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

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.

dbphd, you said:

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.

I also have an Oppo bdp-105. Mine directly feeds into a Parasound A51 amp. Since the Oppo has XLR outputs only for the main R/L channels, the Oppo's surround outputs must necessarily connect to the A51's unbalanced inputs. Using that mixed combination, I have the balanced/unbalanced switch set to balanced. Would it be better to just use unbalanced connections on the main R/L channels? In that case, the XLR's would not be used at all and there would be no question as to where to set the balanced/unbalanced switch.