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

Roman Sowa modular at go2.pl
Tue Mar 21 15:38:49 CET 2023


Well, I don't think about sigma-deltas as DACs but more as noise shaping 
modulators. Actually I don't think about them at all as I have no use 
for them usually. And if we talk about architectures, then not only R/2R 
can work as attenuator, but also resistor-string DACs too.

As for the purpose of 2nd DAC, Tom was kind enough to explain.

This may not going to work in every application though. Imagine Buzz 
Lightyear talking to Woody: "glitches and zipper noise, glitches and 
zipper noise everywhere"

Roman

W dniu 2023-03-21 o 11:42, Mike Bryant pisze:
> 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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