[sdiy] using a dac as a digitally controlled amplifier

[Neil Johnson]

Hi Brian,

Detach your software brain!

You wrap an AGC loop around the oscillator to adjust the gain to meet
the amplitude requirement. Components are imperfect and drift with
everything so you need to actively manage the gain, you cannot just
rely on an exact number. Possibly the most well-known example of this
being HP's lamp AGC in the Wien oscillator.

Neil

On Tue, 6 Aug 2024 at 01:10, brianw <brianw at audiobanshee.com> wrote:
>
> I guess that might be confusing the way I worded it. If I recall, in the software version the sum of the two feedback path gain values is 1.0, so neither gain alone is exactly 1.0 - I imagine that the analog version built around a pair of MDAC circuits would require the two gain settings to be carefully matched, according to the desired sine wave frequency.
>
> Brian
>
>
> On Aug 5, 2024, at 4:58 PM, brian wrote:
> > Thanks, Neil. That's very interesting.
> >
> > I often use a software SVF with feedback gain of 1.0 to implement a sine wave generator, but had not thought of using an analog SVF for the same reason. How difficult is it to get the analog gain at exactly 1.0, so as to avoid distortion or decay? I expect that the two MDAC settings much be matched for good performance, or at least have enough resolution that they can easily be tweaked to match each other.
> >
> > Brian
> >
> >
> > On Aug 5, 2024, at 2:30 PM, Neil Johnson wrote:
> >> Hi Benjamin,
> >>
> >> The Audio Precision System One does exactly this in its SVF-based sine
> >> wave generator.  There are two MDAC hybrids that set the frequency of
> >> the oscillator.  It allows easy control of the frequency from the
> >> controller PC.
> >>
> >> Cheers,
> >> Neil
> >>
> >> On Sun, 4 Aug 2024 at 18:17, Benjamin Tremblay wrote:
> >>>
> >>> 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.
>
>
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