Synth-diy-test

Brian Willoughby wrote: "Which aspects of the Pro-One DAC design do you 
consider to be clever?"

All of the above!

...which reminds me, I need to open mine up to replace the Oscillator 1 
octave switch.

I walked through miles of orchards to get to the SCI office to pick up a 
copy of the technical manual in 1981. Those orchards are long gone.

Thanks,

Olav

On 3/22/26 6:11 PM, brianw wrote:
> 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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