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Mod to add CV control to Frequency on Doepfer a-104 Filter
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Mod to add CV control to Frequency on Doepfer a-104 Filter
I'm hoping to add CV inputs to control frequency independently on each filter on the Doepfer A-104 Trautonium Formant Filter.

The pots for the frequency are A50K pots. I'm pretty new to modding stuff, would this be as simple as routing CV to the middle pin on the pot?

Is this safe to just try out? Would I need additional components? I'm mainly going to use Silent Way software to modulate these so I can provide offset and fine tuning of the CV.

I'm interesting in the same topic, did You found a solution to control A-104 with cv?
It's highly unlikely you'll be able to inject a CV at the middle pin of the pot, as suggested by the OP. It's more likely that the pots are acting as resistors. The usual kludge to gain VC would be to place a vactrol across the potentiometer.

I see this module is made up of three parallel BP filters:

That means you'd need at least three control vactrols. But if that top frequency knob acts as an offset for all three, you might be able to get away with one.

A quick test would be to solder a second potentiometer across the pins of the master (see Doepfer DIY page/ universal vactrol module manual for suggestions of which pins).

If you get stuck, ask Dieter Doepfer - he is always helpful on the subject of mods: hardware (ät) doepfer (döt) de

edit: it's in the Universal Vactrol module manual! lol
thx , very helpful, I hope I can still get somewhere A-101-9
Well, now that you know what is possible and which pins are relevant, making your own expander is relatively easy; you don't need the A-101-9.

If you want to keep it very simple, you can just use a passive circuit. I'd be tempted to do that as there is enough space on the existing panel to add a jack and maybe even an attenuator. If you had to build another 4HP panel, I'd make an active circuit with some buffering and processing of the input CV.

Nice way to mod Navs !

I've tried often for attenuators, just putting an optocoupler between the wiper and one end of the pot and it works, in one way, depending on witch end you choose (up when the pot is down or down when the pot is up).

Nevertheless, not having ever seen the schematic of this filter, there's a possibility for it to be as this kind of architecture for ruling the frequency :

Ref from Ray Wilson, thanks to him ! MATE_EXPANDER/ultexpanderpg2.html&VPW=1219&VPH=632

That seen, (a simpler way ?) i would try to trace back to the summing node, witch must be just behind a resistor following the wiper of the frequency pot, and simply solder a 100k resistor there (as per R9 on this schematic above). then a usual jack setup, with or without an attenuator pot.

Again, i don't know this circuit and there is a tiny risk if it's not this kind of circuit architecture. So this would be at your own responsibility to try that.
I would first trace back what append at both end of the frequency pot.

Cheers !
I have just skimmed the Ray Wilson schematic very quickly, so I may be missing something, but I think the reason why that works is because the filter is already voltage controllable; you're just adding another input to the control scheme. hmmm..... I doubt that will work for the A-104.
Well, as said, i don't know anything about the structure of this filter and i'm not doing anything elsa than supposing. But if i had one circuit in front of me, i'd try to look up the traces and connections, around the frequency pot, with powers rails or other places to figure out what kind of filter is it. Of course, it might be totally useless and i might be in a wrong direction regarding the A-104.
Honestly, thanks for your doubts. It's so i try to do.
You're right.

I just grabbed this image from : r-eurorack-module-quad-series-lp-bp-vcf-filter-bank

And it looks like the frequency pot is a dual gang ! ?
And looking at the components, i doubt my proposition would ever work with that kind.

Then, more your way :
, with a dual vactrol (VTL5C3/2 type)
Perfect for the A-1019 kind !

Cheers !
Wondering if anyone can guess what the difference is between this filter and the fonitronik triple resonator? besides the obvious.... Is there a special troutonium sound to the doepfer A-104 or is it just a quad bandpass filter?
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