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converting CV to resistance?


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I've been lurking around here and at ucapps and I'm really blown away.  some really cool stuff going on here!

I have been wanting to build an all analogue multi-fx guitar pedal for a while, and I have most of what I need together for it...

I've been thinking about controlling it with MIDIbox, and feel that If my good friend (hopefully he has some time) programmed the pic for me, I could put together my pedal board with midi control.

I know that I can keep a digital rail going that can perform switching with relays, and I know that led's and lcd's should be an issue...

also, using pots into analogue in's for the controls should be fine as well, but what I can't figure out is how I could get a CV signal from an AOUT board into a resistance value to take the place of the pot normally in the circuit.    Can this be done?

I'd like to try and do things with rotary encoders and led rings to display travel, and illuminated momentary switches for switching.

And has any one else used midibox apps to control analogue equipment besides synths with CV input?

Thanks for having this great forum, and thanks TK for these great projects!

billy

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Hi Billy,

"digital pots" are provided by some manufacturers in form of integrated chips. I cannot tell you, which one is the best, because I never used such ICs by myself. But so long they provide a serial interface (SPI), they could be controlled from a MIDIbox CV with small software changes in cv_aout_lc.inc (just only the byte encoding has to be changed).

Is there anybody who could recomment a certain digital pot type?

Best Regards, Thorsten.

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I'm also very interested in this matter, retrofitting external control to existing gear to make it more useful.

I'd love to learn more about digipots, but I'm still also very interested in a pure analog way, because then you have the choice of controlling something with MIDI, or with real CV (from say your modular)

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but I'm still also very interested in a pure analog way, because then you have the choice of controlling something with MIDI, or with real CV (from say your modular)

yes thats what i think too

you can also make analog Voltage Controlled Resistors; with transistors/OTA's or vactrols (vactrol is a led/ldr combination)

i think the OTA VCR is the most accurate you should check datasheets of for instance LM13700, and report back if you have anything usefull, because i still messing on howto implement this

the vactrol method is less accurate, but way easier to implement

example:

http://home.comcast.net/~sbernardi/elec/og2/og3_echo.html

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Hi and welcome!

I have been wanting to build an all analogue multi-fx guitar pedal for a while, and I have most of what I need together for it...

There is one Big BUT(T) ;) here.. relay switching is known good method, but replacing the pots:

Captain Hastings of this very forum (and the contributor of aout_AD5204.inc, which is modified aout driver for AD5204 digipots) reported severe interference coming from digital control side to the analog side of the circuit - and his project was to build digitally controlled GuitarFX unit.

The problem probably mostly lies on the input impedance of the guitar FX and Hi-Gain combined also amplifies even small interference coming from the surroundings:

http://www.midibox.org/forum/index.php?topic=2556.msg17995#msg17995

I think pilo has made changes to driver for PGA2310.. this chip has very good specs on the paper:

http://www.midibox.org/forum/index.php?topic=2115.0

illogik's (Hi Marcel!) suggestion of using LED/LDR combo or Vactrol is a good one. These are "slow" anyway, but if You're about to replace a resistor in the circuit LDR (Light Dependant Resistor) will act like one and should be totally "transparent" soundwise. (Althought, you can't get down to zero ohms with this and if you're looking to replace a pot that is in "potential divider" combination, You'll need to do careful adjustments)

OTAs are excellent too.. too bad Our Real SDIYers are not here to post information needed.. I'm just slowly getting my head around this OTA thing.

Moebius

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wow, thanks for the responses!

Going to have look up that LM13700, as searching the other day for ota transistors didnt lead me to anything.  After I posted, I also found some info on vactrol's, but sometimes the low range of its resistance may be too high for my application....

Motorized pots are cool and all, but probably a bit pricey for me...  I'm thinking of using a vactrol or other led/ldr type of unit (or transistor based) and using endless rotary encoders with led rings....  does this sound plausible?

And relays to replace switches of course.

and one more... if all of these things were voltage controlled by the pic, I can save my settings couldnt I?  And if so, how many states could I save (would I need extra memory somehow?)

also, what voltage is available from an AOUT?  Is standard CV 0/+5 or -5/+5 or what?  I have a feeling that I'll have to use vactrols or led/ldr combo's that match up with the available cv from the AOUT.

Thanks for the help guys!

billy

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