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

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.

Thanks for the suggestions. Based on dbphd’s suggestion, I emailed Richard Schram. At first I got an automated reply saying that Parasound is closed until Jan 3, so expect a delay in response. But after a few hours, I was pleasantly surprised to see an email from Richard. This is pretty freaking unbelievable. Wow, now that’s what I call customer care.

Here’s an excerpt from his email:

The Bal-Unbal switch on the A 21 isn’t actually an input selector. Its purpose is to optimize the signal to noise ratio for each type of input. If you leave the switch in the Bal position you can most likely leave both the 851D and AVR connected at the same time. It should work ok if you simply switch off the unit you aren’t using. However, since you’re probably using your 851D with the AVR you don’t have the option of switching the 851D off and you’d hear the player both via the A 21’s XLR input and the AVR pre out via the A 21’s RCA input. This wouldn’t damage the A 21 but I don’t think you would get the results you want.
I just briefly looked on the internet for a simple switch box which accepts XLR and RCA inputs and I didn’t find exactly what you need that wouldn’t degrade the signal quality. I’m sorry we’ll lose you as a customer.

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?

tls49 and almarg, sorry I just now read your responses. Using an analog-to-digital adapter is a great suggestion. I think it should solve the problem very nicely. And I agree with almarg that even if there's some degradation of SQ while watching movies, it's not a big deal for me. Eventually, I might just buy a newer AVR that supports digital pre-outs.

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)

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