[sdiy] Niche ACM-1 how's this thing work?

[Tom Wiltshire]

It's cancelling the variable offset introduced by the first one, in order to make a bipolar MDAC instead of a unipolar one.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 21 Mar 2023, at 10:42, Mike Bryant <mbryant at futurehorizons.com> wrote:
> 
> I think you mean to say any R/2R based DAC with external reference.  Plenty of delta-sigmas have an external reference but won't work like that.
> 
> So what exactly is the second DAC in your scheme doing ?
> 
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman Sowa <modular at go2.pl>
> Sent: 21 March 2023 10:25
> To: Paul Perry <pfperry at melbpc.org.au>; synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] Niche ACM-1 how's this thing work?
>  
> You can make multiplying DAC out of 2 any DACs with external reference 
> input. Both DACs must be controlled by the same SPI. The Vref of 1st DAC 
> is driven with your input of interest, say mxer's channel audio, but 
> with added voltage offset to midscale so the signal fits within DAC's 
> ratings. The Vref of 2nd DAC takes only the offset itself. Then the 
> output of 2nd DAC is reversed and added to the output of 1st DAC.
> Despite the hassle and complexity it's still cheaper than "real" 
> multiplying DAC.
> 
> Roman
> 
> 
> 
> W dniu 2023-03-20 o 23:29, Paul Perry via Synth-diy pisze:
> > This reminds me that I can't seem to find any cheap available 
> > multiplying DACs. Any suggestions? Are they abandoned technology?
> > paul perry Melbourne Australia
> > 
> > On Tue, Mar 21, 2023 at 4:12 AM Mike Bryant <mbryant at futurehorizons.com 
> > <mailto:mbryant at futurehorizons.com <mailto:mbryant at futurehorizons.com>>> wrote:
> > 
> >     That was one of Jim Williams' last app notes for Natsemi before he
> >     left for LTC.  At the time the DAC1020 family was an improvement on
> >     the PMI and ADI devices we used but I recall it was a lot more
> >     expensive at the time.
> >     ------------------------------------------------------------------------
> >     *From:* Ben Stuyts <ben at stuyts.nl <mailto:ben at stuyts.nl <mailto:ben at stuyts.nl>>>
> >     *Sent:* 20 March 2023 16:50
> >     *To:* Mike Bryant <mbryant at futurehorizons.com
> >     <mailto:mbryant at futurehorizons.com <mailto:mbryant at futurehorizons.com>>>
> >     *Cc:* synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
> >     <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>>; mark
> >     verbos <markverbos at gmail.com <mailto:markverbos at gmail.com <mailto:markverbos at gmail.com>>>
> >     *Subject:* Re: [sdiy] Niche ACM-1 how's this thing work?
> >     That reminded me at something I’ve looked at in the past: NatSemi
> >     AN-299 uses a multiplying DAC (DAC1020) for this. See page 5
> >     (panner/attenuator) and 6 (bandpass filter) of
> >     https://www.ti.com/lit/an/snva530/snva530.pdf <https://www.ti.com/lit/an/snva530/snva530.pdf>
> >     <https://www.ti.com/lit/an/snva530/snva530.pdf <https://www.ti.com/lit/an/snva530/snva530.pdf>>.
> > 
> >     Ben
> > 
> >>     On 20 Mar 2023, at 16:10, Mike Bryant <mbryant at futurehorizons.com
> >>     <mailto:mbryant at futurehorizons.com <mailto:mbryant at futurehorizons.com>>> wrote:
> >>
> >>     The NatSemi LM1973N used to be fairly popular for this
> >>     application.  Not as good as the dBX chip but a lot cheaper and
> >>     smaller.  It was three channel, but there were also one and two
> >>     channel versions.  Maybe they prototyped an 8-channel version but
> >>     never released it into volume manufacture.
> >>
> >>     ------------------------------------------------------------------------
> >>     *From:*Synth-diy <synth-diy-bounces at synth-diy.org
> >>     <mailto:synth-diy-bounces at synth-diy.org <mailto:synth-diy-bounces at synth-diy.org>>> on behalf of mark verbos
> >>     via Synth-diy <synth-diy at synth-diy.org
> >>     <mailto:synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>>
> >>     *Sent:*20 March 2023 13:55
> >>     *To:*synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
> >>     <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>>
> >>     *Subject:*[sdiy] Niche ACM-1 how's this thing work?
> >>     Those of us as old as I am might remember this thing from the 90s.
> >>     It is patched into the inserts on a mixer and then adds MIDI
> >>     controlled automation. All the chatter around is about how it is
> >>     passive, so it has “infinite” headroom, 0% distortion, 0% gain. I
> >>     got one recently for a low price. I looked inside, eager to know
> >>     the trick…. Vactrols? Motorized pots? something I don’t even know
> >>     about? Inside the is a big chip 28pins or something with the label
> >>     scratched off. Does anyone know what is actually going on in this
> >>     thing? Is that an 8 channel digitally controlled pot? Just curious.
> >>
> >>     Mark
> >>
> >>
> >>
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