground, some are positive only, some
negative only...

Some of the circutis you're trying to interface will
have a very hard time being redesigned for DC
coupling... I'd suggest some of the analog switches
that can suppoet +/- 15V signals and not bother with
the (4000) CMOS ones. Part cost is higher but design
cost is WAY lower...

H^) harry

--- Dave Kendall <davekendall at ntlworld.com> wrote:

> Hi.
> 
> Hope this is not a dumb post(!), or one that's
> already been done to death,
> but I haven't been able to get any opinions on this
> yet....
> Hi all.
> 
> Are there any big disadvantages in having all DC
> signal and CV paths?
> 
> I'm in the planning stages for a relatively complex
> rackmount hard-wired
> synth -  5 VCOs, 1 sub osc, 2 or 3 VCFs, 3 - 4 EGs,
> at least 2 waveshapers,
> several triangle wave LFOs, a complex LFO, Sample
> and hold, several VCAs for
> CV control and audio level, and (eventually) logic
> controlled
> switching/routing and  PAIA/ELBY/uCApps MIDI
> control. Power will be a
> Power-one Linear PSU, and most modules will be
> EFM/CGS, with a handful of
> homebrew modules. 
> 
> I'm hoping to use CMOS switching IC's for the
> routing, and with 0V to +10V
> max DC and audio, I could power these directly from
> the +15V supply, rather
> than having to work out ±5V or ±7.5V supplies which
> would mean voltage
> regulators, more bits, possibly a triple output PSU,
> compatibility problems
> with some ICs, etc.. I guess it can be done, eg.
> http://www.uni-bonn.de/~uzs159/switch.html>, but DC
> only *might* be simpler.
> 
> Would this be an advantage heat/power/space wise, or
> is there virtue in
> having AC audio/CV signals?
> 
> I've noticed that some VCOs have DC outputs, and
> having DC LFOs/S+H etc.
> would not be a problem.
> 
> Have I missed something really vital?
> 
> I've successfully built quite a few EFM/CGS modules
> over the last year,
> being quite happy at the practical soldering and
> building stuff, but am very
> poor on theory, having never really needed to study
> it until now, so
> apologies if this is really basic stuff.
> 
> Any thoughts will be most welcome.
> 
> Cheers,
> 
> Dave
> 
> 
> 
> 
> 
>