Electrical – connect 2 audio inputs to 1 output using an audiosplitter

The simplest way to balance an unbalanced signal is with an isolation transformer. Unfortunately, a transformer that works well across the entire audio frequency range is going to be expensive ($>100 from Jensen).

Another way to do it is to use a chip similar to the one you found from THAT Corp. Is it overkill, probably. But it'll be cheaper and perform better than a transformer (assuming that you don't really need the isolation that a transformer will give you).

The "typical" way would be to use two op-amps. One is set as a mostly-unity gain non-inverting buffer and drives the + input of the amp. The other opamp is set as a unity-gain inverting buffer and takes it's input from the other op-amp and drives the - input on the amp. Vary the gain on the first op-amp if you need to.

Of course, the real question for this is: do you need to use a balanced signal or can you get away with going unbalanced straight into the amp? Of course, a balanced input will give you better audio quality but the op-amps or THAT chip will degrade quality. If you don't do a good job, the net effect could be a decrease in quality. And then you have to wonder if the change in quality (for good or bad) is even noticeable to your ears. It's very likely that simply going unbalanced into the amp is going to be the better choice.

While I cannot provide a complete answer to your question, I think this info should be helpful for you in understanding the nature of the line requirements of the Plain Old Telephone Service (POTS) which normally drives the old style rotary phone. I think your project is quite intriguing and hope you will post more about your progress.

The POTS line, with all phones on-hook, should measure around 48 volts DC. Taking a phone off-hook creates a DC signal path across the pair, which is detected as loop current back at the central office. This drops the voltage measured at the phone down to about 3 to 9 volts. An off-hook telephone typically draws about 15 to 20 milliamps of DC current to operate, at a DC resistance around 180 ohms. The remaining voltage drop occurs over the copper wire path and over the telephone company circuits. These circuits provide from 200 to 400 ohms of series resistance to protect from short circuits and decouple the audio signals.

To ring your telephone, the phone company momentarily applies a 90 VRMS, 20 Hz AC signal to the line. Even with a thousand ohms of line resistance, this can still pack a bit of a shock so be careful when you are probing around trying to find a POTS line.

POTS Line Characteristics: Bandwidth: 180 Hz to 3.2 kHz

The low end is rolled off early to stay away from the 60 Hz power line noise region. The high end cut off is more critical. Voice on the telephone network is digitized at 8 kHz sampling rate which means that any signal above 4 kHz will be aliased back as noise in the voice band. Most voice CODECs roll off at about -25dB at 4 kHz with a -3dB down point around 3.2 kHz. The phone company decided years ago that the 180 Hz to 3.2 kHz range would be sufficient for speech intelligibility while allowing them to multiplex many calls over coax and twisted pair.

Signal to Noise: Approximately 45 dB

This is not as easy to quantify because noise comes in many forms, such as electrical interference from fluorescent fixtures or hiss from the many amplifier stages in the voice path. Speech correlated noise can be introduced from non-linear speech coding and compression algorithms. Crosstalk from other conversations is another form of noise. The phone company uses 8 bit mulaw nonlinear coding which yields about 12 bits of dynamic range. The bottom line is that you can never count on more than about 45 dB signal to noise ratio.