Repling to my own post (OK, I talk to myself, I admit it) I got a
letter offlist from a person who knows the circuit better than I (I
don't have the schemo for this guy, just repaired one once), and this
gentleman informed me that while my hack it would in fact alter the
freq, it would play havoc on the core symmetry.

My bad. Don't do anything I suggest (this time). I assumed (falsely)
that it was a saw core. From the sounds of it it's a tri core.

Just noticed Deet posted a note on how to do this.

Do everything he says.

Adding a vac would require in the simplist of terms an additional
active summer (two stacked inverting amps for best isolation) for the
pot/CV pair. Thwart off the temptation to use a passive summer, else
have major interference of pot movements altering the CV signal. OK
if that's the only place the CV is going, but if it's also going
somewhere else in the patch, it'll be effected by pot changes there as
well which to my understanding of synthesis isn't optimal.

Use the output of that summer to drive the vac. I would suggest
actually that that summer drive a transitor which is configured as an
emitter-follower, as this will allow for easy biasing of the LED. So
output of the summer amp to a resistor - 2.2K or therebouts should
work - output that to the base of an NPN, emitter of NPN to a 1K,
other end of 1K to +12. Collector straight into the anode of the
vactrol LED, cathode to ground. Then as Dieter suggests, bridge the
resistive elements of the vac over the 100K resistor.

You could incorporate an attenuator to the VC by traditional means or
play around with putting a 1K pot in line instead of the 1k resistor
coming off the emitter, but make sure you slap a 200 ohm resistor ater
the pot to +v or you'll fry the LED when you turn up the juice. SO -
+12 volts to a 200 ohm,other side of tat to outide lead of a pot.
Wiper of pot shorted to the it's other end lead and that as well to
the emitter of the NPN.


Or...get Dieter's VC LFO as well. Best solution. They are both fine
units, one cannot have enough LFOs and it's affordable.



- P





--- In Doepfer_a100@yahoogroups.com , "(i think you can figure that
out)" <peter@...> wrote:

>
> 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
> > _________________________________________________________________
> > Share life as it happens with the new Windows Live.
> >
>

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> >
>