Even in an all-digital facility, there is the need for analog audio to enter the picture somewhere. And analog can bring noise and hum induced from long lines acting as antennae, or caused by ground loops. Enter the Ebtech line of Hum Eliminators.
The HE-8-XLR, with eight channels of isolation and elimination on XLR ins and outs, is reviewed here. For less than eight channels, there is the two-channel HE-2-XLR unit.
The quick-fix for a ground loop has been to break or float the ground connection at one end of the cable. This introduces many problems — especially if buried inside a rack or cabinet and forgotten — and is never recommended on an unbalanced line. The Ebtech Hum Eliminator isolates the ground, the inputs and the outputs from each other, and rebalances the signal for the next stage.
The Hum Eliminator also minimizes the antenna effect, as stray 60/120/180 Hz noise induced on a long cable from all around cancels out once it hits the balanced input. For us, it was a vintage Shure mic mixer feeding a camcorder that gave us problems. Plugging the mixer through the HE-8-XLR on the way to the camera audio in socket cleared things up fine.
The isolation transformers used inside scope out to 70 kHz — a good two octaves better than FM quality. Published THD measurement is less than 0.005 percent; quite good for a passive device. Amphenol XLR sockets are used exclusively, and the housing is cold-rolled steel for durability in the rack and on the road.
More than just a set of 1:1 transformers, the Hum Eliminator includes specialized passive circuitry to keep the transformer from ringing, and a balancing circuit ahead of the primary winding to redistribute incoming signal voltages and assure a ground potential of 0V going in.
Placement of the Ebtech unit in the audio path is important — the unit will not remove noise already introduced further upstream, but stops it where the problem is happening. Work your way backwards through the audio chain to find the offending piece, then drop a Hum Eliminator in line before the next device.
If you are bringing in analog audio somewhere in your path, keep an Ebtech Hum Eliminator nearby.
Price: HE-8-XLR (8-channel): $319); HE-2-XLR (2-channel): $125
For information, contact the company at (800) 284-5172 or visitwww.ebtechaudio.com.
Alan Peterson, KJ4IVD, is a 20-year contributor to Radio World and the assistant chief engineer for the Radio America Network in Arlington, Va. Reach him firstname.lastname@example.org.