Harold Hallikainen wrote:

>A purely passive mixer would just be a couple resistors in series between
>left and right. Take the mono sum from the junction. When you take a
>number of equal resistors and tie them to a common point, the voltage at
>the junction is the average of the input voltages. So, if you have two
>(Left and Right) the voltage at the junction is (L+R)/2 . This is assuming
>ideal voltage sources (zero ohm sources) and infinite resistance on the
>load connected to the junction.
>
>However, if the voltage sources are not zero ohm sources, there will be
>crosstalk between the channels. The crosstalk level would be related to
>the ratio of the summing resistors to the source resistance.
>
>Crosstalk can be eliminated by using a zero ohm summer. Instead of just
>taking the output off the junction of the two resistors, drive an op amp
>current to voltage converter. If the input resistors are 10k, connect them
>to the inverting input of the op amp. The non-inverting input is grounded.
>
>

>Put a 10k (for unity gain) resistor between the output and the
>non-inverting input of the op amp. The output will be -(L+R) . The -
>indicates the phase is reversed (this is an inverting summing amp).
>
>

I think you meant put the resistor between the output and inverting input.
David...

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