Doepfer

-P-,

I don't think this simple mod will work as the A-145 is a simple
non-voltage-controlled oscillator (the standard integrator/comparator
circuit). The frequency pot is not connected to a fixed voltage but to the
comparator output and the position of the pot defines the share of the
comparator output that is used to charge/discharge the integrator. If a
current is added to the center pin (inverting input) of the integrator
(voltage+resistor) this will not introduce voltage control of the frequency
but only affect the symmetry of the integrator and consequently the LFO
triangle signal. I did not try out the mod you described but from the theory
it should not introduce voltage control of the frequency.

See you at NAMM
Dieter


> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com
> [mailto:Doepfer_a100@yahoogroups.com]Im Auftrag von (i think you can
> figure that out)
> Gesendet: Samstag, 5. Januar 2008 06:52
> An: Doepfer_a100@yahoogroups.com
> Betreff: [Doepfer_a100] Re: adding CV to A-145 LFO
>
>
> You shouldn't need a vactrol at all. There is a current limiting 100K
> resistor coming from the wiper (center pin) of the Freq pot. That's
> the only place the wiper goes, this 100K resistor should be easy to
> locate. One end of that resistor goes to the wiper, the other end goes
> to one of the negative inputs of the two opamps. That means either
> pins 2, 6, 9 or 13. That's the first thing you have to find. So you
> buzz out and find the 100K resistor that goes to the center pin of the
> Freq pot, go across the other side of that resistor and buzz out which
> IC and which pin (2, 6, 9 or 13) that side shorts to.
>
> Once you find point where the other side of the 100K goes to either
> of the two op amp ICs at either pins 2, 6, 9, or 13...then stop
> looking. That's the one you need.
>
> Add your VC Input jack to the faceplate, run it's output to another
> resitor you'll need to add (another current limiter, start at 100K and
> work down until you're happy with the response, but try not to go
> lower than 47K), and connect the output of that added resistor to the
> node of the PCB mentioned above. So you run the output of your
> resistor to the other end of the resistor on the Dopefer PCB which
> comes from the wiper of their Freq pot.
>
> That should do it.
>
> You can add a attenuation pot if you wish as well (if there's room for
> both the jack and the added pot on the faceplate, not sure if there is):
>
> In that case, the output of the jack you add goes to one of the end
> pins of the added pot the other end pin of that pot is connected to
> ground. The wiper of the added pot (the center pin)is connected to the
> current limiter resistor you just added which I explained in the last
> paragraph.
>
> If you attenuator works in inverse (less FM when turning the pot CW),
> then reverse the wires coming from it's two end pins.
>
> Not sure how the expo converter works on this particular LFO, so you
> may not get linear VC response, and most definately not 1v/oct, but
> this mod should work OK.
>
> Yes you could do it by added a vactrol, but that would require cuts
> traces on the LFO PCB, a bunch of discrete components to drive the
> vactrol LED, and the VAC itself. Basically another board will be
> required. It would be more prudent just to buy a Doepfer VC LFO in my
> opinion.
>
> It's worth a skipping all that and trying this first.
>
> - P
>
> --- In Doepfer_a100@yahoogroups.com, Anthony Rolando <goldenechos@...>
> wrote:
> >
> >
> > I would like to add Voltage control to my A-145 LFO. I would like to
> do so with a Vactrol. Would anybody be willing to point me in the
> right direction?
> >
> > Would it be possible that I could look at a portion of the schematic
> so that I could add this?
> >
> > Thanx,
> >
> > Tony
> > __________________________________________________________
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