Parasound A21 question - can I use both balanced and unbalanced input on the amp?

I agree with dbphd’s first response, suggesting that you contact Richard, but not with the first part of the second response.

While there may be a few amplifier designs for which that would work, with most designs it would definitely not be a good idea. In many and probably most amplifier designs which provide balanced and unbalanced inputs, one of the two signal pins on the XLR connector is internally connected directly to the signal pin on the RCA connector. When the balanced/unbalanced switch in those cases is set to unbalanced it simply shorts the other signal pin on the XLR connector to ground, so that the amplifier’s input circuit which receives that signal is not left in an unconnected and uncontrolled "floating" state. And when the balanced/unbalanced switch in those cases is set to balanced the switch does nothing.

The fact that the specified input impedance of the A21’s balanced inputs is exactly twice that of its unbalanced inputs reinforces the likelihood that it is designed in that manner. As opposed to having separate input stages for the balanced and unbalanced inputs, with the switch selecting which of those input stages is connected to the rest of the amp (that being what dbphd was apparently envisioning) .

Assuming the design is as I suspect, the result of connecting to both inputs at once would not be harm to the amplifier. But it may very well be harm, eventually if not sooner, to the AVR and/or the 851D, since their outputs would be shorted together. And at best the results would definitely not be optimal sonically.

But contacting Richard, as was suggested, certainly can’t hurt.

Regards,
-- Al

That looks like a very promising suggestion, Tony (Tls49). I don’t think that the concern you expressed would be an issue, as I would expect the numerical values of the digital output of the Gefen A/D converter to be closely proportional to the levels of its analog inputs. If that were not the case it would cause low level harmonics to become grossly out of balance with high level fundamental frequencies, and volume differences between soft notes and loud notes to be significantly altered. And, likewise, I would not expect the D/A conversion performed by the Cambridge DAC to cause any such issues.

The main uncertainty, as I see it, is likely to simply be the sonic quality of the Gefen converter. But given that it would just be used for movies, and that it only costs $55, it certainly seems like an approach that is well worth considering.

Best regards,
-- Al

By the way, I’m still a bit confused about Richard’s response. I am, in fact, only planning to use either the AVR or the 851D at a given time. So when listening to music, the AVR would be off and the 851D would be on, and vice versa for movies.

Is there something I’m misinterpreting in Richard’s email?

Even if you make a point of never having the two components turned on at the same time, the output of whichever component is being used would be loaded by the unknown and uncontrolled output impedance the other component has when it is turned off. Depending on the specific designs that impedance may be low enough to result in adverse sonic effects, in part because when the 851D is being used an imbalance may occur to a significant degree in the impedances and/or voltages and/or noise levels of the balanced pair of signals it provides  (since only one of the signals in that pair would be connected to and loaded by the AVR’s output).

Also, depending on the specific designs injecting a signal voltage into the output of an unpowered component could conceivably degrade its long term reliability, and result in premature failure.

So if I understand it correctly, the setup will be something like this?

AVR (RCA pre-out)--> Adatper --> 851D (optical input)
851D (balanced out) --> A21 (balanced input)

Yes, that’s correct. Alternatively, you could also try connecting the adapter to the 851D via coax, and comparing the resulting sonics with the use of the adapter’s optical output.

Good luck. Regards,
-- Al