Doepfer

Hi Roel,

> Fascinating to hear that when you reach the highest position
> on the input of the 138 mixer, a small feedback resistor comes
> to action on the new models, compared to the older ones, which
> didn't amply!

I didn't manage to understand the set-up of your original problem 
which led on to this discussion on the A-138 mixers. The ones I have  
only attenuate, or at max settings give equal to what you put in, 
i.e. all the resistors are the same so there is no amplification 
possible. Thus I suspect what you experienced must be due to 
something else. All my 138's are marked 'v2 1996', and from what I 
know of the Doepfer strategy, I think it's unlikely there has been a 
new design in the last 12 months since I last bought one. It's such a 
simple circuit it's hard to say why it would have needed a revision 
so quickly after they started making A-100's (one possibility might 
be the inclusion of resistors to counteract the small voltage offsets 
caused by the input currents of the op amps used). Maybe if someone 
like Florian has a really early  one we could find out?

Tim

From:  Florian Anwander <Florian.Anwander@c...>
Date:  Fri Jan 10, 2003  10:01 am

Subject:  Re: [Doepfer_a100] It's sounds like there's nothing wrong with my
VCA's!!

Hi Florian,

Thanks for you explanation. I must say it's an complex picture.
I understand now that a lot of "perculiar" things that happen
when I patch a certain way, are unique to Doepfer and would
I use another brand, they could be totally different.
Fascinating to hear that when you reach the highest position
on the input of the 138 mixer, a small feedback resistor comes
to action on the new models, compared to the older ones, which
didn't amply!

When I understand the lin. versus exp. bit, and I extrapolate that
to the lin and exp. mixer, am I right to say that one type of mixer
is better to use for audio signals and the other for cv?

Roel.

Hi Roel 

> Fascinating to hear that when you reach the highest position
> on the input of the 138 mixer, a small feedback resistor comes
> to action on the new models, compared to the older ones, which
> didn't amply!
This is a misunderstanding. 

An signal transfering operational amplifier circuit _always_ uses
feedbackloop.

                  +-RRRR-<<-+
                  |         | <............This is the Feedbackloop.
                  | |\      |
sig.in >>--RRRR---+-| \     |
                    |  \    |
                    |   +>>-+-->>---Out
                    |  /
                  +-| /
                  | |/
                  |  
                  |
                Ground
                 
      
          RRRR means Resistor
          >> shows the direction of the signal flow


As long as the both resistors have the same value there happens no
amplification. If the resistor in the feedbackloop has a larger value than
the input resistor, then the circuit will amplify the signal.


 
> When I understand the lin. versus exp. bit, and I extrapolate that
> to the lin and exp. mixer, 
Lin / exp for the mixers only means, that the potentiometer have
logarithmic or linear characteristics. You simply can make on eout of the
other if you exchange the pots.


> am I right to say that one type of mixer
> is better to use for audio signals and the other for cv?
That is right. 


Florian