Audioclinic (Apr. 1983)

Passive Radiators

Q. In a loudspeaker system, what is a passive radiator?

-Gary J. Arnold; Elk Grove, Cal.

A. A passive radiator is a speaker cone, with a difference. Normal speaker cones are driven by power from the amplifier, applied to their voice-coils.

Passive radiators have no voice-coils, or other motor parts, but are moved by the back wave from a driven cone, usually the woofer (though there are some midrange passive radiators). This makes it less an extra driver than a substitute for a vented enclosure's reflex port; its advantages over an open port include the elimination of possible wind noises and pipe resonances, plus some additional control of port loading through the mass and compliance of the passive cone. The passive radiator is also sometimes referred to as a drone cone in this country, and is known as an auxiliary bass radiator (ABR) in Britain.

S/N with Shorted Inputs

Q. Why do manufacturers measure amplifier signal-to-noise ratios with the amplifiers short-circuited?

-Bob Robinson; Warrensville Heights, Ohio

A. The source impedance of the device driving a power amplifier is not known. Therefore, to make it possible to repeat a measurement by technicians located in any laboratory, such measurements have been standardized so that all signal-to-noise measurements are made with the input of the device short-circuited. This will make the signal-to-noise ratio higher than it would be with an open circuit at the input. Most devices which ordinarily feed into the input of a power amplifier have such low output impedances that they can be considered as virtual short circuits.

The Rack that Hummed

Q. I have secured my system's components in a metal rack, using round-head bolts. One of my amplifiers hums in this setup, the amount of hum depending on how tight the bolts are! If the amplifier is removed from the rack, the hum also disappears. Is this magic? Why does it occur with only one of these amplifiers? And why does reversing the a.c. plug reduce this hum somewhat?

-Steve Kandell; Santa Monica, Cal.

A. Do you mean a physical hum coming from the rack, or an electrical hum heard through the speakers? From your description, I'd say you had some of each.

A physical hum is usually caused by a loosely assembled or mounted power transformer. If the rack resonates at the 60-Hz a.c. power-line frequency 'or some multiple thereof, it will amplify these vibrations and help radiate them into the room. The more tightly the amplifier is coupled to the rack by its mounting bolts, the better its vibrations will be transmitted to that rack. Try tightening the bolts which hold the transformer together, and those holding to the amplifier chassis.

If it's an electrical hum, you probably have a ground loop between the amplifier and some other component, conducted partly through signal cables and partly through the rack. Insulating the humming amp from the rack may help. So might disconnecting the shields at the preamp end of the cables feeding signal to the power amp.

Equalizer Hazards

Q. I am very interested in an equalizer, but I have read that equalizers can damage a power amplifier by the excess power demand at the boosted frequencies. Is this true?

-John S. Burwell; APO San Francisco, Cal.

A. An equalizer, in and of itself, cannot damage an amplifier. But you can use it in such a way as to damage your amplifier or speakers, by overdriving them. This will not happen, of course, at frequencies where you're using the equalizer to reduce the system's output. But trouble can occur if you're using a lot of boost.

One common use for equalizers is to make up for insufficient bass output from the speakers, by boosting bass with the equalizer. If overdone, this can damage the amplifier or speakers (though you'll generally hear signs of distress, such as bass "doubling" or distortion, before permanent damage is done). Boosting of the lowest equalizer band is more likely to affect the speakers than the amplifier, as there is rarely enough musical content in the bottom octave or two to make the amplifier overheat; it will have time to cool between bass passages.

You're unlikely to cause amplifier damage by boosting mid and upper frequencies, as there's little power there. You can, however, damage some tweeters by over-boosting the treble.

The same cautions hold true, to some extent, when boosting the frequency extremes with conventional bass and treble controls.

Off-Center Pressings

Q. Occasionally I encounter a disc with an off-center spindle hole. I even have one record with an off-center hole on only one side. I would think that this would not occur if the stampers for the two sides were simply made concentrically during the record molding. Why hasn't this problem been solved before now?

-Don Lewis; Sunnyvale, Cal.

A. Off-center spindle holes are produced before the molding process.

The condition is created during the production of the stampers used to press records. When making stampers from master lacquers, the concentric lock groove (cut beyond the outer edge of what will be the finished disc) should be perfectly aligned, using a microscope mounted on a special table and then aligned for zero motion as the table is turned. Next, a punch produces a hole which is supposed to be at the exact geometrical center of the stamper's grooves. When the maker of the stamper is careless, we have off center spindle holes.

I recently had a master blank which had such a large center hole that the disc, when cut, was not centered, would not have given much thought to such a circumstance, knowing that the stamper-maker should not refer to the existing center hole but, rather, to a concentric groove. However, because of the maker's carelessness, the final disc got as far as a "test-pressing" before it was rejected and all necessary work redone.

If the stamper-maker does his job for one side of a disc and is careless with the manufacture of the other side, the result will be a disc with an off-center hole on only one side.

(adapted from Audio magazine, Apr. 1983; JOSEPH GIOVANELLI)

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