[mhennessy]

So you have a stereo, balanced output signal from the Audionics box? And you have a stereo, balanced signal from the computer?

If both signals are already balanced, there is no need to convert back into the unbalanced domain, only to convert back again to balanced.

Simply, 4 op-amp sections is all you need. The a pair 5532s will do the job. These will be Left Hot, Left Cold, Right Hot, and Right Cold.

Non-inverting inputs to ground.

10k resistors between output and inverting input.

10k resistors between Audionics and the inverting inputs.

Outputs of NE5532 sections to output connectors, via something like 51 ohms.

At this point, all you've done is inverted the outputs of the Audionics box. If absolute phase matters in this application, you can swap hot and cold signals at the output connectors.

Now, connect the computer playout system to the circuit - 10k resistors between that and the inverting inputs of the op-amps.

Of course, now we have a summing amplifier. Job done!

If the computer system is unbalanced, you can balance it by inverting each leg. Another NE5532. Very simple.

Mark