XLR cable acting as antenna?

[Jacques E. Bouchard]

Originally Posted by Greg Miller

Like ground loops, RF overload can sometimes be tricky to cure.

Thanks Greg, very constructive post.


J.

[John Willett]

Originally Posted by Greg Miller

Sorry, but I respectfully disagree.

If an input is balanced, then the input is balanced. Both sides, (+) and (-) polarity, should (by definition) have identical input impedance. Therefore, any equal signal presented to the (+) and (-) inputs should cancel out and not get past the input stage. Even if the cable is open at the far end, the input is still balanced.

But we have to wonder whether the output of the Sennheiser receiver is perfectly balanced, "somewhat" balanced, or unbalanced. If the receiver output wiring is unbalanced (even if it's turned off), that will unbalance the entire circuit on that mic input channel (of the mixer) and that could make that channel much more susceptible to RF pickup.

Also, if the Sennheiser has an active output (rather than an audio transformer) then its output impedance will be much lower when it's turned on. That might, indeed, tend to swamp out some RF that's getting into the system at that point.

Unfortunately, a lot of equipment is not designed with strong RF fields in mind. For example, if pin 1 on the input XLR connector is connected to the nearest point of the chassis, then any RF picked up on the shield of the mic cable will be directed immediately to the chassis and won't cause much trouble. But if pin 1 connects to a wire (hookup wire, harness, whatever) that runs for several inches inside the case, before actually reaching ground, then that internal wire will act like a short antenna to re-radiate any RF that was picked up on the shield of the mic cable.

Many ICs (namely those used for the mic input) are pretty good at common-mode rejection within the audio band, but are not nearly as good when the common-mode signal is at RF frequencies, or a very high voltage. This is one instance where transformer inputs on the mic channels will make a big improvement.

Like ground loops, RF overload can sometimes be tricky to cure.

Yes - this is the correct description of balanced.

All Sennheiser mains receivers are balanced out on the XLR.

The pocket receivers EK 2000, EK 3000 and EK 500 G2 are all balanced out - however, all EK 100 receivers are unbalanced.

[Bill Davis]

Interesting, I guess I've mis-understood some of the finer points of "balanced audio" for a long, long time...

It makes sense, now that I sit back and think of this for the first time in a zillion years, that any RF entering the cable would hit the input stage regardless of the nature of the signal being sent. Somehow in my basic audio classes a zillion years ago, I got this basic concept confused.

Which shows that even old dogs (at least those who are willing) must allow their old tricks to be refined!

Thanks.

[Greg Miller]

Bill, don't fret, you were pretty close.

If the source signal is not balanced, then indeed that will unbalance the entire system. For example, if a wireless receiver has pin 2 hot, and pin 3 ground, then by grounding pin 3, the system will no longer be balanced (even if the input circuit is designed to be balanced), so the system will be more susceptible to noise pickup.

However, if you disconnect the source, and leave the cable open, then the input can function as a balanced circuit, as designed.

And yes, to take full advantage of a balanced circuit, the source should obviously have two pins which carry signals that are 180 degrees out of phase with each other (as you stated). But a balanced source only allows the system to remain balanced, it does not cause it to be balanced. Simple, in a confusing sort of way. ;-)

(Just be glad you aren't dealing with RF, where some signal flows on the inside of the coax shield, and sometimes a different signal flows on the outside of the shield. I'm still having a hard time visualizing that.)