Audioclinic (Dec. 1983)

Flickering Lights

Q. I have a receiver rated at 50 watts per channel. I have a small lamp on the same household circuit. Every time I turn on the receiver, the lamp flickers.

Is there any danger to the house from turning on the receiver? Is there any device which can eliminate the flickering? What causes the flickering in the first place? I read that such flickering is a sign of bad wiring, but so far no circuit breakers have tripped.

-Robert Patterson; Des Plaines, Ill.

A. When your receiver is turned on, there is an instantaneous surge of current until the back EMF is established in the power transformer--at which time the receiver draws its normal current. This is quite normal.

Any time that current flows in a conductor (your house wiring in this instance), there will be a voltage drop across that conductor. The greater the current flow, the greater this voltage drop will be. Because the current flow is momentarily high when the receiver is turned on, the voltage on its circuit will be low. Any device connected to this same circuit will also experience low voltage for an instant.

If you have a light-gauge extension cord connected to your wall outlet, and both the receiver and the lamp are connected to this line, this can account for the lamp's flickering, even though your house wiring may be adequate.

With no extension cord in use, a 50 watt receiver should not draw so much current that the voltage drops enough to cause the flicker. You should investigate the gauge of wire used to feed that outlet, replacing it with a gauge at least one size heavier. This will be especially desirable if your house was originally wired with aluminum wire.

When such wiring ages, junctions and splices tend to oxidize, leading to either intermittent circuit operation or high-resistance connections (which will cause excessive voltage losses with light loads). The fact that your breakers do not trip is not an indication of the quality of house wiring. When a breaker does not trip under load, it only indicates that the current drawn is below the rating of the breaker. In rare cases I have seen breakers which, because of repeated tripping, fail to trip at their proper time-requiring greater and greater amounts of current to trip them. If the only equipment on your circuit is the receiver and the lamp, do not be concerned about a defective breaker. The current flows involved are below values needed to trip a normal breaker.

Signal Processors

Q. Is there a limit to the number of signal processors one can put into a tape loop without degradation or distortion of the signal? In what order should one hook up an equalizer, range expander, decoder, and rumble filter in a tape loop?

-Roger Ross; Peshastin, Wash.

A. I do not believe we can say just where the limits are to the number of processors which can be cascaded in a single tape loop. By the very nature of signal processing, however, distortion is added. If the idea of high fidelity is to reproduce faithfully all sounds fed into the equipment, an equalizer distorts the sound by upsetting its frequency balance. A range expander distorts the sound by tampering with its dynamics. Many people consider distortion only in terms of IM or THD and the like, but any alteration of signal is distortion.

A typical playback loop with the equipment you mentioned would be connected from beginning to end as follows: Decoder, rumble filter, range expander and equalizer. By altering the relative positions of these devices, the range expander (for example) might be fooled into acting on a light cymbal crash because of its increased high-frequency content because of treble boost added by the equalizer. The equalizer, ahead of the decoder, will give a false idea of original levels and will result in poor rendition of the original dynamics and perhaps even of frequency balance versus instantaneous program level.

High-Pass Filter

Q. What is a high-pass filter?

-William Kay, Yonkers, N. Y.

A. The high-pass filter reduces response at very low audio frequencies to reduce rumble which may be present as a result of problems with the turntable or with a disc recording. The 78-rpm recording was especially prone to rumble. Such a filter also serves to reduce the amplitude of low frequency pulses which will be present when playing moderately or severely warped phonograph records. Even though no sound may be produced as a result of the warps, the subsonic pulses may cause excessive woofer cone excursions which, if severe enough, could damage the woofer.

Some of these filters begin rolling off low frequencies at a sufficiently high frequency as to reduce bass from such program sources as organ recordings.

If you do not need the filter, don't use it.

75- and 300-Ohm Connections

Q. Please explain the difference between the "hookup" of a 300-ohm antenna to a TV set versus a 75-ohm coaxial antenna.

-S.R. Creacey, Redlands, Cal.

A. Most TV sets (and many FM tuners) can be connected to either 75- or 300-ohm antenna systems.

The 300-ohm system, which is more common, uses flat "twin-lead" cable, the ends of which are stripped to be wrapped around the two screw terminals provided on the receiver. The system is a balanced line, with both sides of the line equidistant from ground, so you may connect either conductor to either input terminal.

The 75-ohm system uses coaxial cable whose outside shield is grounded.

On TV sets, the cable usually attaches via a screw-on "F" connector; prefabricated cables usually have such connectors already attached, but it's possible to cut cable to size and attach your own. On FM sets, you're about as likely to find screw terminals to which the cable attaches directly; if so, be sure to connect the shield to the ground terminal. Since 75-ohm systems are shielded, they reduce interference pickup. If you therefore wish to convert a 300-ohm system to 75-ohm, you can get transformers made for that purpose.

On rare occasions, you'll find 75 ohm r.f. inputs using audio-type phono connectors. More commonly, such connectors are used for direct video signals, which bypass the tuner stages.

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

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