cross-point switches (was : PolyModular)

[Paul Maddox]

JB,

>
>In a great many applications, X-point switches can't be used. For the same
>reason some pin-board matrixs can't be used. This is because on synths such
>as the VCS3, the pins themselves have a resistor in them. This acts like a
>summing resistor into the summing node of an op-amp. Without this you cross
>connect or just plain short out connections. Virtual shorting would occur
>for example, if you x-point was set up to select inputs to an array of
>virtual earth mixers. Where the summing node is at "virtual earth" 
>potential.
>

I thought it was a diode?

>However, although the part number escapes me for the moment, MAXIM have a
>new range of x-points which have a guaranteed 1500 ohm on-resistance. As
>long as 1500 ohms would suit your purpose, or you could taylor your system
>to require 1500 ohms, this would act exactly like the pin-matrix on a VCS3.
>

If the signal fed to the matrix was referenced to 0v and had no -ve going 
cycle (easy enough to do) then you could just use diodes on the 'input' side 
feeding the matix and you wouldn't get any shorting and the diode would 
automatically sum the signals for you too...

   A) -------->|-----+------- (output)
   B) -------->|-----|

My first ascii diagram, sorry if its a bit naff, but you catch my drift? A 
and B are summed to the output (the + could be a Cross point switch) but A 
and B could not short each other...

  Paul



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