Doepfer

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@tstinchcombe.freeserve.co.uk
> Date: Sat, 5 Jan 2008 20:39:26 +0000
> Subject: [Doepfer_a100] 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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