testing distortion/noise on a X'd amp?

[rif]

I had this thought on my ride home tonight: what would happen if you sent the same signal to both + and - inputs on a balanced X'd amp?

Wouldn't the result at the output be the amplified noise of the amp? Seems like if the supersymetric concept is good at eliminating noise between 2 identical inverted signals, it would be excellent at eliminating the signal if passed 2 identical signals.

Does anyone with an o-scope, an X'd amp, and some time to spare want to give it a try?

 

[Nelson Pass]

That's how we get our Input Common Mode Rejection (CMMR)
spec. It is in excess of 60 dB down from what the result would
be from the same voltage applied differentially.