[sdiy] using a dac as a digitally controlled amplifier

[brianw]

On Aug 5, 2024, at 8:46 AM, Rainer Buchty <rainer at buchty.net> wrote:
> On Sun, 4 Aug 2024, Gordonjcp wrote:
>> The Ensoniq DOC chip uses the same trick for the same reason.
> 
> To my understanding, it even saves on the amount of DACs and instead S&Hs the volume voltage, then using that as the waveform reference value for the "main" conversion.
> 
> Can be seen on the first-generation Mirages featuring the Rev A DOC where the S&H stuff is spelled out externally.
> 
> With Rev D, almost everything vanished inside the chip w/ only some external feedback signals pointing to it (VVref=-5V, WVref, Vlfdbk, Vol-).
> 
> Rainer

Thank you. I was going to ask for more details.

Very interesting that the volume CV is isolated as an individual signal from a TDM output.

It is quite common to use several channels of S&H with a single DAC, at least when a DAC was expensive compared to an analog multiplexer, capacitors, and op-amp followers. Synths (like the Prophet-5 and many others) use many channels of S&H this way.

What is the DAC chip used with the DOC? Was it custom or off-the-shelf?

Brian