Doepfer

Ok, that makes sense. Thank you both for all the help. 

I have already one VCLFO. I would like to have both of the LFO in my system Voltage Controlled to explore chaotic CV generation using  2 LFOs  cross-modulating each other.

I figured that rather then sell the A-145 (my suitcase is FULL so I would not have room to add), I would just modify it...

Anybody know what is the maxium frequency of modulation a Vactrol wil be able to respond to?


Tony

----------------------------------------
> To: Doepfer_a100@yahoogroups.com
> From: hardware@doepfer.de
> Date: Sat, 5 Jan 2008 18:25:47 +0100
> Subject: AW: [Doepfer_a100] Re: adding CV to A-145 LFO
> 
> -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 
>> 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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