Mixturtrautonium (more details)

[Ingo Debus]

Dr. Jörg Schmitz wrote:
> I used a thin (15 Ohm/meter) isolated Konstantan-wire.
> This was winded around a string (used vor a cello, normally).
> After winding, the wire was grinded, so that the
> surface of the string behaves like a pot.

Ooops! How many meters of wire did you need? Did you do all the winding
by hand? Or did you use a machine for that?

> A current source on both ends of the resistor-string will
> lead to a specific Voltage on the sheet-steel while
> pressing down the string on a special position (presupposed
> an adequate interface behind the sheet-steel).

I thought there could be problems if the resistor wire needs to have
proper contact to the sheet steel (dirt, corrosion etc.). So I'm
thinking about a way where the player had only to touch the resistor
wire but not to press it against some conductor. Given that the player
is "connected" to ground (via some high resistance of course) this could
be done when the resistor wire is fed by a floating, i.e. not
ground-referred, voltage supply. The voltage between one end of the wire
and ground is sensed.

Like this:
                      | V
 --------------------||--------------------
|                     |                    |
|                                          |
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
       (R-Rx)    ^           Rx            |
                 |                         |
                 -                          -
                | | Rb                        CV out
                | |                         -
                 -                         |
                 |                        GND
                GND


with:
R resistance of whole resistor wire.
Rx portion of that resistance between one end and the point where it is
touched.
Rb resistance between player's finger tip and ground.

The current in the wire is V/R, thus CV is V*Rx/R. Rb can be quite high
if the CV is buffered with a high input impedance buffer. Sure there
will be hum problems, but these could be reduced when a grounded
conductive wire is run in parallel to the resistor wire so that the
player always touches both wires (like the strings on a 12-string
guitar).

Maybe it's even possible to get rid of the floating supply when the
supply feeding the wire is steered somehow so that the point where the
wire is touched is always at ground potential.

Just an idea, I never tried it. What do you think?

Ingo