Cable Question: Why Balanced (XLR)?

[SiliGoose]

This is just something I don't know:

Why do XLR cables exist? What do they do that normal RCA type cables don't do? Are they much more expensive? (I assume so since the balanced MC12 is much more expensive than the 'regular' one)

Are they only used in amp to preamp connection or do high-end components (CD players, DVD players, etc.) have balanced connections too?




-Sili

[LarryB]

Sili:

I am not an EE but will answer your questions as best I can.

The advantage of blanced interconnects is reduced noise. As I understand it, in a balanced configuration the waveform is split into two halves, an upper and lower. Noise, due to its random nature, will affect one half, but not the other so by comparing the two, noise can be distinguished from the real signal.

The answer to your second question is easier: Yes, high-end CD players most definitely have balanced connections.

I should also mention that some components have balanced inputs/outputs but are not actually balanced throughout. (Kinda' sneaky of them.)

Hope that helps.

Larry

[SiliGoose]

Interesting. That's a pretty cool idea.

Thanks!




-Sili

[Kevin P]

Unbalanced cables have two conductors (or one conductor and a shield). The shield doubles as both a signal ground and a shielding ground.

Balanced (XLR) cables have two conductors plus a shield (or 3rd conductor). Two conductors carry the signal, and the third is the shield. This allows the ground to be isolated from the signal path, or in the case of ground loop problems, the ground can be lifted on one end of the cable to break the loop, without affecting the signal. In other words, the signal's ground and the shielding ground are independent.

XLRs are popular where the signal level is low and more susceptible to noise, such as microphones. The connectors are also more robust than either RCA or 1/4" phono plugs.

KJP




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[JonMarsh]

Here comes the fifteen cent explanation (didn't think it was really worth two bits- a quarter


Balanced cables, such as XLR line or mic cables, carry the signal as a set of differential signals (opposite polarity) on two conductors. As described above, the shield is not connected in the signal path, and in fact may be connected only at one end. Then, there is no potential (voltage) induced from current times resistance in the sheild.

Stray magnetic fields (such as hum from a power transformer will induce a common mode voltage in both signal conductors, which, if the conductors are a reasonably tight twisted pair, will be equal. The voltage is common to both conductors, hence why it is called a common mode voltage. As long as the circuit is well balanced in impedance, the influence of the common mode voltage on the net differential mode voltage (actual signal) will be very low, becuase it is in phase, not out of phase as the differential signals are. The ability to reject this common mode voltage is called common mode rejectionl, and tight balance of the circuit impedance is necessary to assure this, particularly if active balanced receivers are used, as opposed to transformers.

Transformers have VERY high CMRR (common mode rejection ratio), and contrary to common opinion, they can be very wideband- for example, in my PXO crossover used with my X-1 Klones, the transformers have a bandwidth of 0.1 Hz to over 10 MHz. No, that's not a typo. I said Megahertz. Therein lies part of the issue; good transformers aren't cheap compared to op-amps- au contraire, they're pretty expensive. But they can work great, and properly implmented, a good balanced system will have great SN and great EMI rejection. Some equipment is built largely around that goal, such as Jeff Rowland Desgin Group.

Balanced signalling systems aren't just used in audio- they're also used in high speed digital connections, such as a balanced differential SCSI bus. They convey the same advantages, including better immunity to noise on the ground system.

One of my CD players (the XA7ES) has a balanced output as well as unbalanced. I have a Denon preamp which has balanced and unbalanced inputs and outputs.

Many pro amps have balanced inputs, but convert the signal to a single ended ampflier with a single output polarity. Some amplifiers, like all Ayre power amps, and the Aragon Palladiums, are fully balanced true differential amplifiers input to output. This does cost more, but it balances the power supply behavior, and reduces noise pickup and distortion. A few preamps, like the Ayre models, are true balanced differential preamps input to ouptut, but many convert the balanced signal to a single polarity at the inputs, and then add balanced output drive. This saves money, but isn't probably as effective.

Some DAC converter chips have balanced outputs; the switching noise is in phase, but the output signal is differential- out of phase. Ask me sometime why I'm designing a balanced DAC buffer....

Regards,

Jon




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