[sdiy] Linear response VCOs?

Sun Mar 22 15:12:30 CET 2026

> Correct me if I'm wrong but old synths using DAC for CV were all
> polyphonic.

If it was programmable using solid state memory, it had DAC with MUX/S&H.
Not limited to just polyphonics.

Oberheim OB-1 monophonic was programmable and used a DAC with MUX/S&H for
CV.  Released in 1977.

Moog Source was another one, released in 1980.

MC

On Thu, Mar 19, 2026 at 7:14 AM Roman Sowa via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Correct me if I'm wrong but old synths using DAC for CV were all
> polyhonic. That means a lot of CV sources needed. So they used S&H and
> muxed DAC. To have PWM with fast enough response to feed MUX and S$H, it
> would have to run at enormouse frequency, not suitable to affordable
> technology back then. And putting separate counter as PWM generator for
> every CV is much more expensive, and takes more space than DAC-MUX-S&H.
> Back then if you wanted a timer, you got 8253 offering 3 timers in one
> package, and I'm not even sure if it had PWM mode at all.
>
> Roman
>
> W dniu 2026-03-18 o 21:39, Mike Bryant pisze:
> > Does anybody know why these old synths didn't use PWM/PDM techniques ?
> >
> > LS-TTL or CMOS feeding a comparator into an analogue integrator gave 12
> > bits performance at audio frequencies even in the 70s so CVs good enough
> > for tuning would have been easy.
> > ------------------------------------------------------------------------
> > *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of brianw
> > <brianw at audiobanshee.com>
> > *Sent:* 18 March 2026 19:02
> > *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> > *Subject:* Re: [sdiy] Linear response VCOs?
> > The Prophet 5, Rev 1 and Rev 2, use a 7-bit DAC made from hand-picked
> > resistors. There is a note in the Service Manual that you should *not*
> > replace these resistors because of the challenge of matching a new one
> > to the network. 1 LSB is calibrated to 1/12 V (0.0833 V) for easy use in
> > 1V/8va scaling. CV ranges from 0 V to 10.583 V (127/12), but the Prophet
> > 5 only uses the lower 6 bits for pitch, limiting the range to 5.333 V
> > maximum and thus 5 octaves. All CV were 7-bit, but the pitch combined
> > coarse and fine with the scale of the DAC changed so that there were 64
> > steps in the coarse range plus another 128 steps in the fine range. This
> > wasn't quite as accurate as a 13-bit DAC, but still quite accurate for
> > the time.
> >
> > The Prophet 5 Rev 3 simply used a 16-bit DAC, but maintained the
> > firmware design with 7 bits per CV, so the pitch did not enjoy a full
> > 16-bit precision. The 13-bit pitch values still have 16-bit accuracy,
> > though, just not 65536 steps of precision.
> >
> > One thing to note, Mark, is that a 6-bit DAC has an LSB that's 1.56% of
> > the total range, so 1% resistors would be quite awful. Then there's the
> > fact that a 1% error in the MSB could throw the whole binary scale off
> > enough that the values are not monotonic (i.e. an increase in the code
> > could actually cause a decrease in voltage!). A 7-bit DAC has the LSB at
> > 0.78% so you definitely need better than 1% precision. These
> > manufacturers were not making a custom resistor array so much as
> > hand-selecting individual resistors that were matched well across the
> > whole group.
> >
> > Today, not only are 1% resistors more readily available than they were
> > in the seventies, but you can even get 0.1% tolerance resistors at a
> > reasonable. Still, that doesn't even get you to a full 9-bit DAC. This
> > illustrates how impressive DAC chip technology is. One of the fasted DAC
> > chips I've designed with can run at a sample rate of 125 MHz (yeah, MHz,
> > not kHz) based on current switching rather than voltage, but it stops at
> > 14-bit precision because the smallest current is only 0.0061% of the
> > largest, and it's difficult to be precise enough at such a large scale
> > factor. Larger DAC precision requires a different technique than
> > binary-weighted digits. Fortunately, there are many ways to implement a
> DAC.
> >
> > Brian
> >
> >
> > On Mar 18, 2026, at 4:34 AM, Tom Wiltshire wrote:
> >> Roland had form for this. SH-101 uses a simple DAC built from a few
> resistors too.
> >>
> >> Like Roman said, it doesn't really make sense nowadays when DACs are
> cheap, but it was worth it then.
> >>
> >> Tom
> >>
> >> On 18 Mar 2026, at 11:31, mark verbos wrote:
> >>> Like a TR-909.
> >>> But, surely it is cheaper to use 1% resistors rather than a custom
> resistor array made.
> >>>
> >>> Mark
> >>>
> >>> On Mar 17, 2026, at 18:44, David Manley wrote:
> >>>> It's interesting to see how PAiA's John Simonton solved some these
> issues in the 1970's by having a custom laser trimmed resistor network
> built for their 6-bit "Equally Tempered DAC" to be used with linear VCOs.
> See the bottom of the schematic on page  18, the resistor values are on the
> last page.
> >>>>
> >>>> https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf
> > <https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf>
> >>>>
> >>>> As is typical for PAiA a very low cost solution: build your own DAC
> with a few components.
> >>>>
> >>>> -Dave
> >
> >
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