[sdiy] Unique sounding modules that can't have voltage control?

[brianw]

If I understand the original question, there has to be a control on the original, vintage synth module. Folks are giving lots of examples of synth circuits that can't be controlled in one aspect or another, but that doesn't count because there's no control to replace with CV.

When it comes to panel controls that might be replaced with CV, there are really only two types: variable resistors (pots and fader) or switches. I'm choosing to ignore variable capacitors because I've only ever seen those in radio tuners. I've yet to see a variable capacitor in a synth module.

Switches can be Voltage Controlled using analog switches, which are based on FET. This is how the Prophet 5 solved the problem in the first programmable patch synth. You can use an individual FET, or one of the many brands of IC chips that have one or more channels of analog switches. The "on" resistance of older analog switches can be quite high, but modern examples are basically as good as a mechanical switch.

Variable resistors should be Voltage-Controllable using the technique that Didrik finally mentioned, after a couple of dozens of messages in this thread so far. I was seriously wondering how long it would take for someone to mention a vactrol - they're all throughout Craig Anderton's "Electronic Projects for Musicians" circuits. The limitation here is that pots can have log or linear response, and some of the non-linear responses can be quite complex. You can't piece together different resistance curves under one wiper with a vactrol, so that might account for some differences in the *response* to the control, but it shouldn't affect the *sound* that is being created. i.e. You can always dial in the exact same sound by just calibrating the CV to a different position on the physical control. Another limitation is that a vactrol cannot change its value too quickly, but that's equivalent to someone who doesn't turn the knobs on their vintage synth too aggressively.

Bottom line: The only difference is cost. You should be able to make anything follow CV without changing the quality of the sound.

Brian


On Feb 8, 2024, at 10:13 AM, Didrik Madheden wrote:
> I suspect that in every relevant "weird" case you can cheat(!) the
> system with some brute force by building a vactrol like circuit.
> Connect one or two LEDs to one or two light dependent resistors, as
> necessary based on the pot connection. This might take some nontrivial
> processing of the signal on the LED side to linearize the response and
> potentially balance the high and low leg of the potentiometer so it
> always matches the full resistance across the potentiometer. Maybe
> there are some circumstances where such a circuit wouldn't work but I
> can't really think of any off the top of my head.
> 
> Maybe some (guitar) amplifier circuit where the potentiometer is a
> rheostat and needs to handle a large amount of power being dumped into
> it? (Stretching the definition of the question, I know.)
> Maybe some of Metasonix' tube modules are doing something wild that
> can't really be voltage controlled? Maybe ask Eric Barbour directly
> for suggestions.
> Again hypothetical and stretching the definition of the question, but
> maybe some circuit involving an adjustable capacitor? Although you
> would find those mainly in radio tuners and I doubt you'd find one in
> an audio frequency circuit.
> 
> /Didrik
> 
> On Thu, 8 Feb 2024 at 18:21, cheater cheater <cheater00social at gmail.com> wrote:
>> 
>> Hmm so let's try to narrow down the question
>> 
>> What are circuits where:
>> 
>> 1. the circuit is available as a modular synthesizer module, or a
>> similar subcircuit in a non-modular synth
>> 
>> 2. the module has a resistive potentiometer or slider on its front panel
>> 
>> 3. the module does not have a cv input for the same control
>> 
>> 4. adding a cv input is either impossible or would change the sound of
>> the circuit in an important way
>> 
>> On Thu, Feb 8, 2024 at 5:55 PM Didrik Madheden <nitro2k01 at gmail.com> wrote:
>>> 
>>> Interesting question, and the answer really depends on the definition
>>> of the question. Low hanging fruit would be a theremin. Not a module
>>> as you requested, but just to make a point. The body of the performer
>>> is an integral part of the circuit, so it would qualify at least in
>>> that regard. But, what if you built a voltage controlled robotic arm
>>> that controlled a replica of a human arm, and you now have a bonafide
>>> voltage controlled theremin. :)
>>> 
>>> You might be interested in the diverse work of the artist Gijs
>>> Gieskes. His works are not always eurorack modules, and they generally
>>> do feature control voltage inputs. However, I feel like much of his
>>> work would fit the spirit of your question. Some of his modules use
>>> mechanical and electromechanical devices like relays, motors, optical
>>> detectors modulated by a physical cam and so on to generate sound. The
>>> sound would often depend on the exact physical layout of the product,
>>> so you could argue that the sound is uniquely characterized by that. A
>>> Gieskes module might in that case have a VC input for the speed of the
>>> motor rotating the cam to take an example. Technically a voltage
>>> control, however what's controlled is a mechanical device and not a
>>> purely electronic device.
>>> 
>>> https://gieskes.nl/
>>> 
>>> /Didrik
>>> 
>>> On Wed, 7 Feb 2024 at 19:23, cheater cheater via Synth-diy
>>> <synth-diy at synth-diy.org> wrote:
>>>> 
>>>> Hi all,
>>>> I'm looking for synth modules or circuits which have parameters that
>>>> cannot be voltage controlled - and which have a unique sound that
>>>> similar circuits compatible with voltage control cannot recreate.
>>>> 
>>>> Thanks!