AUDIOCLINIC (Mar. 1993)

Simultaneous Stereo and Mono?

Q. If a receiver or integrated amplifier has outputs for five pairs of loudspeakers, is it possible to bridge the outputs and have stereo separation on one pair of loudspeakers while the other pair is in mono?

-Jeremy Whitfield, McGregor, Tex.

A. Many amplifiers are designed for either normal stereo or bridged monophonic operation. In bridged mode, the two amp channels work together to produce a monophonic signal, usually from a mono input (though some amps might bridge two stereo inputs into mono). Normally, connecting a load between the channels of an amplifier reduces separation--all the way to mono, unless the cross-connected load's impedance is extremely high.

Many car stereo amps permit simultaneous stereo and bridged mono operation, with the mono speaker being a subwoofer.

This should lead to reduced channel separation at low frequencies, but that's no problem if you're using a mono woofer anyway. The low-pass filters that keep the woofer from handling upper frequencies also keep those frequencies from being bridged across the woofer, so separation is maintained in the satellite left and right speakers.

Can Hot Music "Fry" Tweeters?

Q. Everyone knows that pushing a power amplifier into hard clipping will “fry" tweeters, because the clipped waveforms contain much more high-frequency energy than a typical music signal. What happens, however, if the music itself contains heavily distorted, clipped sounds-like those from some electric guitars? Will this burn out the tweeter's voice-coil even though the amplifier itself is not clipping? What is the difference between these two conditions?

-Steve Lindenfeld; St. Joseph, Mich.

A. Distortion produced by "fuzz" guitars is not nearly as destructive as amplifier clipping is. I don't believe that distortion created by guitarists and their special equipment is nearly as rich in harmonics as the distortion produced when a power amplifier clips. The upper frequency of the harmonics produced by the guitar is limited by the instrument amp and by the medium on which the music was recorded.

Just listening to such music makes me feel that much of the distortion occurs at midrange frequencies, rising into the treble range but decreasing in amplitude as frequencies of harmonics increase. I have not measured the spectrum of such music.

Further, if tweeters were being destroyed during the playing of such music, readers would have deluged me with tons of letters about this. I have not received one.

Headphone Hiss

Q. I recently purchased a pair of headphones and found, to my surprise, a very noticeable hiss. It is 11,051 apparent with nothing playing, but I still hear it under most conditions. I also hear hiss through my loudspeakers, but it is not distracting. I can reduce the hiss somewhat with any graphic equalizer, but at the expense of highs. Is all of this just a limitation of my equipment? Can I add something to reduce this hiss?

-Philip C. Hagemann, North Babylon, N.Y.

A. Hiss is more noticeable through headphones than speakers for several reasons. Since headphones lie close to the ears, any hiss they reproduce is readily transferred to the eardrums. Headphones are far more sensitive than speakers, so they're more likely to pick up faint hiss in your system. And headphones that isolate your ears from outside sounds also keep such sounds from masking hiss.

If the hiss you hear comes from your signal sources, your recordings, or the portion of your preamp or amp that precedes the volume control, there's not much you can do about it other than to find its cause and cure it. But if the hiss originates in the circuits following the volume control (in which case it won't vary with the control's setting), there's a cure.

In such a case, turning up the volume will raise the signal level without affecting the noise. Unfortunately, that will also overload your headphones. You could switch to less sensitive 'phones, which would require such high volume settings, but an easier solution is to attenuate the signal between your amp and headphones.

Before headphone jacks were common, many stores carried headphone extension boxes with ¼ inch phone jacks and level controls for the headphones. If your 'phones have 1/8-inch (3.5-mm) mini-jacks, you could use Radio Shack's headphone extension cord with built-in volume control (No. 42-2459, $4.95). If you can find an old extension box or can use the Radio Shack cord, plug it in between your headphones and your preamp or receiver. Listen to the 'phones with no signal, and turn down the attenuators until the hiss virtually disappears. Then play your equipment to see if the hiss is still a problem and if your equipment can provide all the signal the headphones now need.

Leaving a CD Player On

Q. Is it necessary to press the stop button after the last CD has been played? Can you just press the power button when a disc is still playing? 1 was using my player late at might and fell asleep. I woke up wondering if it is harmful for the machine to be left on once a disc has been played. Should 1 use the repeat button so that, if I fall asleep, the system will be doing something specific rather than just sitting there?

-Joseph Barbera, Cheektowaga, N.Y.

A. If you just turn off the machine while the disc is playing, you can't usually open the player to get the CD out. Other than that, though I prefer to press "Stop" first so the mechanism won't be left somewhere in mid-cycle, none of my players has malfunctioned when I turned them off without hitting "Stop." Leaving the system running in repeat mode will increase mechanical wear and shorten the laser's life.

Don't worry if you fall asleep; the player's mechanism will stop itself, and the laser will turn off when the disc ends. The electronics will remain on but won't be harmed by running all night.

(adapted from Audio magazine, JOSEPH GIOVANELLI, Mar. 1993)

= = = =