Chuckled because I thought you were joking. Laughed harder because I realised you weren't. Amazing idea!

@MathematicalOrchid try taking a look at Kirchoff's Voltage Law - it should hopefully help you to understand why you can't divide a voltage with just a single resistor, and is normally taught in conjunction with RC networks (in my experience anyway)

I thought though as in my second circuit I take the signal post X1 before removing the offset (which will be half supply) the NONIN_IN can be grounded through the terminating network of resistors and capacitors. That way even if there is no signal at V3, the non-inverting input to X3 is always 1.55 volts (due to the potential divider network). Maybe I have completely misunderstood....I am a little out of my depth here.

Hi Andy, thanks so much for your help so far. As you can see from the original question I have added a modified circuit with the suggestions you made. It works fine when there is only one source generator, however if a signal is present from the other side of the line the output from X3 is still just my offset voltage...any ideas?

Thanks for your answer Andy. I understand that, but the problem that I'm having is getting the diff-amp to operate on a single-rail supply, rather than a dual-rail. Also, would this work with impedances other than 600 Ohm? I'm building it for an EU phone line, which uses a termination of 370ohm + (620ohm||310nF).

@AlfredCentauri except that 'resonant' does not apply to the frequency, but to the construct which is resonating - it is the 'frequency at which the construct is resonant', as such, the 'resonant frequency [of the construct]' is perfectly valid and correct. Also, as a side note, our language evolves through the acceptance of idioms which become common usage. Try insisting that no-one calls a 'vacuum cleaner' a 'Hoover'

@NickAlexeev For mass production, I assume they don't apply the glue by hand though! That was my logic behind saying don't glue it - not that it's a bad thing to do, I know it's done all the time. It was that if you're glueing it by hand, then you'll need the same amount of care (if not less because of the solder-resist on the board) just to hand solder it

+1 for the most dangerous thing being a short circuit. Can't count the number of times I've accidentally put one probe a bit too close to both Vcc and Gnd! - it never stopped the circuit from working though