<html aria-label="message body"><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Very interesting. I just came across two different dc coupled eight channel DAC's for the Raspberry 5 <span style="background-color: rgb(255, 255, 255);"><font color="#0a0a0a" face="Google Sans, Helvetica Neue, sans-serif" size="3"><span style="caret-color: rgb(10, 10, 10);">supporting up to 24-bit/192kHz or 32-bit/384k, for around $50, though probably overkill.</span></font></span><div><br></div><div><div><br><blockquote type="cite"><div>On Mar 19, 2026, at 5:11 PM, Tom Wiltshire <tom@electricdruid.net> wrote:</div><br class="Apple-interchange-newline"><div><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">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.<div><br></div><div>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.</div><div><br><div><br><blockquote type="cite"><div>On 19 Mar 2026, at 19:30, Mike Bryant <<a href="mailto:mbryant@futurehorizons.com">mbryant@futurehorizons.com</a>> wrote:</div><br class="Apple-interchange-newline"><div><div class="elementToProof" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt;">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.</div><div id="appendonsend" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"></div><hr tabindex="-1" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; display: inline-block; width: 722.25px;"><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;"></span><div id="divRplyFwdMsg" dir="ltr" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><font face="Calibri, sans-serif" style="font-size: 11pt;"><b>From:</b><span class="Apple-converted-space"> </span>Roman Sowa <<a href="mailto:modular@go2.pl">modular@go2.pl</a>><br><b>Sent:</b><span class="Apple-converted-space"> </span>19 March 2026 11:10<br><b>To:</b><span class="Apple-converted-space"> </span>Mike Bryant <<a href="mailto:mbryant@futurehorizons.com">mbryant@futurehorizons.com</a>>; <a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a> <<a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>><br><b>Subject:</b><span class="Apple-converted-space"> </span>Re: [sdiy] Linear response VCOs?</font><div> </div></div><div class="BodyFragment" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><font size="2"><span style="font-size: 11pt;"><div class="PlainText">Correct me if I'm wrong but old synths using DAC for CV were all<span class="Apple-converted-space"> </span><br>polyhonic. That means a lot of CV sources needed. So they used S&H and<span class="Apple-converted-space"> </span><br>muxed DAC. To have PWM with fast enough response to feed MUX and S$H, it<span class="Apple-converted-space"> </span><br>would have to run at enormouse frequency, not suitable to affordable<span class="Apple-converted-space"> </span><br>technology back then. And putting separate counter as PWM generator for<span class="Apple-converted-space"> </span><br>every CV is much more expensive, and takes more space than DAC-MUX-S&H.<br>Back then if you wanted a timer, you got 8253 offering 3 timers in one<span class="Apple-converted-space"> </span><br>package, and I'm not even sure if it had PWM mode at all.<br><br>Roman<br><br>W dniu 2026-03-18 o 21:39, Mike Bryant pisze:<br>> Does anybody know why these old synths didn't use PWM/PDM techniques ?<br>><span class="Apple-converted-space"> </span><br>> LS-TTL or CMOS feeding a comparator into an analogue integrator gave 12<span class="Apple-converted-space"> </span><br>> bits performance at audio frequencies even in the 70s so CVs good enough<span class="Apple-converted-space"> </span><br>> for tuning would have been easy.<br>> ------------------------------------------------------------------------<br>> *From:* Synth-diy <<a href="mailto:synth-diy-bounces@synth-diy.org">synth-diy-bounces@synth-diy.org</a>> on behalf of brianw<span class="Apple-converted-space"> </span><br>> <<a href="mailto:brianw@audiobanshee.com">brianw@audiobanshee.com</a>><br>> *Sent:* 18 March 2026 19:02<br>> *To:* <a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a> <<a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>><br>> *Subject:* Re: [sdiy] Linear response VCOs?<br>> The Prophet 5, Rev 1 and Rev 2, use a 7-bit DAC made from hand-picked<span class="Apple-converted-space"> </span><br>> resistors. There is a note in the Service Manual that you should *not*<span class="Apple-converted-space"> </span><br>> replace these resistors because of the challenge of matching a new one<span class="Apple-converted-space"> </span><br>> to the network. 1 LSB is calibrated to 1/12 V (0.0833 V) for easy use in<span class="Apple-converted-space"> </span><br>> 1V/8va scaling. CV ranges from 0 V to 10.583 V (127/12), but the Prophet<span class="Apple-converted-space"> </span><br>> 5 only uses the lower 6 bits for pitch, limiting the range to 5.333 V<span class="Apple-converted-space"> </span><br>> maximum and thus 5 octaves. All CV were 7-bit, but the pitch combined<span class="Apple-converted-space"> </span><br>> coarse and fine with the scale of the DAC changed so that there were 64<span class="Apple-converted-space"> </span><br>> steps in the coarse range plus another 128 steps in the fine range. This<span class="Apple-converted-space"> </span><br>> wasn't quite as accurate as a 13-bit DAC, but still quite accurate for<span class="Apple-converted-space"> </span><br>> the time.<br>><span class="Apple-converted-space"> </span><br>> The Prophet 5 Rev 3 simply used a 16-bit DAC, but maintained the<span class="Apple-converted-space"> </span><br>> firmware design with 7 bits per CV, so the pitch did not enjoy a full<span class="Apple-converted-space"> </span><br>> 16-bit precision. The 13-bit pitch values still have 16-bit accuracy,<span class="Apple-converted-space"> </span><br>> though, just not 65536 steps of precision.<br>><span class="Apple-converted-space"> </span><br>> One thing to note, Mark, is that a 6-bit DAC has an LSB that's 1.56% of<span class="Apple-converted-space"> </span><br>> the total range, so 1% resistors would be quite awful. Then there's the<span class="Apple-converted-space"> </span><br>> fact that a 1% error in the MSB could throw the whole binary scale off<span class="Apple-converted-space"> </span><br>> enough that the values are not monotonic (i.e. an increase in the code<span class="Apple-converted-space"> </span><br>> could actually cause a decrease in voltage!). A 7-bit DAC has the LSB at<span class="Apple-converted-space"> </span><br>> 0.78% so you definitely need better than 1% precision. These<span class="Apple-converted-space"> </span><br>> manufacturers were not making a custom resistor array so much as<span class="Apple-converted-space"> </span><br>> hand-selecting individual resistors that were matched well across the<span class="Apple-converted-space"> </span><br>> whole group.<br>><span class="Apple-converted-space"> </span><br>> Today, not only are 1% resistors more readily available than they were<span class="Apple-converted-space"> </span><br>> in the seventies, but you can even get 0.1% tolerance resistors at a<span class="Apple-converted-space"> </span><br>> reasonable. Still, that doesn't even get you to a full 9-bit DAC. This<span class="Apple-converted-space"> </span><br>> illustrates how impressive DAC chip technology is. One of the fasted DAC<span class="Apple-converted-space"> </span><br>> chips I've designed with can run at a sample rate of 125 MHz (yeah, MHz,<span class="Apple-converted-space"> </span><br>> not kHz) based on current switching rather than voltage, but it stops at<span class="Apple-converted-space"> </span><br>> 14-bit precision because the smallest current is only 0.0061% of the<span class="Apple-converted-space"> </span><br>> largest, and it's difficult to be precise enough at such a large scale<span class="Apple-converted-space"> </span><br>> factor. Larger DAC precision requires a different technique than<span class="Apple-converted-space"> </span><br>> binary-weighted digits. Fortunately, there are many ways to implement a DAC.<br>><span class="Apple-converted-space"> </span><br>> Brian<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> On Mar 18, 2026, at 4:34 AM, Tom Wiltshire wrote:<br>>> Roland had form for this. SH-101 uses a simple DAC built from a few resistors too.<br>>><span class="Apple-converted-space"> </span><br>>> Like Roman said, it doesn't really make sense nowadays when DACs are cheap, but it was worth it then.<br>>><span class="Apple-converted-space"> </span><br>>> Tom<br>>><span class="Apple-converted-space"> </span><br>>> On 18 Mar 2026, at 11:31, mark verbos wrote:<br>>>> Like a TR-909.<br>>>> But, surely it is cheaper to use 1% resistors rather than a custom resistor array made.<br>>>><span class="Apple-converted-space"> </span><br>>>> Mark<br>>>><span class="Apple-converted-space"> </span><br>>>> On Mar 17, 2026, at 18:44, David Manley wrote:<br>>>>> 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.<br>>>>><span class="Apple-converted-space"> </span><br>>>>><span class="Apple-converted-space"> </span><a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a><span class="Apple-converted-space"> </span><br>> <<a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a>><br>>>>><span class="Apple-converted-space"> </span><br>>>>> As is typical for PAiA a very low cost solution: build your own DAC with a few components.<br>>>>><span class="Apple-converted-space"> </span><br>>>>> -Dave<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> ________________________________________________________<br>> This is the Synth-diy mailing list<br>> Submit email to: <a href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br>> View archive at:<span class="Apple-converted-space"> </span><a href="https://synth-diy.org/pipermail/synth-diy/">https://synth-diy.org/pipermail/synth-diy/</a><span class="Apple-converted-space"> </span><br>> <<a href="https://synth-diy.org/pipermail/synth-diy/">https://synth-diy.org/pipermail/synth-diy/</a>><br>> Check your settings at:<span class="Apple-converted-space"> </span><a href="https://synth-diy.org/mailman/listinfo/synth-diy">https://synth-diy.org/mailman/listinfo/synth-diy</a><span class="Apple-converted-space"> </span><br>> <<a href="https://synth-diy.org/mailman/listinfo/synth-diy">https://synth-diy.org/mailman/listinfo/synth-diy</a>><br>> Selling or trading? 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