[sdiy] using a dac as a digitally controlled amplifier

[brianw]

This is a great question, and one that occurred to me as a question around 1984 when I built an 8-bit DAC and was working on digital synthesis. At the time, I had not discovered MDAC technology, and it was only years later that I found out that it's both possible and practical, having been used in commercial products.

Most DAC designs will not pass audio frequencies from the VREF input to the output, though.

Another question I had at the time was why a single transistor could not be used as a VCA. The eventual answer that I discovered is that the TR-808 does have single-transistor VCA stages, but they require AC filtering because a DC offset is inherent. As folks have pointed out in this thread, many MDAC designs would require similar DC manipulations.

Brian


On Aug 4, 2024, at 10:13 AM, Benjamin Tremblay wrote:
> Is there any practical application for synthesizers to pass an analog signal in as a reference voltage to a DAC?
> 12 bit DACs are pretty cheap these days, but VCAs aren’t expensive either. 
> Aside from cost, why would I want to use a DAC as a “DCA”? 
> 
> I can imagine making a state variable filter using a dual DAC as the ‘gain' stages.  
> It’s the kind of thing I would try just to see how well it worked… 
> 
> I suppose you inject one or more DACs into an oscillator to directly control pitch and waveform, some kind of DCO where stability and simplicity are cost-effective.