SE vs Balanced volume

As I stated before it provides excellent noise rejection and that is the purpose of XLR connection. Amplifier does not have to be fully balanced to take advantage of inherent XLR noise rejection (and most of them are not). Some manufacturers use input transformers (that add distortions at low frequencies) while others use instrumentation amps. Instrumentation amps are widely used in measurement equipment to provide better noise immunity. As for the use of op-amps - many wonderful amps, preamps and DACs have them, including gear from Jeff Rowland.

Like I said the only time balanced is an advantage is for noise rejection, but is only an advantage over rca with very long interconnects, and there is no sound advantage, if anything it’s a worse sound in this fake balanced connection case above with an opamp in the signal path, the $2 INA134 was never designed for high-end audio use, more like PA audio.

Cheers George

 I agree that if you don't need noise rejection you might use RCA input.

To answer OP question - It is true balanced input stage with gain of +1 and normally you should get 6dB gain. Perhaps there is some gain compensation (divider) after this instrumentation amp?  I would also suspect that your preamp might provide +1/2 and -1/2 instead of +1 and -1 outputs, but it is not likely since you already stated that it sound louder with tube amp.  Again this Instrumentation amp has gain of one and reponds to the signal difference between inputs. If you feed input with +1 and -1 then it will produce +2 (+6dB) on the output.

Given Steakster’s mention that the solid state amp is described as being fully balanced, a possibility that hasn’t yet been considered is that the INA134, rather than converting balanced input signals to an unbalanced signal, might be used to invert the unbalanced input and thereby provide the rest of the amp’s circuitry with a balanced pair of signals when the unbalanced input is used. That would be consistent with his observation that the balanced and unbalanced inputs provide the same gain, as well as with the “fully balanced” description of the amp.

George indicated that the whitish cable appearing at the right side of the photo appears to be a one conductor shielded cable. That could certainly be the case, but it is also possible that it is a two-conductor shielded cable, conducting a balanced pair of signals. For example, something like Belden 8761 (although that particular cable is a different color), which incorporates a pair of 22 gauge conductors and a shield yet has an overall diameter of only 0.175 inches.

And note that one of the suggested applications listed in the INA134 datasheet is "unity-gain inverting amplifier.”

Also, given that three wires are connected to the XLR/RCA switch it is possible that the switch is wired to select whether XLR pin 3 or the output of the INA134 is routed to the subsequent circuitry in the amp. With XLR pin 2 and the RCA center pin being connected together and to the other polarity of the subsequent circuitry.

Just a thought, and a possibility as I said. If what I’m speculating is the case, though, using balanced cables between that amp and the preamp would certainly seem likely to be the best way to go.

In any event, Steakster, do let us know what make and model the amp is, which may allow us to provide further insight.

Regards,
-- Al

"Some manufacturers use input transformers (that add distortions at low frequencies) while others use instrumentation amps.

True but there are ways to reduce the distortions using input transformers.  Depends on the design.

"As for the use of op-amps - many wonderful amps, preamps and DACs have them, including gear from Jeff Rowland."

True again but that depends on what you consider "wonderful"

Happy Listening.