Convert XLR to TRS Headphone Cable Issue

@052rc 

I’ve confronted several manufacturers on this, and every one of them reluctantly admitted I was right

I am 100% with @cleeds on this one, unless you can get Musical Fidelity to agree with you .in writing.

Here is the confusion.  The headphone amplifier outputs a fully balanced signal for headphones, using two 3-pin XLR connectors, one for each channel. Each channel uses two wires to carry the balanced signal, for a total of four active wires, probably in two separate cables.

Unfortunately, as a bit of a gimmick, it also allows a standard headphone jack to be inserted into the middle of either of the XLR connectors. I presume this is a three-section jack designed to carry a non-balanced stereo signal over three wires (left, right and common).  So when the OP plugs his jack in, he gets unbalanced stereo.  My educated guess is that it does not matter which side he uses (left channel may become right, who knows)!

To get the undoubted benefits of truly balanced operation, he needs to rewire each side of his headphones directly to its own 3-pin XLR connector and completely remove any jack.

It is London to a brick that when Musical Fidelity says one of its products is fully balanced, its active circuitry is doubled from input to output. 

Ideally the attached player should output in balanced mode too.  Then the signal path from the output of the player to the coils of the headphone will be fully balanced.

Note:  balanced means the signal is duplicated on two paths, where one path is the opposite polarity of the other.  The combined signal is twice the magnitude of an unbalanced signal.  More importantly, Electro-Magnetic Interference (EMI) acts equally and oppositely on each path, and is cancelled out in the combined signal.

Balanced circuitry, explained clearly and concisely by Uncle Paul: 

https://www.psaudio.com/blogs/pauls-posts/true-balanced

There's a half dozen consecutive posts on the subject.

052rc

I made 2 main points in my post. The first is most people fall for the balanced headphone scam. You clearly fell for it. The second was the technical reasons to back it up. I’m not changing a word of my post because every word of it is correct. You fell for the advertising. Instead, get the schematics for this MF headphone amp, or any other one that claims to be balanced, and then we’ll see who knows what they’re talking about. But I know this will never happen. Once you see how it works, You’ll have to admit you were wrong. I’ve confronted several manufacturers on this and everyone of them reluctantly admitted that I was right.  

That is most remarkable! You have a very special way of assessing how balanced lines work and how effective that can be. Good for you! That you have convinced "several manufacturers" of your views is especially impressive.

I’ve long considered Bill Whitlock to be a particular authority on this topic. As an expert, I’m sure you’ve seen his AES paper "Balanced Lines In Audio Systems: Fact, Fiction, and Transformers," which is free to AES members here. It’s peer reviewed and as an AES member I’m sure they’d welcome your comments. If you’ve let your AES membership lapse, you might be able to find a copy of the paper at academia.edu. (You may need to sign up for an account.) 

Another excellent treatment by Whitlock is "Design of High-Performance Balanced Audio Interfaces." Also peer reviewed, published by AES and open for comments. A free copy is here, at least for the moment.  

Given your insistence that balanced audio is: 

... a marketing gimmick. There is no such thing. The people that make this stuff use the term balanced, put xlr connectors on the gear and hope you don’t ask too many questions ...

... I’m sure the professionals would welcome your thoughts. Perhaps they will be more accommodating than me. For those who might miss your AES comments, please also report back here. Have a Great Day @052rc.

Oh boy...

@052rc 

Please do as @cleeds says.

I think your second level of confusion is caused by marketing.  There is a world of difference between the claims of balanced and claims of fully balanced.

Balanced inputs and outputs can be achieved by adding extra components to accept or generate an additional inverted signal at the input and output stages of a component.  Fully balanced means that the entire signal path through the component is duplicated.  This pretty obviously increases both the cost and the ability to reject EMI.

An example, which mixes both balanced and fully balanced in one component, is my SoulNote phono stage.  This supports Moving Magnet (MM), Moving Coil (MC) and DS Audio optical cartridges.  MM signal levels are typically more than 10 times higher than MC, and DS Audio optical are more than 10 times higher again than MM. 

Most cartridges and tonearms inherently support 4-wire balanced operation, and the MC section of the stage is fully balanced (with a pair of 3-pin XLR connections for input). 

DS Audio optical cartridges need a DC power supply, so two of the wires are used for that, meaning there are insufficient signal wires for balanced operation.

The MM input uses the same balanced connectors as the MC input, but SoulNote has decided that the extra cost of fully balancing the MM input is not worthwhile. 

RCA inputs are provided, but they just hook to two of the three XLR input pins, so they cannot be used simultaneously.  Output of course is balanced via two 3-pin XLR connections.

@wncrob 

The headphone amp is model MX-HPA...it was sold as new and heavily discounted.  ... I’m beginning to think that there is an issue with the AMP. ... It’s at the selling dealer today to be analyzed ... but I doubt it can be "fixed" for XLR functionality, and a replacement is likely to have the same issues.  

I am positive there is absolutely nothing wrong with your amp, but it seems the dealer has little idea of how it should be connected to realise its sonic benefits of fully balanced operation.  Maybe this is why it was heavily discounted 

Ultimately, Musical Fidelity will have to educate the dealer.  As I posted earlier, you need to connect your headphone to both XLR outputs, one for the left channel and one for the right.  If you post your headphone make and model, we can probably work out exactly how the headphone wiring can be modified.