MIDI Control (ASM1)

[Harry Bissell]

Here's a good tip... if you locate the mux switches at the summing point
(inverting opamp input) of an inverting op-amp... with the summing resistors
at the other side (away from the opamp) you can minimize the voltage drop
across the mux and make it virtually flat...
(picture a 100K resistor and an analog gate of 100 ohms... one side of the
mux is at "virtual" ground and the other is a 1000:1 divider)

You can in this case use the 4000 series muxes (4051???) IF you bias the
negative supply pin 0.7 volts below ground. (one diode drop) This is so
close that you can drive the control input with a normal 0-12V (or 15 etc)
signal.

This does add a pile of opamps, and it inverts the signals so you might need
two of them... for audio you might not care about the phase.

IMHO this is a better soultion than trying to get gates that work over the
large control range.  :^) Harry

Andy Main wrote:

> Hi all,
>
> >
> > >Also does anyone know of cheap analog MUXs suitable for my patch
> > >design - or can you think of something better/cheaper?
> >
> > Go over to the Maxim site and check out the DG family of analogue
> switchez
> > and MUXez. Harris also make a range of analogue cross-point switches.
> > These, if you don't know them, are a matrix of up to 64 analogue
> switches.
> > 8 in 8 out. There's also a 12 x 8 and there's probably a 16 X 16 but I
> > can't be sure right now.
> >
> > Be careful to choose analogue switches with a good transfer function
> > otherwise it'll bend your CV/Audio too much.
>
> Magnus mentioned these switches, I looked at the time but didn't think
> them relavent, however the more I think about it it seems better.  You
> would need one per patch lead (which is better than one MUX per output!)
> Can someone give me an idea of how many leads I may need (sorrry I don't
> have a modular - yet, and I use a Yamaha AN1x and 303 clone that I made
> myself a couple of years back - incredably unstable drifts to much but
> makes fantastic sounds fo sampling) and I need to cost up and also think
> up some control logic
>
> Cheers
>
> Andy
>
> PS - does anyone have any experience with turning pots into MIDI
> controllers?