Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

Dear almarg:
Your posts and advice deserves historically (at least from my perspective) a complete and proper answer.

If I am interpreting correctly (and I’m not sure that I am), I believe Luis is saying that completely disconnecting the "looped," shielded, and balanced preamp-to-amp cable from the preamp (not just disconnecting the shield) resulted in hum and buzz. 

Correct

While doing the same thing with an unshielded cable resulted in buzz but no hum. 

Not exactly, the unshielded cable poses no hum and no buzz (on the true balanced connection) as tested

And replacing the "looped," shielded, and balanced DAC-to-preamp cable with an unshielded one essentially resolved the remaining buzz. 

Correct

Also, I believe that when Luis refers to a shield being "looped" he simply means that it is not connected at the corresponding end. 

Also correct, looped means the cable shield is not connected to ground at one end as usually should be, but connected to itself at the ends by means of an external wire or outer added shield over the cable jacket which is isolated from everything else. JSSG for wire loop or JSSG360 for external braid loop.

If those interpretations are correct I would not attribute the differing results to differences in capacitance

Well I'm not an EE but somehow I thought the cable jacket will resemble and act as a dielectric and the shield will get "charge" as a capacitor hence increasing the Mogami speced capacitance for such cable, but I could be wrong on my assumptions.

I can certainly envision that a cable of any type hanging off of the input of a component, while not being connected to a signal source, could result in hum as a result of EMI effects

My actual testing with an unshielded cable showed no hum or other effect. Of course to have rigorous testing I should have tested a shielded cable not looped over itself but shield connected to ground at the ends "mea culpa"

As I and others have said in past threads it is often ***very*** easy in audio to attribute an observed difference to the wrong variable. And that is especially likely to be the case when the observations involve a very limited number of components, cables, and circumstances. I believe that in this case differences in cable capacitance were not the cause of the observed differences. 

You are correct I did not "used" all the available variables, now the shield loop in the cable it is my understanding it will affect capacitive coupling per the article below

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwio...
Note that I am not a John Swenson detractor, it is completely feasible that the looped shield on certain specific applications (DC cables, signal cables, digital cables etc) and with certain signals/currents (DC, AC, digital streams etc) could provide some "enhancement" (I would even dare to say tone control) on certain cables and possibly be of detriment on others.

I think we have deviated enough from this thread :)

My conclusion for balanced connections on my equipment (per manufacturer's recommendations) standard Mogami console cable with no tricks will do the trick :)
And this is one of the reasons I wanted to go "seriously" into analog as well, digital is "so convenient" but "so hard" to get it right and there is just no real reference on what to look for that I decided to listen to both (without spending a fortune)
OMG I hijacked this thread, my apologies to the OP, I will shut up now

luisma,

This shielding "concept" is flawed and I am going to assume comes from a lack of understanding of what is happening. The person who came up with it is giving himself a bit too much credit.


https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwio...


When you run a signal through one "loop", and measure the induced voltage in another magnetically coupled loop, what you have done is created a transformer.

• Leave the shield floating, and you just have a piece of "metal" in the middle. It will do nothing beyond a bit f magnetic shielding.
  
• Ground one side of the shield, and you now have an electrostatic shield. It will break the capacative coupling from one loop to the other in the transformer.
  
• Add an external wire to form a loop, or complete a loop as in the experiment, and you have now created a "shorted winding" in your transformer.
  

So what is the problem with this?

• Transformers work both ways. If it is a shorted winding to the noise source, it is also a shorted winding to the signal in the wire. That’s not a good thing.
  
• Since the shield is floating, it provides no electrostatic shielding which is of course also important.
  
• On a "normal" shield, the inner conductor and the shield are actually both "windings" to external magnetic fields, and will have similar induced voltages which can be negated with differential inputs.
  
• Twisted pairs, and star-quad already reduce magnetic coupling, so adding electrostatic shielding addresses both noise sources.

Twisting a pair of wires works great against capacitive or electromagnetic pickup, exposing both wires evenly to electric or magnetic field.  That way induced noise currents are exactly even and cancel.  It works fine as long as twist is even and its pitch is much shorter than the wavelength of offending signal.  Shield in addition to twisting adds more protection against electric or magnetic field by being a Faraday cage, but even some induced currents flow on the surface (shield) only if frequency is high enough (skin effect) while some are lost as eddy currents.  Both Faraday cage and skin effect becomes less effective for electromagnetic radiation below 100kHz.  Combining both provides good protection against low frequencies (twisting) and high frequencies (shielding).  Of course keeping cables short is very important, since cable as an antena for electromagnetic field becomes very ineffective when shorter than 1/10 of the wavelength.  If shield creates ground loops, then I would at least ground it at the source end only.  It is much better, than not having shield at all, IMHO.

kijanki,

My last post was specific to the link luisma31 posted. I am not disputing anything you said, just not sure you were aware I was making a specific response to that.

roberttdid, Of course.  I posted only to state that shield, IMHO, plays important role in defense against electrical noise (when connected properly).  I also strongly agree with you that shorting open ends of the shield with wire creates large loop that picks-up electrical noise.  Noise currents in such loop produce magnetic field that will "transform" noise into signal wire.