[Dailow]

Hi head-fi gurus,

I need some help wrapping my head around this concept. I did some searching to no avail, just more confusion

First off, I understand that the signal itself consists of hot, cold and ground. Also, your typical connectors are XLR and TRS plugs. My questions are concerning balanced TRS plugs.

One of my searches led me to this page:
Everything you were Afraid to Ask about Cables

One interesting snippet to me:
Quote:

Q) What is the difference between balanced and unbalanced connections? A)TS--tip and sleeve (1 conductor plus ground) is for unbalanced signals. TRS--tip ring sleeve (2 conductor plus ground) is for balanced signals. With balanced cables, you can travel longer distances without picking up hum or noise... Note: It is the signal that is unbalanced or not. Using a TRS or XLR cable does not make an unbalanced signal balanced. However, if you use a TS (or RCA cable with adapters) on balanced signal, the signal will become unbalanced. Keep in mind that both the source and destination gear must pass balanced signals and you must use TRS or XLR cables to maintain the balanced connection.

Taking that at face value, that would make it seem like any standard TRS headphone cable would be capable of carrying a balanced signal. However, the web page also mentions the following snippet:

Quote:

Q) Is a headphone balanced? A) No, it uses a TRS cable but in a different way. A balanced audio signal only carries one mono channel of audio. A headphone jack carries 2 signals for stereo and both these signals are unbalanced. Make sure you get this straight as it is perhaps the most embarrassing way to show off your noob-ness.

Now when this is mentioned, it makes me wonder. In the case of XLR connectors, it would make sense because you have two plugs - one for each channel of audio. But is it possible for headphones to have a balanced signal connected via TRS? According to Wiki, normally the Tip and Ring carry the stereo channels. I'm assuming if that was the case, then it would not be possible to run stereo audio AND a balanced signal at the same time?

Which brings me to another question - assuming balanced TRS connectors only carry one audio channel, you would need two TRS connections to your amp/source. If I bought an adaptor (such as this one at Headroom) that converted 2x XLR to 1x TRS, would that then be carrying a balanced mono signal or unbalanced stereo signal? Or would it be better to just buy two mono XLR-->TRS adaptors to achieve the desired effect?

Thanks for any insight anyone can provide!

 

[JohnFerrier]

One XLR connector will handle Left +, Left -, and ground. A second XLR connector will handle Right +, Right -, and ground. Simply put, inverted signals (+ and -) are used so that common mode noise cancels. This is commonly used for long cables (microphone, guitar, mixer, etc.) during performance.

A TRS connector could handle one channel of balanced signals left (+ and -) or right (+ and -). If you did wish to use TRS for balanced, you would need to use two connectors (just as two XLR connectors are used). It's probably best to use XLR connectors, if you are using balanced signals.

In the balanced to stereo convertor, the - side of each channel is grounded. So, it's not "balanced".

a balance to stereo convertor

diagram depicting noise cancellation


.

 

[infinitesymphony]

Balanced cables have two (or more) conductors in each cable. Your headphone cable only has two conductors, one for each channel. Any time you only have one conductor per channel, the signal is unbalanced.

In order for a headphone to be balanced, it would need two two-conductor cables (two XLR-terminated or TRS-terminated cables), one for each channel. You would also need a headphone amplifier capable of driving a balanced load--it must have balanced outputs. If you want a fully-balanced system, your source or DAC will also need balanced outputs.

 

[sejarzo]

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif Balanced cables have two (or more) conductors in each cable. Your headphone cable only has two conductors, one for each channel. Any time you only have one conductor per channel, the signal is unbalanced. ....

Ummm.....I think I know what you really meant to say, but it didn't come out right, perhaps?

A headphone cable has four conductors...one side is left signal and ground, the other right signal and ground. The grounds that go to each earpiece are connected at the sleeve of the TRS plug.

Balanced cabling always uses a minimum of a twisted pair, one conductor each for the hot or cold/non-inverted or inverted signals; realistically, it almost always includes a braided or "served" shield that is connected to signal or chassis ground. Star quad balanced cables use two conductors for hot and two for cold, plus the shield to enhance noise rejection. Star quad is useful for recabling headphones because it can be easily split into two pairs; the presence of the shield is pretty much irrelevant.

Two really excellent references on the topic are available on the Rane Audio site. They explain the differences and problems that exist with misconceptions about balanced versus unbalanced cabling and the fact that shielding and grounding are different things:

Sound System Interconnection

Grounding and Shielding Audio Devices

 

[infinitesymphony]

Quote:

Originally Posted by sejarzo /img/forum/go_quote.gif Ummm.....I think I know what you really meant to say, but it didn't come out right, perhaps? A headphone cable has four conductors...one side is left signal and ground, the other right signal and ground. The grounds that go to each earpiece are connected at the sleeve of the TRS plug.

Yes! I was thinking about TRS cables in general, which only need two conductors and a shield. I guess it's a special case for headphones, since each driver needs its own ground connection.

 

[Dailow]

Thanks for the replies everyone. You've pretty much cleared up any misconceptions I had

 

[Sovkiller]

The differece between the balanced and single ended in heapdhones cables is just that in balanced mode the ground of both channels is not shared, that's it, the shielding that balanced cables used to reject noise is not 100% a must unless a very nasty environment, and is rarely used...

 

[sejarzo]

You know, a light just went on in my head when re-reading the OP......we might all be answering the wrong question for Dailow, so here goes!

My EE friends in college used to tell me that a prof would always tell them "Remember, voltage is relative, never absolute!" That's important to think about here.

The issue that may confuse you, Dailow, is that balanced cabling for headphones isn't exactly like balanced cabling intended to transfer line-level signals between electronics.

In the case of headphone or speaker drivers, there are always exactly two connections to the voice coil in the transducer--regardless of how it's being driven.

In single-ended mode, the "ground" conductor is connected to an output terminal that stays at the ground voltage of the power supply, and the signal conductor is connected to a terminal that swings both postive and negative voltages with respect to that ground potential.

When driving cans in balanced mode, what would normally be the "ground" conductor becomes the "cold" or inverted conductor, and it's connected to an terminal that is always at the opposite voltage of the terminal to which the "hot" conductor is connected. There isn't any "ground" needed, because the voice coil reacts to the difference in voltage, not the individual values of the voltages present on either end with respect to ground.

If a single ended amp plays a single frequency tone into headphones such that the signal varies from +2V to -2V with respect to ground, the difference across the voice coil varies across that same range.

If that same set of headphones is attached to a balanced amp, the balanced outputs need only swing +/-1V from signal ground to produce the same volume (because when the "hot" is at +1V, the "cold" is at -1V, and the difference is +2v....and when the "hot" swings to -1V, the "cold" swings to +1V, and the difference is -2V.)

The ICEpower monoblocks in my speaker rig actually run the "ground" or black output terminal at roughly 30 volts with respect to the neutral of the power supply, and the "signal" or red connector swings the audio signal from 0V to 60V versus the power supply neutral. If the black terminal is mistakenly connected to the ground of the power supply, the amp can be damaged in a hurry. That's not the case for most SS amps, in which the black output terminal is at the same potential as the power supply ground.

So......"it's all relative."

Gee, I now realize that while I was typing the above, you posted that your misconceptions were cleared up. Hope that didn't re-confuse you!!!!

 

[brainsalad]

As usual, you are right on target, very good sejarzo