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 |
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 |
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 |
Read PS: above.
The amp in question is the solid state amp, not the tube amp. Regards, -- Al |
Sorry Al, but you assume the amp itself is fully balanced, where and who said this.
It was stated in the original post at the start of the thread, and reaffirmed in the OP's post just above. Regards, -- Al |
Hi George,
Please carefully re-read my various posts above. As I have explained in precise detail multiple times, and has Kijanki has also stated multiple times, **if** the amp is in fact fully balanced, as claimed, and if the rest of my hypothesis is correct, when the amp is provided with a balanced pair of input signals via its XLR connector there would be no need for the INA134 to be in the signal path at all, and setting the XLR/RCA switch to "XLR" would remove it from the signal path. Aside, that is, from the slight loading effect its input impedance would present to the signal on XLR pin 2.
And when the RCA input is used, the balanced pair of signals provided to the amp's input board would be comprised of the input signal on the RCA connector together with the inverse polarity of that signal, generated by the INA134 configured as an inverter.
I’m not sure how to explain that any more clearly than I already have.
Regards,
-- Al
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Hi George,
If you are referring to the three solder pads that are above R1, as far as I can tell those aren't necessarily where the cable is connected. And I could be wrong, but I was thinking they might be for the RCA connector.
Regards,
-- Al
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In that case this amp is in the signal path for RCA input, but not for the XLR. Exactly! To be sure it's clear to others, "amp" in this sentence refers to the INA134, not to the entire amplifier. Best regards, -- Al
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Al, it is possible that they use this amp to invert signal, but why?
What are they going to do with two signals of the same polarity?
Hi Kijanki, Not sure what you mean by this. I have not referred to two signals of the same polarity. What I am describing is a possibility that the amp has a fully balanced signal path, and when its RCA input is selected the signal from the RCA connector together with its inverse (generated by the INA134 used as an inverter) form a balanced pair of signals that is provided to the rest of the circuitry in the amp.
It seems much simpler just to use INA134 as differential amplifier and
use SPDT switch to select between its output and RCA input.
But if the amp is fully balanced, as has been claimed, this would not provide the rest of its circuitry with a balanced pair of signals.
In addition, summing of the signal when only half of it passes thru inverting op-amp compromises CMRR at higher frequencies.
I have not referred to any signals being summed. But yes, if the amp is fully balanced and only one of the two signals provided to the rest of its circuitry goes through an inverter when the RCA input is used, that can be expected to result in some degree of sonic compromise. But arguably some degree of sonic compromise will result with any fully balanced amp if it is provided with an unbalanced input. The bottom line is that my hypothesis would explain why the amp may be fully balanced, as has been claimed, while at the same time providing equal gains regardless of whether its RCA or XLR input is used, as the OP has reported finding. Best regards, -- Al |
It maybe Al but not used as such when the fake balancing opamp only has a
single ended output. Only if the opamp was the INA2134 not a INA134
George, note the following statement in my previous post:
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.
So if my speculation is correct, when the RCA input is selected with the switch one conductor in the whitish cable would be conducting the output signal of the INA134 (with the INA134 being used as an inverter for the RCA input, not as a balanced-to-unbalanced converter), and the cable would be conducting the signal from the RCA connector in the other conductor. When the XLR input is selected the output of the INA134 would not be used, with the signal from XLR pin 3 being selected by the switch and used instead.
The result, if that speculation is correct, being that even though the amp may be fully balanced it
would provide the same gain regardless of whether its RCA input or its XLR
input is selected.
Regards, -- Al
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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
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