question about Balanced signals.

@dukebdevil I got that. I was pointing out that in addition to the cables, there's a benefit in the electronics itself if fully differential and balanced.

@atmasphere  sorry was trying to sum it up without going into details (because that is where we tend to make our technical mistakes).  In this last post, I should not have said "cable" .  As you clearly are aware, it is the whole design that contributes to less noise relative to signal (conversely higher signal to noise ratio).

@dukebdevil There are other advantages too- for example, differential amplifiers are more immune to power supply noise and they will have, in theory, a 6dB better noise figure than the same circuit executed single-ended. 

@atmasphere Thank you for the xlr cable suggestion.  I too like to save money and even if on short runs, an unbalanced cable is able to sound "clean", that cable is likely to be much more expensive to get similar performance to the balanced path.

audio-gd responded that "The preamp convert the SE input to balanced output.".  

without getting into the weeds, it seems there is one highly desirable outcome of balanced/differential circuits.  A balanced cable will have less noise, giving a higher signal to noise ratio.

@dukebdevil If there is a 6dB increase in gain by going balanced, then the equipment does not support the balanced standard (AES48). The reason this is important has to do with your opening question about the best interconnect for the money.

If your equipment supports the standard then the interconnect will cease to be important. It will simply work and sound correct. And if the equipment supports the standard then there will be no 6dB gain increase as well.

There are three ways to do a balanced output: output transformer, a solid state circuit that can sink current and float at the same time, and the method we patented using a Circlotron. Inductive sources, like a phono cartridge are balanced sources. So think about this: if you apply a phono signal to the input of a differential amplifier, will it make a difference if one side of the signal is at ground? The answer is ’no’ the total Voltage is exactly the same whether one side is at ground or not.

A phono cartridge can meet all the requirements of AES48 as a result. Does this clear things up?

So with this all in mind, Mogami Neglex is a good balanced cable for not a lot of money (I use them in my home system) that will keep up with any high end audio balanced cable of any price so long as your equipment supports AES48. To me this is a big deal since high end audio cables can cost north of $1000/foot! My cables at home are 30 feet long and cost about $300/pair. If $1000/foot I’d be looking at $60,000!! For no difference in sound that’s pretty significant. Capishe?

@rauliruegas thank you for locating that info.  agreed, everything I have read about Balanced says you gain about 6dB of gain over unbalanced, not lose.  this helps explain why we were confused when trying the PSA Dac Jr as a preamp.  it was too wimpy with SE, someone suggested that it should be stronger with balanced so I got a pair of XLR and it was even lower output.  I guess to Rotel, dual mono (who isn't) and the inclusion of XLR jacks with a "balanced" or "unbalanced" switch is "Rotel Balanced". 

The best man to answer your question is Atmasphere. He practically made 'True Balanced' equipment the norm when most manufacturers only offered RCA.

Bob

Dear @dukebdevil :  I have  serious  doubts that your amplifier be a TRUE balanced design as you posted. Please read these:

" The voltage gain (into 8 ohms) was 28.4dB via the unbalanced RCA jack. This is slightly on the high side for a THX-certified design, but 6.7dB lower via the balanced XLR jack, which is very unusual. "

That was measures by in real time by STHP review ( J.Atkinson. ).

Additional to that in the manual operation Rotel stated:

" Rotel's balanced design. "

A true balanced design is only that a true balanced design but not Rotel's or Luxman's or whatever manufacturer name.

I think could be better that you forget about your OP with this amplifier.

R.

All of those criteria are met in a proper balanced circuit. 

What is the purpose of a balanced circuit. If you want it to reduce hum and noise you need a differential input. The signal does not have to be differential but both legs need to have the same output impedance to ground. The input must have the same impedance on each leg relative to ground but should be lower than the input impedance of the driven device.

Hmmm.  I've not come across that before.

mashif is correct. You would need the first balanced device to have a transformer at its input so as to convert the SE input to balanced.  It would then pass a balanced signal to the downstream balanced device.  But most often, the inputs on balanced devices that can also handle an SE input will simply amplify the SE input; the negative phase is internally grounded for SE and the positive phase (of the balanced circuit) carries the signal.

Depends on the preamp. Some convert to balanced and some just pass a single ended signal down two legs of the XLR. 

my question is regarding sources? what if they only have SE output to the preamp SE inputs?  will I at least get some benefit of balanced signal from the preamp? 

Sure.  They will be carried balanced from preamp to power amp.