Thanks for mentioning the Pro-One DAC, Olav, because I had not looked at these details before (and I own one!) Which aspects of the Pro-One DAC design do you consider to be clever? Choosing an 8-bit DAC, but only engaging the upper 6 bits? Stealing the 6-column 8021 Port 0 outputs from the keyboard matrix to double as DAC parallel data inputs? Calibrating a laser-calibrated DAC, where each step used is normally 40 mV, so that it actually has 41.667 mV (one quarter step)? * Placing a CPU between the keyboard scan and the VCO input, so that sequencing and transposing are simple to achieve? [Further Ramblings] I believe that reading multiple notes from a keyboard for polyphonic voice assignment requires a CPU. At the very least it requires matrix wiring of the keys, some sort of digital scan, and a method to deliver a unique pitch CV to a selected voice. That's probably extremely difficult without a CPU unless the voice assignment algorithm is baked into the logic design. I think that explains why most polyphonic synths use both a CPU and a DAC, even though nothing requires that they (CPU & DAC) can't each be of benefit on their own. I'm excluding duo-phonic keyboard wirings that can read both a low-note-priority and a high-note-priority CV from the same set of keys for a two-voice architecture; and I'm excluding full-polyphony keyboards where each key has a dedicated voice. Those designs do not require a CPU, of course. The Pro-One is a bit of an exception, here, since it's monophonic but still has a CPU to read the keyboard. One side effect of this is the ease with which a sequencer with transposition can be implemented. I suspect that the fact that the Prophet 5 was designed (in 1978) before the Pro-One (in 1981) meant that they were already familiar with using a CPU to scan a keyboard, so the fact that it wasn't necessary for a monophonic keyboard was moot. They just used the technology that they already knew, and enjoyed the advantages that come with that design. In fact, I recall that Dave Smith came up with the idea to use a CPU at a time (1975 for the Sequential Circuits Model 800) when the synth industry was not doing that. Brian * Note that the AD558 DAC incorporated laser-trimmed resistors. Normally, the Vout, Vout-sense, and Vout-select pins are all shorted together to produce exactly 10 mV per step in full 8-bit mode. By placing a resistor and trim pot in series between Vout and Vout-sense, I assume that the Pro-One tweaks this to 10.41667 mV. Then, using only the upper 6 bits of the input, this can output in steps of 41.667 mV, up to 2.635 V total. I haven't figured out the gain of the 3280 + TL082 op-amp pair, but they might double that voltage to the expected half step resolution. On Mar 22, 2026, at 3:44 PM, Olav Kvern wrote: > The Sequential Pro-One is another mono synth with a DAC. I still think that the way it's done is clever. > > Thanks, > > Ole > > On 3/22/26 7:12 AM, Michael E Caloroso wrote: >> > 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 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: brianw >> > *Sent:* 18 March 2026 19:02 >> > 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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Re: [sdiy] Linear response VCOs?
2026-03-23 by brianw