SE vs Balanced volume

"So what your saying then is the RCA input uses the IN134 to provide an extra 6db of gain to keep both inputs the same level, but there’s no feedback network to set the gain, and as the data sheet says it’s fixed gain at unity."

No. The IN134 is not used to get an extra 6db. If you have a fully balanced component that needs to work with other single ended gear, you need to configure it to do so. Think of it as an adaptor. Also, you can't run a fully balanced component SE and Bal at the same time. The circuitry changes. You can't just use 2 pins of an xlr connector to make a SE connection. The pins do different things. If you've ever seen a component that has a little jumper connecting 2 pins of an xlr cable, that's why.

"Also then wouldn’t the switch have to have to be more than just a simple two pole switch? "

Since there's only 3 signal path/pins for balanced, the only change you can make is a simple 2 way switch. You're going from 3 pins to 2.

"If not then and I’m wrong the rca is at a disadvantage having to go through the opamp."

Correct. The opamp is another component in the single path, so it has to have some effect. In many cases, though, the complete opposite is true. If you have a singled amp but want it to work with balanced cables, the opamp is used on the xlr and not the rca.

Sorry Al. You put your last post up while I was typing mine, so I didn't see it. You're much better at explaining this type of thing than I am, so I'm sure you would have had to clarify some of my posts anyway.

because the signal coming in on the RCA connector together with the inverse of that signal provided at the output of the INA134 enables the input board to be provided with a balanced pair of signals.

That means one phase only going to the "balanced" input board is going through the INA134, as it’s clearly just a single ended output, pin 6 on the data sheet. If that's the way it’s no wonder some think the balanced sound better. This should be called fake single ended. Why sacrifice sound quality of the rca for the sake of the same gain.
http://circuits.datasheetdir.com/164/INA134-pinout.jpg

I can see it using the AD8476.
http://www.mouser.in/images/microsites/low-power-diff-con-fig01.jpg

Cheers George

No problem, Sfall. Explanations stated from a second perspective can only add clarity.

Georgehifi 3-12-2017
Why sacrifice sound quality of the rca for the sake of the same gain.

Some fully balanced architectures will not work properly unless they are provided with a balanced pair of signals. For example, some or all ARC Reference series power amplifiers, which do not provide RCA connectors, are designed such that only half of their balanced signal path will see a signal if their XLR connector is provided with a signal on pin 2 but pin 3 is grounded. In that situation the result would be a drastic reduction in power capability, as well as various adverse sonic consequences. So in this case there may be a more important reason for the approach I’ve envisioned than just adding gain for the RCA input.

As Sfall said, if my hypothesis is correct the use of the INA134 can be thought of as an adapter. And it could be viewed as analogous to the many designs having unbalanced internal signal paths in which a balanced pair of output signals is created by means of an op amp configured as an inverter. Is that an ideal approach? No. But it is a simple and inexpensive way of providing a feature that can be useful in some systems.

Regards,
-- Al

George, it is interesting that you picked AD8476 that I use in my projects in 5V applications to convert single ended output to bidirectional A/D converter  (ADS1271) shifting common mode by 2.5V