AUDIOCLINIC (Feb. 1990)

CDs and "Emphasis"

Q. My CD changer has an indicator which lights up with the word "Emphasis" when certain Compact Discs are played. I have noticed an audible decrease in high-frequency response when I listen to these recordings. I would appreciate your comments on this.

-Colin C. Hall, Merrimack, N.H.

A. All else being equal, there should be no difference in sound quality between a CD marked "Emphasis" and one which is not so marked. CDs marked "Emphasis" are recorded with a bit of equalization in order to obtain about 6 dB better S/N ratio than would otherwise be possible. The player must sense the presence of such a disc and roll off frequencies to compensate for the rise during recording, thereby maintaining a flat frequency response.

This is an automatic action, just as is true of the display you have described.

It may be that your player is overcompensating for this frequency pre-emphasis when it attempts to play these Compact Discs.

Distortion at Low Volume

Q. I have a baffling problem with my audio system. I first noticed this problem after getting new loudspeakers.

When I listen to FM programs at a low volume and the DJ comes on, the voice is distorted. Female voices tend to bring out more of this distortion than male voices do. When I raise the volume, the distortion disappears. Music generally comes through clear at all volume levels. However, both on FM and when playing LPs, I hear this same distortion with guitar music or with acoustic piano; most electric music is not affected. I notice no distortion when playing tape recordings, and I never hear it when listening through headphones, regardless of volume level.

-Steven R. Sanchez; Mt. Clemens, Mich.

A. I appreciate your having given me lots of detail because it really gives me more to go on. I always prefer having too much information over too little.

I believe that the distortion you describe is present most of the time, regardless of volume, but is masked by the music. Although you mention that playing tapes does not produce this distortion, I think that if you were to record one of the DJs, the subsequent playback of this tape would again bring out the distortion.

The clue as to the true nature of your problem lies in the fact that you never hear the distortion when listening to headphones. Because everything else in the audio chain remains the same, the loudspeaker must be responsible for the distortion. The voice-coil of at least one of the drivers is rubbing against its pole-piece. This, in turn, restricts the free movement of the cone.

Not only that, but the mechanical friction produced by this rubbing is transmitted to the cone and is reproduced as a "buzz." When I manufactured loudspeakers some time ago, I made a special test recording whose main content was acoustic guitar and acoustic piano. For me, these two music sources made this sort of distortion very apparent and provided a quick method by which I could determine if my completed loudspeakers were free from this defect.

Overdriven Audio System

Q. During the playing of certain symphonic passages, we hear clipping. Can we stop the clipping by adding another power amplifier to be used as part of a biamplified system? How about adding a powered subwoofer?

-Allan and Alice Walter; Chatsworth, Cal.

A. Because the bass consumes most of the power required by your sound system, using a separately powered subwoofer should solve your problem. Of course, you could also do what you had suggested: Use a second amplifier to drive the woofer as part of a biamped setup. You may run into difficulties similar to those you already have. Your woofer needs a given amount of power, so if the second amplifier cannot provide more power than your present amplifier can, the second one may also clip.

Another thought comes to mind: Some listeners have said they heard amplifier clipping when they actually heard a "pop" caused by the voice coil hitting the bottom of its pole-piece assembly when forced to carry more bass than it was designed to handle. If this is what you heard, you would certainly need a different woofer as part of a biamped setup-a condition which is met by using a powered subwoofer or a pair of them.

Amp and Speaker Concerns

Q. I am considering buying a receiver that has a rated power bandwidth of 5 Hz to 60 kHz. The speakers I intend to buy have a rated frequency response of between 20 Hz and 20 kHz. The speakers can handle about twice the amount of power which can be supplied by the power amplifier.

Will this arrangement damage the loudspeakers? If the receiver is able to produce frequencies above and below what the speaker can deal with, will this damage the tweeters and woofers when I play the system at high volume levels? Also, I have heard that amplifier clipping can damage tweeters. What is amplifier clipping, and how does it ruin the tweeters?

-Richard Cramotte, Jr.; Knoxville, Tenn.

A. In order for me to give you an answer as to whether or not this equipment can work together without destroying the loudspeakers, I need to know just a bit more. When you listen to your system as loud as you are likely to, is the amplifier working somewhere near its maximum rating? If not, then you can use the system without problems. If you are likely to drive the amp to its maximum, damage to the tweeters can result because of amplifier clipping.

Clipping occurs when the amplifier is forced to perform beyond its capabilities. The amp generates spurious audio frequencies (frequencies other than those intended by the nature of the program). These frequencies are quite high and, hence, will damage the tweeters rather than the other drivers.

The fact that the amplifier has a wider bandwidth than the speakers is of no consequence. Even though your amp is capable of producing frequencies above or below those which can be reproduced by the speakers, it is unlikely that these frequency extremes will be found in most program sources you will encounter. Regardless of frequency, if the power fed into the loudspeakers is less than they can handle, all is well.

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(Source: Audio magazine, Feb. 1990, JOSEPH GIOVANELLI)

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