Hi,

the VCAs in our modules have been realized with different circuits over the years (like CA3080, LM13700, CEM3381, CEM3382, CEM3360, SSM/V216, THAT2159). Each of these circuits has advantages and disadvantages and there are a lot of parameters one has to take into consideration: e.g. type of control scale (linear or exponential), noise, control voltage feedthrough, distortion, maximum attenuation, crosstalk and matching (if two or more VCAs are in one circuit) and some more. Unfortunately there is not "THE VCA circuit" that meets all requirements and one has to find a compromise for each application.

E.g. the old A-132-1 was made with the old fashioned CA3080 which has an excellent minimum attenuation but generates a lot of noise and distortion compared to newer circuits. The new A-132-1 is made with SSM/V2164. This is actually a VCA with an exponential control scale but it's possible to use two of them to built a linear VCA. This solution has much better noise and distortion parameters but the max. attenuation is not that good.
Another important parameter is the control voltage feedthrough, especially for FM applications (otherwise one obtains an unwanted frequency shift when the control voltage changes).
Currently we are experimenting with a new circuit that seems to meet most of all requirements in the best way. But we still have to do some tests before we decide to use this new circuit.

In the meantime I instructed our manufacturer to replace the four 10M resistors of the A-135-2 by 4M7 to be on the safe side.

Best wishes
Dieter Doepfer

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com [mailto: Doepfer_a100@yahoogroups.com ]
> Gesendet: Samstag, 3. August 2019 21:18
> An: Doepfer_a100@yahoogroups.com
> Betreff: Re: 1 A-135-2 as quad VCA
>
> Hi Dieter,
>
> thanks for the information! However, it wasn't clear to me in your reply if
> the VCA should be closed with no control voltage, or if the current behaviour
> is intended.
>
> I finally had the opportunity to do the soldering today thanks to the kind
> help of a friend. As you suggested, I soldered four 10M resistors in
> parallel. Now the signal which passes through with no control voltage is
> between -96db and -102db. I notice my "new" edition A-132-1 also lets the
> signal through at -102db with no control signal, so after the modification
> the behaviour of the A-135-2 is the same as that.
>
> I tested my "old" edition A-132-1 and there is only noise at -120db when
> there is no control signal. It seems that the new higher quality VCA
> components just behave in this way. It is certainly nothing which could be
> heard in a mix anyway.
>
> Are there plans to reduce the resistance in future editions of the A-135-2 I
> may get another one at some point, as it's one of my favourite modules, but
> would rather not have to solder again.
>
> All the best,
> CB
>
> On Mon, Jul 22, 2019 at 1:27 PM yahoo@... 1
> < Doepfer_a100@yahoogroups.com > wrote:
>
> Hello,
>
> indeed the VCAs used in the A-135-2 tend to close not fully with 0V CV. But
> actually we have added a small negative offset voltage in the circuitry to
> each VCA CV to avoid this flaw (resistors R3, R7, R11, R15, all 10MOhm). If
> you are familiar with electronics you may reduce the values of these
> resistors a bit to increase the negative offset (e.g. by soldering another
> 10M resistor in parallel). The four resistors are all at the upper rim of the
> pc board.
> Otherwise please return the module for modification/repair to the dealer
> where you purchased it.
>
> Best wishes
> Dieter Doepfer
>
> > -----Ursprüngliche Nachricht-----
> > Von: Doepfer_a100@yahoogroups.com [mailto: Doepfer_a100@yahoogroups.com ]
> > Gesendet: Montag, 15. Juli 2019 12:34
> > An: Doepfer_a100@yahoogroups.com
> > Betreff: 1 A-135-2 as quad VCA
> >
> > Hello,
> >
> > I got an A-135-2 a few months ago to use as a quad VCA. I notice that with
> > all the knobs turned completely to the left and no CV signal plugged in, if
> I
> > connect a signal to an IN socket on any one of the VCAs, the signal is
> still
> > present at the output for that individual VCA, albeit very quietly. (I'm
> > using a sawtooth from a Dixie VCO as a test signal. If I connect the output
> > of the VCA to my RME Fireface UCX with no extra gain on the soundcard
> input,
> > the signal is present at roughly -84 db and can clearly be seen and scoped
> in
> > the mixer channel of Ableton Live.)
> >
> > This surprised me, as with all the other Doepfer VCAs I own, the output is
> > fully closed when the knobs are turned all the way down and there is no
> > control signal.
> >
> > Is this a problem with my unit, or have I misunderstood the module Is it
> > possible to adjust some trimmers on the unit to fully close the VCA in this
> > situation
> >
> > Thanks in advance for any help!
> >
> > All the best
> > CB
>
>
>
>