On Tue, 27 Jan 2009 11:22:30 -0000, "TonyL"
wrote:

Phil Allison wrote:


** I take it you want a mixer whose gain is a function of the number
of inputs actually connected.


I want to add/remove inputs without having to adjust levels.

So forget using the "virtual earth" summing method.

Feed each input to a high impedance summing point via a suitable
resistor ( like 10k ohms) and the job is done.


Tried that....

As each new source is connected, the gain will drop in proportion to
the number.


..and the level fell as each new input was added. Which was why I was
looking at a virtual ground input instead.

Folks, I seem to be having problems communicating what I'm trying to do.
Here's the requirements:

I have a single audio conductor (+ground), length from 6 feet upwards.

A variable number of inputs are to be fed *to* the conductor at different
points along its length.

A variable number of outputs are to be taken *from* the conductor at
different points along it's length.

Level changes on the outputs to be zero, or small, as inputs/outputs
added/removed.

How would you do it ?







If you know the maximum number of inputs you will need, you can do
this by putting an op-amp buffer on each input, then putting a
resistor in series with each output to feed the common inner of the
coax. Set the gain of the buffer so it cancels the loss caused by all
those commoning resistors that appear in parallel with each other. As
long as the outputs feed a high impedance, you can connect as many of
those as you like.

d