[sdiy] Linear response VCOs?

Wed Mar 18 12:31:15 CET 2026

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 via Synth-diy <synth-diy at synth-diy.org> 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
> 
> As is typical for PAiA a very low cost solution: build your own DAC with a few components.
> 
> -Dave
> 
> On 3/17/26 4:12 AM, Tom Wiltshire wrote:
>> While you might not need 8 or 10 octaves at once to play a melody, many V/Oct analog synths *do* have a lot more than five as a continuous range. Arguably that's a problem, since it makes the tuning issues worse and it necessitated the High Frequency Trimmer, but that *is* how it's been done many times. And that is probably thanks to the 3340 and its temperature compensated expo convertor, as you say!
>> The Sequential Pro-One is one example. It has a three octave keyboard and an octave switch, and all the octave switch does is feed nicely calibrated doses of 1V to the oscillators to switch the octave.
>> 
>> We certainly both agree that trying to cover less range in one go makes life technically simpler for an oscillator designer, almost whatever oscillator design is used. That would be true for a V/Oct oscillator too. If we only have to cover 4 octaves in tune, then our expo convertor has a lot less to do and scale errors are much less of an issue. That's not specific to linear oscillators. Another example where this sort of range switching makes life easier and hides the limitations is the Juno 106's divider-based DCOs. Without range switching, there wouldn't be enough divider resolution to get really low notes, and the stepping at the high end would be even worse than it is already!
>> 
>> For a linear oscillator run from a DAC, the errors would be worse for the lowest frequencies where you have the fewest DAC steps. As you say, our pitch perception is pretty poor down there, so as long as you've got plenty of resolution by the time the oscillator has got up to the sensitive midrange frequencies, you can probably afford to lose some further down. This definitely helps, I agree!
>> 
>> Tom
>> 
>> 
>>> On 16 Mar 2026, at 16:01, Roman Sowa <modular at go2.pl> wrote:
>>> 
>>> Your point is valid, but rises fundamental question:
>>> who needs 8 or 10 octave continuous tuning range anyway?
>>> No analog synth has more than 5 and today most popular is 3-4.
>>> That means smaller range is enough and you can add octave switch as separate control to cover all frequencies of interesrt. Just like a knob doing that in any synth basicaly.
>>> And octave switch in DAC controled Hz/V VCO is just switching the resistance on the way from DAMC to VCO core. This is also exactly how it was done in vintage linear VCO synths like MS20 and many more.
>>> And tuning it further down with modulation or pitchbend doesn't require much of a precicion, especially when it happens at so low frequencies where ear pitch perception is rather limited.
>>> 
>>> Roman
>>> 
>>> W dniu 2026-03-16 o 16:13, Tom Wiltshire pisze:
>>>> That's a thought-provoking point of view, Roman.
>>>> I thought the fundamental problem with linear VCOs was that it's hard to cover an 8 or even ten octave range in a linear fashion - that's 256 or 1024 times the base value. For a CV, that requires very small offsets at the bottom. Say we're going with 1V/KHz. We have a maximum CV somewhere up around 10 or 12V giving us an impressive 10 or 12KHz maximum frequency. But we have an system of 1mV=1Hz, so errors will need to be in the sub-mV level at the low end to keep things sounding in tune.
>>>> uPs and DACs do help in some respects, and since DACs are always always linear these days, it's kind-of an obvious way to go. But again, the accuracy requirement bites at the bottom end. Since the width of each octave doubles every time, if I have 1024 steps in a low octave, then I have 262144 steps in an octave eight octaves higher! That's a massive waste of DAC resolution, and eats up DAC accuracy in a way that makes finding a suitable device a lot harder and more expensive.
>>>> Still, you've given me something to think about, and I shall go and do some calculations to see how well this might work. Thanks.
>>>> Tom
>>>>> On 16 Mar 2026, at 11:36, Roman Sowa via Synth-diy <synth-diy at synth-diy.org> wrote:
>>>>> 
>>>>> IMHO linear response VCOs are simply better and easier to use. No temperature dependency by design and easier to tune - as long as all offsets are small or trimmed, whatever you do, an octave is always an octave, not like with exp VCO where you have to turn 3 to 5 trimmers in several iterations to get them sound remotely acceptable.
>>>>> 
>>>>> One may say that summing few CV inputs is difficult for linear VCOs, but today the VCOs are controlled by microcontroller anyway, so the problem of the 1970's is no longer any concern.
>>>>> 
>>>>> Roman
>>>>> 
>>>>> W dniu 2026-03-16 o 03:49, Thomas Hudson via Synth-diy pisze:
>>>>>> My first modular synth was a Paia 4700/j synth. Yes I know, cheesy design, but the one thing I miss is how linear response VCOs have a very different detuned chorusing effect with each interval. Just wondering if anyone has experimented with this?
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