[sdiy] Linear response VCOs?

Thu Mar 19 22:11:11 CET 2026

The whole "Mux and S&H" thing was only because decent quality DACs were damn expensive, so it was better to buy one good one and re-use it a lot of times. If there'd been a sixteen channel DAC for the same price, you can bet they've have used it.

Incidentally, I once worked on a PWM-DAC based CV transposition module. We used one 3-bit PWM to generate octave voltages, and one 4-bit PWM to generate 12 note voltages. Since each PWM was very low resolution, they were able to be very high frequency and able to respond fast. And since the two channels were entirely separate, trimming was a simple matter of tweaking one to give octaves and the other to give semitones and done. Once added together, the whole thing could produce good quality Note CVs over a wide range without using any expensive DAC. Of course, the secret here is that it was semitones only, so the full MIDI range suddenly boils down to seven bit accuracy.

> On 19 Mar 2026, at 19:30, Mike Bryant <mbryant at futurehorizons.com> wrote:
> 
> Ah okay, the poly requirement makes sense.   But we ran PDM DACs at 8MHz using LS-TTL to give better than 16 bit audio for a single channel so you could still feed that to a MUX and S/H.
> From: Roman Sowa <modular at go2.pl>
> Sent: 19 March 2026 11:10
> To: Mike Bryant <mbryant at futurehorizons.com>; synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] Linear response VCOs?
>  
> 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> 
> > <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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