[sdiy] Generating a large number of CV outputs

[Roman Sowa]

That's exactly what was done in some Oberheims, OBX or something, can't 
recall exactly. One big cap tied to a MUX, then a resistor and much 
smaller capacitor tied to that cap. So with slow update rate the 
stepping was smoothed out enough not to be heard as zipper noise. 
Envelopes and LFOs done this way, the synth is considered one of the 
best sounding classics.

Placing RC after the buffer sounds like cleaner idea, and since every CV 
output is hardwired to a node in the circuit with known and constant 
impedance, even high impedance of the RC filter will not matter at all, 
once it's considered as part of circuit's total input impedance.

Roman

W dniu 2023-12-10 o 20:36, brianw pisze:
> A passive RC link between the mux cap (a.k.a. S/H cap) before the buffer op-amp will drain the S/H storage cap. I'm super-paranoid about selecting op-amps with JFET inputs and tens of gigaohms of input impedance to avoid draining that S/H cap. Putting a passive RC filter between the cap and op-amp buffer would defeat that.
> 
> Wouldn't the same RC filter work equally well *after* the buffer op-amp?
> 
> p.s. After the op-amp is where I placed my single-pole low-pass. So I guess you've answered my question in respect to whether some folks consider single-pole to be enough.
> 
> Brian
> 
> 
> On Dec 10, 2023, at 3:50 AM, Mattias Rickardsson <mr at analogue.org> wrote:
>> brianw <brianw at audiobanshee.com> skrev:
>> On Dec 8, 2023, at 6:50 AM, Mattias Rickardsson <mr at analogue.org> wrote:
>>> Roman Sowa <modular at go2.pl> skrev:
>>>> OTOH I would love to see someone doing 100-outs MUX with single DAC
>>>> spitting 16-bit precision at 1Msps
>>>
>>> That's basically what we do since 2012.
>>> 16-bit 1MSPS DAC running at 0.96 MHz, multiplexing its output to 40 CV channels, each at 24kHz update rate (no zipper noise in the audio band).
>>> Works well, but I wouldn't want to try controlling that DAC/mux timing with a microprocessor, or build it on a single-sided board. :-)
>>
>> One question I have is, "How much reconstruction filtering is necessary?"
>>
>> It was enough with an extra RC link directly after the mux cap, before the buffer op-amp. Added cost: ~0.008 $ per CV channel.
>>
>> /mr
>>
> 
> 
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