Differential Balanced Sound Quality

Thanks for all the discussion folks.  It's always nice to read a thread that actually educates, or at least presents the opportunity...

@dentdog Do note that the technology that makes power "balanced" is different from the technology that makes amplification equipment (i.e., sources, preamps, and amplifiers) balanced. 

If you care for an explanation, read on. 

Apologies for length, but I'm just now starting to understand all this and wanted an excuse to type it out so that I could consolidate the learning.  Please do offer corrections anyone...

--Balanced Power--

Traditionally, AC power is delivered with 120v on the "hot" line and 0v on the neutral return.  In balanced power equipment, the hot line is run at +60v and the neutral is run at -60v.  

--Balanced/Differential Equipment--

Alternatively, the balanced equipment discussed above separately amplifies the positive and negative parts of the electrical signal.  This requires double the circuitry because you have to amplify two signals (0-to-positive and 0-to-negative) instead of one (negative-to-positive) as in single-ended equipment.   If you imagine music as a sign wave, then balanced equipment is separately amplifying the upper and lower halves of the wave.  The benefit of this added complexity is that any noise introduced into the signal by your equipment will be cancelled out when these two "half" signals are re-combined to make the full-wave signal that drives your speakers. This is because any injected noise will appear positive going in one amplification circuit and negative going in the other... add them together and you get automatic noise-rejection because they'll be in exact opposition. 

--Distortion & Preferences for Balanced vs Single Ended Equipment--

Interestingly, in addition to external noise sources, every amplification circuit also creates its own "ringing distortion." If, for example, you play a sign wave at 1000Hz, the circuit will also produce a signal of reduced amplitude at different harmonics. Thus, you'll see small spikes at 2000Hz, 3000Hz, 4000Hz, 9000Hz, etc. These are called even (2000, 4000, etc.) and odd harmonics (3000, 9000, etc.). The balanced circuitry also eliminates all even harmonics when the signal is combined to re-create the music. This is because their peaks line up directly with the peaks of the original signal and they get cancelled just like any other noise/distortion that affects both the positive and negative amplification circuits at the same time.  

While this is a good thing in principle because your music has less distortion, about 1/3rd of human ears actually find the second harmonic distortion pleasing.  It seems that this harmonic ringing creates the perception of increased warmth and 3-dimensionality in stereo listening.  Alternatively, signals with a dominance of 3rd harmonic ringing is often perceived to have increased clarity and detail... and again is preferred by about 1/3 of listeners.  For more discussion from Nelson Pass, read this link:  https://positive-feedback.com/audio-discourse/the-pass-h2-harmonic-generator/

I believe that variances in preference for these different distortion profiles is likely one reason why you see such different (and strongly held) recommendations about the importance of balanced vs. single-ended gear from different posters. 

While my training as a scientist initially biased me towards a "balanced is better" perspective (why wouldn't you want less distortion???), I have subsequently found that I have a consistent preference for gear with a 2nd harmonic dominant profile.  At this point I'm confident enough in that preference that I'll only buy single-ended gear.  

Sorry for all the typing folks.  I'm bored.  Thanks.

Also, I took the opportunity to read the article I linked. It has lots of good information about the 2nd harmonic, but none about the 3rd. Here’s some more Pass writing that discusses both:

https://www.passdiy.com/project/articles/audio-distortion-and-feedback

And here in the introduction (on p3): https://www.firstwatt.com/pdf/art_ba_3.pdf

@oldears Great quote.  There's someone over on diyaudio who's posting tag is "The power supply of my power supply is my FRIEND!"

I believe the only benefit is that you can connect the circuit to other balanced equipment.  

(Unless you really like the sound of a signal passed through a transformer of op-amp as is used to create a balanced output from a single-ended device.  And some people do swear by the sound of going through some iron.)

@atmasphere Yes, indeed... the half terminology could lead people astray in that way, thanks.  

A related question that has been bothering me.  If your circuit isn't perfectly duplicated after signal splitting, doesn't this introduce timing errors upon recombination? 

I see a lot of people building balanced designs over on diyaudio, but don't see much talk about controlling wire lengths, and otherwise perfectly matching the two amplification circuits.  Obviously it's not a deal-breaker given the experienced success, but I can't help but imagine it's introducing error if there are any post-splitting differences...

Thanks.

@atmasphere Ralph, apparently we posted at the exact same time.  Any facts of operation I got wrong in mine?  As always, thanks for your contributions...