Doepfer

Hi, thanks for the post. Unfortunatelly, I don't have any old A-131 VCAs! 
Both units I had problems with are 2 to 7 months old. Please don't get me 
wrong, but what I am experiencing on both of these A-131s is not occasional 
distortion at the loudest points, but pitched hum that follows the melody. I 
have samples to illustrate this for both VCAs I owned (returning second one 
today, first one was returned already).
So what is my solution than? First, I tested all three available VCAs (A131, 
A132, A109) last night little bit more, to confirm results. And here it is:
- A-131 - cannot use it (pitched hum)
- A-109 - barely noticable distortion on loudest notes in bass line (your 
"normal" distortion) - I am keeping this module, because VCA on  it can do 
the task, and filter is very good

So you might be asking: look at this guy! He says both A-131 and A-109 are 
at least imperfect but he still takes one anyway. And this is my point 
exactly: imperfection between two is very different in character and 
loudness (I am looking for the right word - while you really have to try 
hard to notice distortion on A109, on A131 it is more like a second voice 
:-).

- I also checked A-132 - no noise whatsoever. But dynamics (volume changes) 
were slightly different than what I am used to (I guess because it is linear 
VCA).

So, my choice is A-109 for audio. What would have happened if A-109 was not 
good enough? I would play more with A-132, and if they were not 
satisfactory, I could always use them for CV duties. And than I would have 
ordered Blacet VCA and see where that takes me.

Regards

Zoran


>From: Florian Anwander <Florian.Anwander@consol.de>
>Reply-To: Doepfer_a100@yahoogroups.com
>To: Doepfer_a100@yahoogroups.com
>Subject: Re: [Doepfer_a100] Noisy A-131 VCA
>Date: Wed, 04 Feb 2004 11:29:05 +0100
>
>Hi Zoran
>
>The reason for the noise is quite simple to explain. The old A-130/131
>used Curtis CEM-VCA-Chips (which is out of production as far as I know),
>which had low noise. The new 130/131 uses CA3080 OTAs (like the 132
>always did). To my knowledge there is no difference between the Audio
>circuit of the new 130 and the 132. The new 131 is noisier, because its
>logarithmic behaviour requires a higher amplification. More
>amplification -> more noise.
>
> > This noise disappears when using A-109 VCA.
>This is clear, because the A-109 uses again another Curtis CEM-Chip (but
>a different from the one in the old A131).
>
>If you have an old 131, you should think about an extension as it is
>mentioned in the service manual of the module. The CEM VCA-chip in the
>A131 contains two(!) VCAs but only one is used. The pcb of the A-131
>offers the possiblity to access the second VCA, you need a second A-131
>pcb and frontplate, but without the second CEM-chip.
>
>It should be possible also to implement this as a quite simple diy project.
>
>
>Florian
>--
>Florian Anwander                  |ConSol
>Tel.   +49(89)45841-133           |Consulting&Solutions Software GmbH
>Fax    +49(89)45841-111           |Franziskanerstr. 38, D-81669 M�nchen
>email: florian.anwander@consol.de |http://www.consol.de
>

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