I have a few Vactrols on hand Tim. Been experimenting with the VTL5C3 though, as it is "the fastest" of my choices.

The Mod works, but it is not to my likings, and I think I might have to just buy the 146.

I have tried several circuits to drive the Vactrol, but all seem to have too much slop in them, the response just does not feel right to me.

Perhaps I have not tried the magic circuit... but I feel that more likely the 145 just was not designed with this type of control in mind!

Thank you everybody for your help on this,

Tony


----------------------------------------

> To: Doepfer_a100@yahoogroups.com
> From: timothy@...
> Date: Sat, 5 Jan 2008 20:39:26 +0000
> Subject: 1 Re: adding CV to A-145 LFO
>
> Hi Tony,
>
>> Anybody know what is the maxium frequency of modulation a Vactrol
> wil be able to respond to
>
> That will depend quite a lot on just exactly what type of Vactrol you
> use. Take a look at the second page of this datasheet, and at the top
> you'll see two 'response time' graphs, which you will see have quite
> different scales for the 'C3' and the 'C4':
>
> 4.pdf>
>
> That we are talking 10's to 100's of milliseconds shows they are
> pretty sluggish - this might (and I only say 'might', as I have no
> experience with these devices!) be good enough up to a couple of
> Hertz or so. I skimmed the A-101-9 pages, but couldn't see any
> mention of just what device is used inside that module - thus if you
> go this route, be prepared to have to juggle some component values
> along the way (although this may be made easier by having the LFO
> range switch to play with too).
>
> Since using a Vactrol will mean providing support circuitry to drive
> it, it might not be much more work to replace resistor R4+the
> frequency pot with an OTA, and do it that way: off the top of my
> head, the scaling for the input to the OTA should be pretty straight
> forward; harder will be how to drive its control current, as this
> would probably need some sort of expo converter, and since you need
> to source current for the OTA, you won't be able to use the simple
> pnp+npn transistor combo, I think it might need an op amp or two
> (although I guess the pnp+npn route with a simple current mirror
> might do it, it doesn't really need to be super-accurate after
> all...).
>
> In conclusion, lots of possibilities: the deciding factor is likely
> to be how much time you are prepared to spend on it! (At least it
> should be relatively easy to graft anything onto the 145 by merely
> removing resistors etc,. i.e. without having to cut any tracks.)
>
> Tim
>
>
>
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