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Understanding balanced headphone connections

Discussion in 'Sound Science' started by Karister, Oct 18, 2019.
  1. Karister

    I am trying to understand current flow in balanced and unbalanced headphones connection. I have read numerous articles on this topic and threads here but I am still wondering some things. Best article I found is this one: https://www.headphones.com/pages/balanced-headphones-guide. I also read this thread: https://www.head-fi.org/threads/con...lr-balanced-output-to-headphones-work.830205/. But I am still hungry for more info.

    I would like to start with stuff that I understand:

    Unbalanced connection with 3pin/TRS (amp) - 2x 2pin/TS (HPs) cable. There are two circuits (for left and right channels) with separate hot wires and common return wire.

    Balanced connections for DACs/AMPs/etc. Each channel has two hot wires with reversed signal polarity and return wire. Receiving device has OpAmp that reverses reversed signal back to original. This leads to doubling the amplification power and cancels noise (as it is not inverted in any of hot wires).

    What I don't understand:

    Balanced connection od HPs to amp. I believe HPs are passive element and do not have any OpAmps to reverse signal. So it is like a simple circuit with amp and load (HPs). I read that advantage here is not having shared return wire for L and R channels. Why?

    If I understand correctly, each channel is powered by two amps that produce inverted signals (thus wires ale also return ones for each other). But the reason for this is not noise cancelling but better performance of amps that do half of work each. Am I correct?

    What about noise then? Let's focus on one channel. Let's say that one amp pushes 1V to a wire and another amp -1V to another wire. So voltage is 1V-(-1V)=2V. Will noises from both wires cancel or sum up? Let's say something produces 0.1V noise in both cables. Will those cancel from both cables [1V+0.1V-(-1V+0.1V)=2V]? Or add due to opposite flow direction of current in both wires [1V+0.1V-(-1V-0.1V)=2.2V]?

    Another stuff I do not get is different number of pins in various cables. Aqudioquest Nighthawk has TRS to 2xTS cable. Sounds fine, it is simple unbalanced connection. Hifiman Arya also has unbalanced connection but with TRS to 2xTRS cable. What's the point of additional ring in 3.5mm jack plugs?

    Similarly for balanced cables. Focal Clear has 4pin XLR to 2xTS while Hihiman HE-1000 uses 4pin XLR to 2xTRS. Again, why additional ring? It is even more complicated when i look into aftermarket cables. There are balanced cables offering TRRRS to 2xTS or TRRRS to 2xTRS. 4pin/TRRS to 2xTS sound reasonable to me. Two separate circuits for each channel with no shared wires. But I do not get idea of stuff like TRRRS to 2xTS or TRRRS to 2xTRS.

    Hope someone is able to clarify this. I am really interested in how it works.
  2. Acke
    Return wire isnt used by the HPs in a balanced (i.e bridged) connection to the amp, only hot and cold, you can have a shield which can be connected to return, thats all.
    The return is only required between a balanced amp input and balanced DAC output.

    The correct term for this kind of amp is a bridged amp and it can have single ended inputs with an internal circuit to make an inverted copy of the signal or a ''native'' balanced input

    In one channel, the positive and negative amps will each add unique noise to the audio signal, which will not cancel. but any external noise picked up by both hot and cold signals will cancel passively at the headphones.
    However even though you will have twice the noise from the 2 amps combined, the SNR will not worsen because you also have twice the amplitude.
    While the noise of the 2 amps wont cancel, some of the 2nd harmonic distortion of the hot and cold amps will cancel, though I dont understand the science behind it. 2nd harmonic distortion is the kind of distortion associated with sounding warm and pleasant, but it is distortion nonetheless and only clouds the music beneath.

    There are different possibilities with balanced interconnects and bridged amps which may yield better results depending on the situation.
    For example, if you are using very very long balanced interconnects between the amp and DAC, and you also are using a bridged amp, that means all noise picked up by the balanced interconnects will go straight into the hot and cold amps and possibly degrade performance. The noise will still get cancelled at HPs/speakers, so you wont hear it directly, but you could hear the side effects it may have had on the amp.
    So in this situation, the balanced signal from the DAC could be unbalanced at amp input to cancel this noise, and then immediately ''rebalanced'' by making an inverted copy of the signal so the bridged amp has the signals its needs to operate. you have the benefits of both balanced interconnects and bridged amplification... the downside? you now have 2 more stages in the signal path that will add their own noise and distortion.
    For Home use, considering you already have shielding on balanced interconnects just like single ended interconnects, there is really no need to do this... instead you can reap all the benefits of a bridged amp with native balanced signals from the DAC: most simple signal path (less internally generated noise+distortion), bridged amplification (lower distortion from headphones due to better amplification, lower 2nd harmonic distortion) and external noise cancellation.

    Also know that all modern DAC chips have differential or native ''balanced'' outputs, any DAC with a single ended output will have the same unbalancing circuit inside that you'd find at the balanced input of a single ended amp, so dont think single ended output to single ended amp input has a simpler signal path than balanced DAC to balanced amp input.
    OTOH The implementation and quality of this unbalancing circuit will vary and could well be worse at the balanced amp input than the DAC, and for short runs there is much less benefit to balanced interconnects, which are bulkier and more expensive than a single ended interconnector...

    There is no reason for them using 2XTRS and I believe Hifiman must use them because they were cheaper than TS plugs or something.
    In theory if you were building a custom cable with 2xTRS plugs and added shields you could solder the shields to spare terminal on the plug to keep it secure, but it would have no electrical effect and doesnt explain why they are on Hifiman cables.

    the extra ring is for the return in TRRRS if the cable had a shield, 4 pin XLR actually has another contact on the outside of the connector for the return.
    Last edited: Oct 20, 2019
    Karister likes this.
  3. Karister
    Thank you! Very helpful. I would like to ask for a bit more explanation on one thing that remains unclear to me. Regarding custom cables build.

    Did you mean balanced connection here?

    Ok, so shield is soldered to additional ring of TRRRS. And assuming you meant balanced connection above, additional soldering shield to dual TRS mini jacks gives no benefit. It sounds like TS is always sufficient for mini jacks on HP side for both single-ended and balanced connections. Am I correct?
    Last edited: Oct 20, 2019
  4. Acke
    yes and yes
    Karister likes this.

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