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

[Dave Kendall]

Thanks for the reply Harry.


on 12/10/05 22:40, Harry Bissell Jr at harrybissell at prodigy.net wrote:
>Removing DC offsets inherent in some designs will be
>a challenge as well...

Yeah, I'd worried about that, The Ken Stone VCO has DC outputs, but most of
the EFM gear has AC outputs...Not having the extra offset circuits should
also make things simpler (and smaller) I guess.

>I'd suggest some of the analog switches
> that can support +/- 15V signals and not bother with
> the (4000) CMOS ones. Part cost is higher but design
> cost is WAY lower...

I hadn't considered that. Just had a look at the DG441/442, and DG408
datasheets - they look like they should be up to the job.
Are these pretty common, or can you recommend any others?
With the recent stuff with the CA3080, I'd hate to start a new design with
rare chips....

Nose back in the books for me then :-)

Cheers,

Dave

> 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
>> 
>> 
>> 
>> 
>> 
>> 
>