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

@kijanki I'm fine, buzz and hum are gone, the point I was making is that shield although indeed blocks RFI / EMI could create other problems which balanced lines really don't need.

There is little, if any, advantage to XLR on short runs.

danvignau

There is little, if any, advantage to XLR on short runs.

That’s a pretty broad statement and it isn’t clear exactly what you mean. As I’ve already pointed out, the use of XLR connectors does not necessarily mean that the unit is true differentially balanced.

There is great potential advantage in using differentially balanced circuitry and connections in a phono preamp. In addition to the increased immunity to noise, there’s a 6 dB increase in gain. That’s a huge advantage in a phono section!

As a counter to that cleeds, and not taking anything away, it is also true that because an RCA connection is used, does not mean that the input is not true differential.

With great gain, comes great responsibility. But seriously, w.r.t. phono, differential gives you the ability to increase gain while decreasing common mode noise, but you are still amplifying differential noise as well as noise internal to the amp. You are not getting 6db for free. You could always crank the gain 6db on a single ended connection, which again, for a cartridge, is already pseudo-differential due to the floating source.

roberttdid4

As a counter to that cleeds, and not taking anything away, it is also true that because an RCA connection is used, does not mean that the input is not true differential ...

Quite so, and I made that same point earlier in the thread (fifth post). The Audio Research Ref Phono preamps are examples of RCA connections into a differentially balanced preamp.

... differential gives you the ability to increase gain while decreasing common mode noise, but you are still amplifying differential noise as well as noise internal to the amp. You are not getting 6db for free.

That's a fair point. There's no such thing as a free lunch. It's still 6 dB.

You could always crank the gain 6db on a single ended connection ...

Actually, no you can't "always" - unless you change preamps. At maximum output you do gain 6 dB with a differential circuit.

Except with a phono input, you don't gain 6db, as it is an application specific input. It amplifies the difference between the two leads. There is no "differential" output that has 2x the signal level (6 db).

roberttdid

Except with a phono input, you don’t gain 6db, as it is an application specific input. It amplifies the difference between the two leads. There is no "differential" output that has 2x the signal level (6 db).

I do not know what you mean by "application specific input" or why this matter isn’t more clear to you.

My phono preamp gain is adjustable and is spec’d up to a maximum of 68 dB single ended and 74 dB balanced. That seems consistent with my experience.

 To clarify, I can only add the specs for my Ayre P-5xe: the gain for the balanced output is 6 dB higher than single ended out for each of the 3 gain selections. However the gain does NOT change with the choice of RCA or XLR inputs. You should be able to look up the specs and recommended phono cable wirings on the Ayre website in the manual download.  The specs are minimal and no S:N specs are given.

oldears

...the specs for my Ayre P-5xe: the gain for the balanced output is 6 dB higher than single ended out for each of the 3 gain selections. However the gain does NOT change with the choice of RCA or XLR inputs ...

Thank you! Exactly. That’s been one of my points from the beginning of this thread; the choice of connector has nothing inherently to do with whether the circuit is balanced (or not) or differential (or not).

And one of the best places to use differential circuits is in the phono section, for both higher gain and better immunity to noise.

I have a very fine Audio Research PH-2 that is balanced in and out....And it works like it should.........

Luisma31 7-9-2020

... playing with the Mogami interconnects I noticed no hum, not even the faintest hum with the XLR in place BUT when unplugged one of the XLR’s from the preamp and amp on, disconnecting from the pre there was a nasty hum, this is not supposed to happen, then I remembered when ordering these Mogami’s interconnects everyone said better to be shielded and at no cost I got them JSSG (looped shield at the ends), there was one guy don’t remember where online which told me before all this, for balanced interconnects never get these shielded but since it was only ONE guy I did not.Replaced the balanced interconnects (between pre and amp) with unshielded ones and when unplugging one end the hum was gone, but the buzz was still there.My source DAC was connected to the pre via XLR shielded, looped interconnects and even with the DAC OFF there was buzz, replaced these and the buzz was gone almost completely,

... I described hum with a shielded balanced cable when unplugged from the pre side, when an unshielded cable was used there was no hum, my understanding is the shield adds capacitance and IMO is not needed and undesirable.

Kijanki 7-9-2020
You keep saying "unplugged from pre side" but you probably mean that shield was disconnected, otherwise what are you listening to if nothing is connected to pre?

@luisma31 @kijanki

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. While doing the same thing with an unshielded cable resulted in buzz but no hum. And replacing the "looped," shielded, and balanced DAC-to-preamp cable with an unshielded one essentially resolved the remaining buzz.

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

If those interpretations are correct I would not attribute the differing results to differences in capacitance. Consistent with one of Kijanki’s comments I cannot envision a means by which high interconnect cable capacitance (within reason of course, and the capacitance of Mogami, even in stock form with the shield not looped, is certainly well within reason) could result in hum or buzz. But on the other hand 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, depending on the design of the specific component and the specific cable, as well as on the surrounding EMI environment. And likewise when the cable is connected at both ends but the source of the signal is turned off (assuming the component providing the signal is not one of the relatively few designs in which the output is grounded via a relay when the component is turned off).

Also, while Luis describes the shields as being "looped" I’ll mention FWIW that in a conventionally designed balanced cable (i.e., one in which the shield or some other kind of conductor connects XLR pin 1 at one end to XLR pin 1 at the other end), the resistance of that shield or other kind of conductor can significantly affect the degree of hum and buzz that may result from ground loops. That would also depend on whether the components connect pin 1 to chassis (as they should) or to circuit ground (as is often incorrectly done), as well as on the relation (i.e., the impedance between) circuit ground and chassis ground within the components.

The bottom line: 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.

Regards,
-- Al

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.    

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.

roberttdid, Sorry I read it wrong and I understand argument about shorted turn, but transformer works both directions. Any closed loop will pick-up ambient electrical noise.  Induced electric charge will flow in this loop thru/along the shield producing magnetic field around signal wire, that will induce electric noise current in it.

In the experiment presented, and the "concept" for shielding presented, their is an external magnetic field noise source. As the shorted loop is passive, and it's current is only due to the external magnetic noise field, the field it would create cannot be larger than the field already created by the magnetic noise source. It cannot effect the other wire more than the noise field would have already. It would direct the field away from the wire.

Perhaps it works, but I would think that it can do more harm than good in comparison to plain grounded shield.  High frequency noise currents induced in the signal wire flows on the surface (skin effect) - shield to ground.  I'm not sure replacing it with shorted turn will be better, but who knows.  I'm just skeptical, but if it works it works.

Oh absolutely it will do more harm than good. It acts as a shorted loop to the signal wire, which means a parasitic load on the signal wire, bigger than anything else one could do.  It also provides no electrostatic ground. It is a terrible idea .... some "cable" guy came up with it :-)