[Scott Dorsey]

My sig gen signal only appeared before R20, and on the output pin 1.

I recall learning about virtual ground in this topology.

** Yes, that is what you had.

Think about it this way. R20 and R24 are identical, and both are coming
from sources that have effectively infinite current available.

Pin 1 is reverse polarity from the input, so pin 2 is part of a summing
junction with equal parts of the input and inverted input summed together.
So in normal operation there's nothing there.

On the other hand, pin 3 has the inverted input coming in from the source,
so you should see signal there. R21 and R25 just form a divider to halve
the voltage.

It's not really a very good differential input topology, because you wind
up having to use fairly high value resistors in order to get the same load
impedance on both sides (which you want for noise rejection). But it works
well enough for the job and it only takes one op-amp. Line level is high
enough that noise isn't a concern.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."