midification of an analog filter

[moxi]

hi,

after my seq will be finished ,i plan to build an analog filter of "oakley sound" web page ,and before starting this project i would know if it would be difficult to add midi for the main parameters.

If i know how midi work ,my knowledge of analog control is very limited ,maybe someone can give some links to page where i could learn about midification (what i need is very similar to midification that some shop propose for vintage synth) ,because if i understand how you can convert midi to analog signal for triggering or how you can transpose the value of the resistance of a pot into digital data ,i don't see how you do to change the value of the resistance  of a pot without using motorized pots (i suppose that on vintage synth ,they don't replace all the pots by motorized ones!).

thanks

[arumblack]

If the filter has CV inputs you can simply add an aout module to your seq and use it to control those parameters with CV. The filtrex 1 has them, as do the modular synth modules(of course ;) ).If you need to control functions other than those provided with cv inputs, things get trickier. You would need the schematics and some knowledge, I don't even know if I could do it. But I was told to look into OTA's for replacing Pots... Still a little above my level at the moment. Anyone else got any suggestions?

[moxi]

one time again i feel me stupid : yes is the filtrex-1 i plan to build but i didn't see that there is cv input on it ,and what i want is just to have midi in/out on the unit .So i think it will be not too much difficult to use simply a core module and an AIN one as midi/cv interface? i'm wrong?

on the filtrex there is only the cutoff freq that you can control ,but as soon as i will receive the kit ,i will try to analyse how it work and if it's possible to replicate it for other function (the Rezo mainly ,others are not very usefull as they can stay "fixed")

what's the better midibox appz to use?

maybe it's possible to use the midimon appz ,so i will be abble to display the value of incoming midi data and why not ,using a DIN module to display directly value of pots? i think it will be not too much difficult to do it ,adding some resistance to get the 10k ohms the DIN need?

[illogik]

hi,

one time again i feel me stupid : yes is the filtrex-1 i plan to build but i didn't see that there is cv input on it ,and what i want is just to have midi in/out on the unit .So i think it will be not too much difficult to use simply a core module and an AIN one as midi/cv interface? i'm wrong?

That would be a core and an AOUT (not ain) as a midi/cv converter; MidiboxCV (is quite expensive but worth it if you like analog). Then you could connect 1 of the 8 cv outputs to the cv input(s) of the filtrex. It does have a cv input, this is from the filtrex page;

CV and gate inputs are available for connection to a modular synth or midi-CV convertor. The CV input will control the filter frequency, and the gate control will trigger the envelope generator if selected.

So you can also connect 1 of the 10 gates of mbcv to the filtrex. If you do all this you can; control the cutoff frequency with a midi CC/controller and gate (trigger) the filters envelope generator with midi note on/off messages.

If you want resonance under control of cv and thus midi, i've seen this trick but i dunno for sure this works with each VCF.

resonance is mostly some kind of feedback path of the amp part of a filter (more experienced people are welcome to correct me here), more feedback>more resonance. if you want to control this you can "insert" a VCA (voltage controlled amplifier) instead of the pot in the feedbackpath. With the vca you now can control the amount of feedback by cv thus by midi!

@@rumblack; you can also use OTA's as voltage controlled replacements for pots, but i haven't studied it enough to really undersand how it works and how to use it.

The datasheet of LM13700 OTA http://www.national.com/pf/LM/LM13700.htmlhas some examples of voltage controlled resistors; maybe someone can shed a light on this?? How to replace a pot with the VCR circuit, how to get the right values, the right cv-range?

You can also use LDR's (light dependent resistors) combined with (a voltage divider and) a led (vactrol) to implement a VCR's

this is way mor easy however really unaccurate (forget 1V/Oct tracking), and maybe not so fast response. check this for more info;

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

cheers, marcel

[moxi]

hi ,

thank ,great explanation!

The AOUT  is needed to send Control Voltage , you're right , but when i said "just a core and an AIN "  , i want  to know if i can "catch" the value of the pots and , using the "AIN" module , convert them to MIDI (and then  output it through the midi out).

i think the use of a Voltage Controlled Resistor is possible ,but such a thing won't help me in "catching" the position of the pot and i think it must be  a way to do this without modification on the audio signal patch (without replacing parts of the original design.

what i'm thinking : just replacing the pot i want to "midify" by a two pole one ,and so using one pole as planned in the original design ,and the second pole to drive the ain module ,using an additionnal circuit to get the voltage range that the ain can handle.

it would be easier , no ?

In this way , i don't have any matter even if the pot are "log" ,the midi data will represent the physical position of the pot ,not its "electric value" ,and i can simply use a table in the mios to match incoming and displayed  midi value to "outputed" cv  .

(what i mean : if the  pot is half of its "run" ,so the second pole send a value that mios display -and send out - as "63") ,and so if the same value is received @ the midi IN , the AOUT must send a cv that drive the pot to half of its "run". it's the purpose of the "tables" ,i'm wrong ?)

[illogik]

hi

what i'm thinking : just replacing the pot i want to "midify" by a two pole one ,and so using one pole as planned in the original design ,and the second pole to drive the ain module ,using an additionnal circuit to get the voltage range that the ain can handle.

it would be easier , no ?

maybe this is easier but if you make the voltage-range outputted by AOUT right, so that it sweeps the whole spectrum like the original pot would do (i think you can adjust each of the 8 voltages on AOUT with it's own trimpot so this can probably be done); you could even replace the original pot with a pot hooked to AIN. This pot sends midiCC, midiCC signal can change the voltage of the cv output on AOUT, thus you can change your filter cutoff directly by your midicontroller(or whatever is voltage controlled in the analog gear). or am i missing something here?

the reason i'm saying this is because i don't know if this is tru;

In this way , i don't have any matter even if the pot are "log" ,the midi data will represent the physical position of the pot ,not its "electric value" ,and i can simply use a table in the mios to match incoming and displayed midi value to "outputed" cv .

are you sure you can use log pots? and even if this doesn't matter can you find (make?) dual

pots with different values (or are the pots used in the filter also 10K?) im curious  ;D

good luck, marcel

[moxi]

yes ,there is  two lin pots 10k and one log pot 10k ! for the other ,it's more difficult ,right!

[illogik]

ok then! ;D~ thats being lucky

better check out the thing bout log pots though; you said;

the midi data will represent the physical position of the pot ,not its "electric value"

i thought the pot acted as a voltage divider; when a lin pot would be halfway, the resistance between the middle leg and each of the other poles will be the same which divides the voltage exactly in half. But a log pot wont have the resistance halfway when it's phisically halfway>> it wont divide the voltage in half which results in that mios doesn't think the pot is halfway>>"false" mididata

i hope this makes sense.. ;)

good luck, marcel

[moxi]

yes ,the log pot won't work too... :(

i will unmount two steréo pot of the same serie to see if i can mix them ,of find an fast mechanical solution :

http://fr.farnell.com/productimages/farnell/standard/42253821.jpg

as you can see ,even if the two shaft are "one" in this type ,i think you can find pot where you can replace one of the resistor part.

.

[illogik]

yup, should be possible ;D , just bend some clips and mix some values!

let us know if you make it work!!

[moxi]

esonance is mostly some kind of feedback path of the amp part of a filter (more experienced people are welcome to correct me here), more feedback>more resonance. if you want to control this you can "insert" a VCA (voltage controlled amplifier) instead of the pot in the feedbackpath. With the vca you now can control the amount of feedback by cv thus by midi!

you're right ,the man from Oakley said me the same thing ,and i need two VCAs for the Rezo  ,may be you get some link to schématic for that ,the ones i've found seems to be a bit complicate for me !

Secondly ,will the VCAs modify the sound  in any way ( must i choose a great circuit in order to preserve the signal as good as it is before the mod  ?

[moebius]

you're right ,the man from Oakley said me the same thing ,and i need two VCAs for the Rezo  ,may be you get some link to schématic for that ,the ones i've found seems to be a bit complicate for me !

Secondly ,will the VCAs modify the sound  in any way ( must i choose a great circuit in order to preserve the signal as good as it is before the mod  ?

VCAs do change the sound.. but much less that your filter does! ;D I wouldn't be too worried.

While only customers have access to Oakley schematics, the guy has also released some to the public. And as the "Super Ladder Filter" probably has more or less same design as Filtrex-1 and has voltage controlled resonance, it's probably the best source of information for this modification:

http://www.physicsenterprises.andrews.edu/diy_archive/schematics/oakley/super.html

In the super.gif - circuits around U11 (the CV input processing and OTA current source) and U6 & U7 (OTAs used as VCAs) are used for CV resonance.. - more or less replacing the resonance pot. See if you can find enough similarities in the circuits to use this method..

Bye, Moebius

[moxi]

thanks ,i wait for receiving the pcb and the schematic ,and i give you my result .

the idea that i got to unmount double pots could be only usefull to monitor the value ,not to drive the filter to "replay" recorded event .I try to learn more about OTAs ,it's seems to be the good way .

[illogik]

Hi,

Secondly ,will the VCAs modify the sound  in any way ( must i choose a great circuit in order to preserve the signal as good as it is before the mod  ?

the quality of the VCA is (mostly) defined by the quality of op-amps/OTA's you use. so use low-noise/high quality opamps etc.

here are some easy DUAL VCA's i've built, they work fine...

http://www.musicfromouterspace.com/analogsynth/vca.html

http://www.musicfromouterspace.com/analogsynth/newdualvca.html

cheers,

marcel

[moxi]

great thanks , the Oakley man tell me that he will send me the PCBs in one week , the "full-midified-stereo-analog-filter-with-LCD-and additional distortion-and-ribbon controller-and maybe more"- is coming soon!

[moxi]

my fiirst pcb is arrived , and i need an advise to add the "Another Dual Voltage Controlled Amplifier" , but i don't see how i must do to keep the control pot with the vca circuit : if i put the VCA after the pot ,i will be obliged to "open" fully the pot ,and if i put the VCA before ,my pot will work with the signal still driven by the VCA.

i see there is pot on the VCA module build on the page http://www.musicfromouterspace.com/analogsynth/newdualvca.html ,so where have I to connect them?

[moxi]

solved :-X

[cheater]

How do you make the ribbon controller? :)

[illogik]

hi,

google for "diy ribbon controller"

for instance;

http://www.wiseguysynth.com/larry/ribbon/ribbon.htm

but i've seen other (simpler) designs before too. I think you can buy them ready made as well..

hybrid synths  ;D

[moxi]

it's that.

[13rian]

So i was going to post a question, but this is closely related so i'll start here and see where it gets me.....

I am building my first mbSid and i want to add in 2 additional toys to the box - a roland phaser and an mxr delay.

each of these fx boxes has an on/off and 2 pots for parameters. is it possible to replace these pots with a voltage controlled resistor?  without using motorized pots - i actually prefer no pots at all and only want midi control. any tips or ideas would be very helpful.

i have been using the miditron made by eric singer (http://eroktronix.com) and it has been very good to me (working on my 4th miditron device) but i discovered this midibox project while searching for a way to use my c=64 sid chip and now i see myself spending a lot of time here as well....

thanks.

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