[sdiy] A question of CV ins

[Magnus Danielson]

On 01/22/2012 10:52 PM, Mattias Rickardsson wrote:
> On 22 January 2012 22:17, David G Dixon<dixon at mail.ubc.ca>  wrote:
>> For CV inputs, I think you really need to
>> figure out how you use your modules, and customize them accordingly.  I
>> would personally never require 5 CV inputs for VCO pitch, as Harry suggests,
>> but peoples' needs differ.
>
> Me neither, I actually see a point in being forced to use other
> creative unplanned connections once in a while. :-) Do any commercial
> modules at all (except perhaps ARP2500) have 4 or more parallel
> inputs?

Well, the VCS-3/Synthi-A/Synthi-100 can be said to have this. The 
Synthi-A for instance has a 16 by 16 patch matrix and thus allowing up 
to 16 sources into the signal and CV inputs of any module. What makes 
these stand out is that the level mixing is done on the output rather 
than the input, and to some degree it is also done by choice of pins 
used. It works completely backwards to what a normal modular would do, 
but for some strange reason I can work quite intuitively with it 
regardless even if it doesn't give me the same level of control as a 
more classical modular would do. It also serves to provide an unbeatable 
economics since the number of level pots is 18. That's a low count for 
the amount of modules you have.

I think the Synthi patch matrix and level control has its benefits, and 
it would not be hard to mimic it as a separate module for a frac rack 
modular or something. It's just a different way of operating.

Hmm. Now I have a DIY project more on my hands. :)

So, I just wanted to bring in a completely different way of thinking.

Another variant would be to use stackable bananas on the input, 
essentially making it a current input and then use outputs. Again to 
shake the world-view a little. To explain this a bit, a typical op-amp 
summer uses a resistor in the negative feedback path over an op-amp. The 
voltage inputs is then added by having a scaling resistor between the 
voltage input and the negative input of the summing-opamp.

This is what the modified patch matrix of Synthi-A actually do.

For DC-stability with nothing applied, a resistor to ground should be 
installed.

The beauty of this approach is that you can combined voltage inputs with 
current inputs. You can also let the current limiting resistor of an 
output act as a unity gain scale for the input, so a combine 
voltage/current output is possible. Scaling can be done either with 
passive resistors/potentiometers or by simple active circuits (providing 
buffering and output resistor).

I agree that many inputs per module is better, it makes better sense to 
build it in than having gazillions CV-mixers as separate modules. I 
rarely use CV mixers normally, but they are not unwelcome into my modular.

I hope I have shaken your tree a little to see things a bit differently.

Cheers,
Magnus