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

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.    

I was not saying that shorting the shield with a loop will pick up noise kijanki. It will act as a shorted turn and will direct an external magnetic field away from inside conductors to a degree. However, it will also act as a shorted turn to the current carried by the internal conductor, which could seriously impact it's performance.