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In Reply to: RE: Ladder DACs for volume controls. posted by joerosen1 on October 02, 2014 at 05:37:46
"DAC's operating in the analog domain???"
Yup. Some are, or at least were, just R-2R networks and transmission gates so you could use them for four quadrant multipliers (one axis being digital, the other the "reference voltage" port) with a little biasing.
I would like to offer you more detail but I'm not sure that I still have any information (or if I do, that I can find it!) as it was something I did over a quarter century ago.
Since this wasn't an audio project I have no idea what it might have "sounded" like but it is an example of using an R-2R DAC in the analog domain.
Regards, Rick
Follow Ups:
> Some are, or at least were, just R-2R networks and transmission gates so
> you could use them for four quadrant multipliers (one axis being digital,
> the other the "reference voltage" port) with a little biasing.
I always thought this was an elegant piece of lateral thinking: rather than
connect the DAC's voltage-reference pin to a steady DC voltage, you feed it
the audio signal The value of the DAC's internal resistor ladder is then
set by applying an 8-bit word to the data port.
I first knowingly saw it used in the Mark Levinson No.38 preamp from the
early 1990s, so I am astonished to learn that the Acoustic Research SRC,
which I used 30 years ago, also featured it. I believe that this kind of
control gives an improvement in S/N ratio as the volume setting decreases.
John Atkinson
Editor, Stereophile
Yup. My wife bought this for me as a surprise because I once mentioned that I was curious about it. It got some rave comments from the Boston Audio Society, iirc. I got a schematic, did some opamp swapping and upgrading of the passive componets and it was quite respectable as a simple preamp back then.
Yup. AR implemented that in their System Remote Control decades ago. They used the R-2R ladder as a controllable voltage divider.
"Yup. AR implemented that in their System Remote Control decades ago. They used the R-2R ladder as a controllable voltage divider."
Thanks! I thought I had seen it done in audio and that was probably where since I remember knowing of the product.
Rick
I still have one in the original box sitting in a closet. Cute idea.
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