SE vs Balanced volume

And also Al while were waiting for the OP's answer, I use two s/s amps with fully balance discrete input stage. This amp has rca inputs and xlr inputs with a switch between the two, no opamps involved. When switched to rca it just put’s pin 3 of the xlr to ground.

Cheers George

I use two s/s amps with fully balance discrete input stage. This amp has rca inputs and xlr inputs with a switch between the two, no opamps involved. When switched to rca it just put’s pin 3 of the xlr to ground.

Yes, that approach is used fairly widely, of course. And since the difference in voltage between the two input lines would be twice as much when XLR pins 2 and 3 are provided with a balanced pair of signals, compared to when pin 3 is grounded, there will be a 6 db difference in the gain of the amp between the two conditions. But in the case of the OP’s solid state amp there is no difference in gain when the XLR input is used (and provided with a balanced pair of signals) compared to when the RCA input is used, which was the reason for his question that started the thread.

Regards,
-- Al

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. Also then wouldn’t the switch have to have to be more than just a simple two pole switch?
If not then and I’m wrong the rca is at a disadvantage having to go through the opamp.

Burr Brown INA134
FEATURES ● SINGLE ● FIXED GAIN = 0dB (1V/V) AUDIO DIFFERENTIAL LINE RECEIVER ● SUMMING AMPLIFIER ● UNITY-GAIN INVERTING AMPLIFIER ●

Cheers George

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

No, that is not what I am saying. Again, please carefully re-read my earlier posts.

My hypothesis, as previously stated multiple times and clearly understood by Kijanki, is an attempt to account for the fact that the amp provides the same gain from its XLR and RCA inputs, while apparently being fully balanced. Under that hypothesis the INA134 is not being used to provide 6 db of gain, it is being used as a unity gain inverting amplifier, which as you indicated is one of the ways it is intended to be used. But that use results in the power amp’s overall signal path having 6 db more gain from its RCA input than it otherwise would, 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. With the voltage difference between those two signals being essentially the same as the voltage difference between the balanced pair of signals that would be provided to the amp if an XLR connection were used between the preamp and the amp (for the same setting of the volume control).

And under that hypothesis, the switch would be a single pole double throw type, which as I stated earlier multiple times would select between the output of the INA134 and XLR pin 3, depending on whether the switch is set to the RCA position or the XLR position.

And yes, under this hypothesis the RCA input is at a disadvantage sonically, at least theoretically, since the INA134 is in the signal path when the RCA input is used but is not in the signal path when the XLR input is used (aside from the slight loading presented to XLR pin 2 by its input impedance). I and Kijanki both stated that earlier.

Regards,
-- Al

"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.