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Cable immunity and lower noise

Posted by Ralph (M) on July 24, 2012 at 10:55:36

In Reply to: What is the benefit of balanced inputs/outputs posted by tz7 on July 23, 2012 at 19:44:00:

Should be the correct answers.

If you have ever heard a 3 foot cable make a difference in your system, then you know what I am talking about. In theory, balanced operation means no cable artifact at all.

In practice, you still can hear differences but they are very slight compared to single-ended cables if the standards of the balanced line system are adhered to by the equipment manufacturer.

Unfortunately, in high end audio, adherence is very rare. If adherence was common, single-ended would have long ago been relegated to the entry level of the market.

Here are the standards:

1) pinout: pin 1 is ground, pins 2 and 3 carry the signal in inverted and non-inverted form. USA industry standard is pin 2 is non-inverted.

2) the signal is driven between pins 2 and 3; ground is ignored- instead ground is used for shielding only. Termination standard is 600 ohms.

3) The circuit is low impedance.

#1 is so things hook up right.

#2 is so signal current does not go through the shield. This reduces the artifact of the cable quite a lot, and is one of the areas where most high manufacturers don't keep with the program.

#3 is also to reduce cable artifact by swamping out minor capacitive and inductive effects that are the result of the construction geometry of the cable. This area is also a problem for most high end audio manufacturers. The termination standard is what the preamp can drive; to do that its actual output impedance will be a lot lower.

In effect this means that the cable can also be very long and expense of the cable will have little to do with how it sounds.

Even if the cable is only 3 feet long this can still be a benefit! An excellent example is LOMC phono cartridges, which are a low impedance balanced source- if the cable is set up according to these standards, you will not be hearing any difference between a $200 cable and a $2000 cable.
Try that with single-ended!

The lower noise is obtained both by the common-mode rejection ratio of the equipment (the ability to reject signals that are the same on both the inverting and non-inverting inputs, like a hum field imposed on the cable) and also by the internal noise-rejecting aspects of the balanced circuit. It is also lower distortion as IM is much lower due to superior power supply noise rejection and even ordered harmonic distortion is gone.

So you can expect a fully differential balanced preamp to be very low distortion and very neutral compared to the same topology in a single-ended embodiment.

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