Fine Tunes #10

Last month (click here for previous Fine Tunes) I tipped you on how to check the AC plug orientation for transformer current leakage—the best sound is often found at the lowest voltmeter reading. Roger Skoff of XLO suggests another technique that's worth passing on to you, if only because the imagery is so . . . piquant: Leave the speaker cables hooked up and pull the interconnects from the power amplifier. Turn it on and "stick your head in the speaker," as Roger puts it, checking for the level of hum. (Imagine a pair of bony audiophile legs waving crazily out of the bell of an Avantgarde Acoustics horn speaker.)

Reverse the polarity of the amplifier's AC plug and stick your head in there again. Whichever orientation gives the lowest hum level is the one to use. Connect the preamplifier to the amplifier and do it again, this time reversing the preamp's AC plug. Same again for your digital front-end. And if you're of the analog persuasion, do your phono preamp (assuming it's a separate chassis) and turntable. I guarantee that even modest high-end systems will benefit from the exercise.

Remember—you're trying to minimize the voltage potential between the chassis of the components in your system and ground. Current flows from high to low voltage as it equalizes the potential. The further apart the potentials, the more they'll "talk" to each other through the common ground. Lowering current leakage from the transformers is one way to attack the problem directly rather than just treat the symptom.

Speaking of which, let's get something straight, as I begin to chart and demystify one of the biggest pains in the collective butt of audiodom: Gremlin Grounding. Some of the underlying causes of buzz and hum can be traced back to the 120V AC power line, but be advised: a little knowledge can be dangerous. Unless you know what you're doing, call an electrician. Don't go rooting around your socket or breaker box if you haven't a clue. I do not want to hear about anyone Slam Dancing at 120 volts. Really, don't fool around, and be careful. (It's happening. I'm becoming my parents! Arghhhhh!)

So what, exactly, is that pestilential buzz? Ground loops occur when you have two paths to ground in your system of unequal resistance. The small difference in chassis voltage induces a current flow that causes hum. By plugging the normal 3-pin AC plug of one of the problem components into a 2-pin cheater plug, you "lift" its ground, thus breaking the loop. But for safety's sake, the ground wire of the cheater should be attached to the cover-plate screw of the AC outlet or another good ground.

If you "lift" a preamp and ground the amplifier, the preamp is, in effect, connected to ground by the interconnects. But think about it: Do you really want your interconnects carrying a current flow across their shields so close to the conductors? I think not. Anyway, the preamp is the center of the system, dealing with the lowest-level signals. It is a desirable point at which to make the overall system ground.

And, he said, waggling his fingers in warning, if you disconnect the lifted component's interconnects, it's obviously no longer grounded. If, through some fault or failure, high voltage appears on the chassis as you ramble over and give it a hug, ZOTTT! Careful . . .

Another source of hum and buzz are the rectifiers that convert AC to the desired DC voltage to power a component's circuitry. These are nasty little buggers, radiating lots of high-frequency splash. Rectifiers conduct only during a narrow portion of the AC cycle (120 times a second), explains BAT's Victor Khomenko. The pulsing diodes create spikes that can be heard as buzzing. "Rectifiers have to be strong to charge typically large capacitors very quickly," he continued. "That type of noise is pervasive and covers a broad spectrum, so it's very easy to hear. All that switching on and off sends the chassis potential jumping all over the place on amps, and even preamps!" That's one reason even a big amplifier notching or flattening the AC waveform with current draw may be less harmful to the circuit than a chorus of ground loops and rectifier noise riding the ground plane.

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