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

BTW:  I'm pretty sure you can connect both balanced and single-ended without problem, but I'm also pretty sure you will need to switch the A 21 input each time you switch from music to movies.  But you really should contact Parasound.

PS:  An obvious solution is to buy an A 31 and sell the AVR.  Bidding ends 1/8 on an A 31 listed at eBay. 

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

A possible solution might be to use this on the front L&R pre outs of the AVR.

http://www.bhphotovideo.com/c/product/660217-REG/Gefen_GTV_AAUD_2_DIGAUD_GTV_AAUD_2_DIGAUD_GefenTV_Analog_to.html

This will give you a PCM digital signal to input to your 851D. You would need to set the 851D's VC to a somewhat loud setting, make channel balance adjustments on the AVR, and then use the AVR's VC.

The reason I said possible is due to fact that the signal for the fronts is being converted to digital then back to analog. I am not sure if the front's volume will remain perfectly balanced with the center and rears as the AVR's VC is turned up and down. Hopefully someone else might have an idea if this could work.

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

The answer to the OP original question is NO, unless the AVR is completely disconnected from the A21 when using the balanced inputs.

I have a schematic of the A21, and the #2 pin of the balanced XLR is directly connected to the single end RCA "hot" input. This means any active amplifier connected to the single ended RCA will be directly connected to the (+) output of the amplifier connected to the balanced input.

You should NEVER connect the outputs or two active amplifiers together as they will fight each other. The signal that actually gets to the A21 will be a sum of the two different amplifiers, modified by the source (or output resistance) of each amplifier.