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

In my second system, I would like to double the duties for both HT and music. For preamp, I'm using a Cambridge Audio 851D, which unfortunately doesn't have any HT bypass or even analog inputs.

Originally, I was thinking that I might be able to connect the pre-outs from my AVR to the 851D. And then from 851D to Parasound A21 via balanced/XLR connection. But the problem is that my AVR has only RCA pre-outs and the Cambridge has only digital inputs.

Another option that comes to mind is something like this:
1. Cambridge 851D --> Parasound A21 via balanced/XLR -- for music listening
2. AVR --> Parasound A21 via unbalanced/RCA -- for movies

Is this even possible? Assuming it is, can this cause any potential harm to the amplifier if both balanced and unbalanced inputs are connected to different sources? What should I set the switch (Balanced/Unbalaned) to on the A21?
Send an email to [email protected]  Richard Schram, the CEO, is very helpful and prompt in replying.  

The A 21 is a very nice amp.  I use mine with balanced input from an Oppo 105 to drive a pair of KEF LS50s with excellent results.  The Oppo does double duty as a disc player and processor, and does it well.

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.

-- Al

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

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.

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

Thanks Al, for your comment on my suggestion. This type of adapter can be found for less than $10 on either one of those huge online shopping sites, however, I chose to show the Gefen because I have experience with a few of their video products and they always worked very well with no signal degradation. The Gefen was the only one that showed specifications for SNR, THD, Frequency Response, and Crosstalk, which are reasonably good. Also, B&H Photo has a 30 day return policy.

Arafiq, if you want to look at the cheaper ones, just make sure it is the A to D, and not D to A. The majority of the cheaper adapters available are D to A. I did notice a review about someone being sent the wrong one.

And if you try this, I hope it works well, and glad to help.

almarg, thanks for the detailed explanation. I didn't realize the impact of impedance even when the unit is turned off. I looked at a few other adapters on amazon but it seems that the Gefen recommended by tls49 is a better choice. It's a bit more expensive but looks to offer more in way of quality and has decent reviews. 

However, having said that, I am now in two minds whether to just stick with the Parasound P5 and return the Cambridge 851D to AudioAdvisor as I'm still within the 30 day return period. I only bought the 851D thinking that the DAC is considered to be much better than the one in the P5. Since I mainly stream my music, it made a lot of sense. But the HT bypass, bass management and tone controls on the P5 are features that are quite useful. I don't know if improvement in the DAC quality is worth losing the other features of the P5. Tough decision.