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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?
128x128arafiq
tls49's avatar
tls49
1,351 posts
 
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.
almarg's avatar
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almarg
9,670 posts
 
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

dhl93449
455 posts
 
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.


arafiq's avatar
arafiq OP
1,599 posts
 
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.




arafiq's avatar
arafiq OP
1,599 posts
 
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?
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