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Cv control in place of a pot techniques?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Cv control in place of a pot techniques?
juicepatrol
I was wondering the method of getting cv control over a pot of a module. I have done this with vactrols in the past but was wondering if any one had a better solution. Any thoughts/help is much appreciated.

MY ASS IS BLEEDING
guest
theres been a few threads on this in the past, and it really boils down to how the pot is being used. some pots are just variable DC voltage sources, so you can just substitute CVs. other pots act as attenuators for AC sources, and these can usually be substituted with a multiplier (LM13700, etc). and some pots are just variable resistors, and these are difficult to replace. you can use a multiplier or two, a vactrol, a microcontroller and digipot, or a PWM and analog switch setup. theres no silver bullet for this. vactrol is the easiset, though.
juicepatrol
gotcha. I am pretty sure that many of the things i want to try are attenuators. I will def try that. I maybe could also try a vactrol with faster response time as well. thanks
mskala
I wrote a Web log article about exactly that the other day: https://northcoastsynthesis.com/blogs/news/voltage-control-modificatio ns
roglok
mskala wrote:
I wrote a Web log article about exactly that the other day: https://northcoastsynthesis.com/blogs/news/voltage-control-modificatio ns


great article and a pleasant read, thanks a lot!
guest
yeah, meant to say it was a good article. definitely useful information.
notmiserlouagain
guest wrote:
yeah, meant to say it was a good article. definitely useful information.

+4 Homer Simpson Marge Simpson Bart Simpson Lisa Simpson
The writeup on state variable filters is also neat Drinking
MrsWedge
Interesting blog post. I believe, however, that you have some incorrect information on vactrols and CdS sensors. It it true that cadmium is toxic but CdS sensors are only toxic if you grind them up and eat them. Also it is possible to import audio products into the EU with vactrols as there is an exemption for audio products where the vactrols would be hard to replace. It is also possible to get a decent response from homemade vactrols. Just have to know how & what to drive them with.
glennfin
These use vactrols but I thought it worth mentioning.

https://www.synthrotek.com/products/lo-fi-synthesizer-circuits/vac-pak  /

I'm experimenting with these to convert circuit bent devices for use with eurorack.
flts
MrsWedge wrote:
Interesting blog post. I believe, however, that you have some incorrect information on vactrols and CdS sensors. It it true that cadmium is toxic but CdS sensors are only toxic if you grind them up and eat them.


I've understood that the problem with toxicity of materials is eventual proper handling of electronics waste (or, rather, lack of it), rather than the fact that people would get a cadmium poisoning from using Buchla modules.

Which kind of makes sense, seeing and hearing reports from places like Guiyu.

Quote:
Also it is possible to import audio products into the EU with vactrols as there is an exemption for audio products where the vactrols would be hard to replace.


Is it still like that - can you give a source on this? I'm not saying this is false, just haven't yet managed to find a reliable source on this one way or another.

I know some RoHS exemptions have already ended, and some are still in place. I just always found it kind of an amusing idea that channel switching vactrols in amplifiers etc. would be important enough to be actually granted an exemption, whereas I DO understand it's important with eg. medical devices.
mskala
MrsWedge wrote:
Interesting blog post. I believe, however, that you have some incorrect information on vactrols and CdS sensors. It it true that cadmium is toxic but CdS sensors are only toxic if you grind them up and eat them.


Tell that to the EU Parliament. As flts said, the main concern is with waste disposal - it's not particularly dangerous to *use* the photocells, but they're worried that the cadmium will eventually get into the environment after the device has been thrown out. Same reason they banned lead solder.

MrsWedge wrote:
Also it is possible to import audio products into the EU with vactrols as there is an exemption for audio products where the vactrols would be hard to replace.


As I understand it, this exemption only existed from October 2012 to December 2013, and despite the efforts of the community to beg for it to be extended further, it wasn't. If you've information on a more recent exemption I'd be interested to hear about it. See https://www.lexology.com/library/detail.aspx?g=06b3bc64-d2c6-4283-90ea -638588021101
trip
mskala wrote:
I wrote a Web log article about exactly that the other day: https://northcoastsynthesis.com/blogs/news/voltage-control-modificatio ns


Great blog post! Excellent blog generally - a lot of really useful, clearly written info, thanks for sharing.
ablearcher
another +1 for great article by mskala

The only nitpick is I wouldn't be so quick to write off homemade vactrols (at least in the US). I know people using them in production runs, the labor required to test photo cells for correct resistance levels and heatshrink one with an LED is comparable and sometimes easier and cheaper than all the trimming and cost that goes with a similar JFET circuit.

This is for the very specific cases where it has to be a resistance and you can't use a VCA, OTA, etc. and there is no microcontroller handy for the digipot.

I'll have to look at the environmental stuff though, thats probably something I should be more up on. It's easy to get addicted to ebay bags of questionable providence for LEDs, photoresistors, and other such noncritical items hihi (I am careful about where I source ICs)

note: I was dead set against vactrols of any form for a while because I don't like using hard to find or obsolete parts, but a couple JFET circuits later and my attitude is considerably softened.
guest
i have a bag of vactrols from elctronic goldmine that i use when i just want to fix something quick. they work really well, especially with a simple expo converter in front of them. and it seems electronic goldmine will never run out of them, because theyve been selling the same lot for over a decade.
juicepatrol
Wow thanks mskala thats wonderful.
juicepatrol
In some cases would it be possible to use an analog multiplier and a op amp to control the depth of signal.

"In order to control the amount of resonance, our CV shouldn't be applied directly, but should control the extent to which the signal makes it through the feedback circuit. What we need is exactly a VCA."
mskala
A VCA is basically the same thing as an analog multiplier. Calling it an "analog multiplier" instead of calling it a "VCA" may imply that you want it to be especially accurate, or to do four-quadrant multiplication (whereas many VCAs are only two-quadrant), but VCAs multiply and multipliers do voltage-controlled amplification; there's no firm distinction between the two.
Troubleshooter
+1 for the Voltage controlled resistor cirquit using LM13700.
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