AUDIOCLINIC (Jan. 1991)

Damaged Subwoofer Dust Cover

Q. During the installation of my subwoofer, my daughter punched in the dome of the woofer cone. After I had calmed down, I repaired it. This was done very carefully with a needle pulling out the dome by pricking the outer layer of material and pulling up.

The system sounds great and I can't tell that there has been any loss of sound quality because of this. I am concerned, however, about any long term effects that may ultimately de grade the performance of the woofer. Do I have to worry?

-Jeff Mason; Alta Loma, Cal.

A. I don't think you have anything to worry about regarding your subwoofer.

By the way, if the center dome (dust cover) is ever pushed in again, try sucking it out with a vacuum cleaner.

That approach works well. As you know, your solution also works, but there is a real danger of tearing the dome with the needle. Even if that happened, the sound would likely remain unchanged. I suppose that over a period of time, dust could find its way into the voice-coil structure. The main purpose of this dust cover is to prevent just that from happening.

Noisy Power Transformer

Q. I recently purchased a new amplifier-very expensive! After it is operating for about 5 minutes, I hear a buzzing coming from the power transformer, not the loudspeakers. For the next half hour, the sound increases in volume and finally levels off. If the room is quiet, I hear the noise very well. If there is music playing, I would have to stand next to the unit to hear the noise. Is this a normal occurrence or is there something wrong with my equipment?

-Robert Bigaouette, Brooklyn, N.Y.

A. In most instances, very little sound will emanate from the power transformer. Certainly if the unit is still under warranty, you should ask for a replacement or for a repair.

The sound you hear is the result of the vibration of the laminations that compose the transformer's core.

Transformer cores are dipped into a compound to bond each lamination to its neighbor to prevent just the kind of sound you are talking about. When the bond between laminations is poor, each one is free to vibrate in accordance with the a.c. in the primary winding. As the transformer heats up, expansion may permit the laminations to loosen, making them freer to move.

In some instances, it is possible to stop or reduce this annoyance by tightening the screws that hold the elements of the transformer together. (Unfortunately, not all power transformers have such screws; some are held together with clips.) It is also sometimes possible to improve matters by tightening all of the bolts that mount the amplifier to the chassis.

Sometimes the noise can be reduced by placing the amplifier on a different surface--one that has less tendency to act like a sounding board.

If relocating the equipment is not practical, try placing small pieces of foam under each corner of the chassis. The foam must be dense enough so as not to completely collapse under the weight but must still give a bit. These blocks will isolate the amplifier from the shelf and reduce the sounding board effect.

Subwoofer Boominess

Q. My subwoofer is enclosed on the bottom level of a Formica cabinet. Every time I use it, the sound is boomy. I can't remove the subwoofer from this cabinet because I just had the cabinet made specifically to house the subwoofer and 16 other audio and video components. The enclosure housing the subwoofer is felt-lined; there is approximately 1 foot of space between the rear of the subwoofer and the back of the cabinet, and no space on either side. Just the face shows at the front of the cabinet. I should add that this cabinet is next to a wall. Is there hope of solving the boomy sound?

-G. Lipton, North Woodmere, N.Y.

A. I am sorry that you can't remove the subwoofer from the enclosure in which you mounted it. Loudspeakers, including subwoofers, are designed to work with enclosures specifically de signed to match them-they must be considered as a system and cannot be chosen independently of one another. This does not seem to have been done here. The resonance of the woofer and enclosure is obviously too high, and there is probably too little damping (too high a Q) in the system. It might help a little to temporarily remove the sub-woofer system, fill the space behind it with sound-absorbent material, and then put the driver back.

You might also consider replacing the subwoofer driver with one whose characteristics are better matched to the small dimensions of your enclosure. That might well turn out to be a smaller woofer than the one you're using now. If so, mount the new woofer on a piece of heavy plywood or high-density particleboard large enough to cover the original hole, paint the new board black, and use a grille large enough to cover the original hole completely. You may also find that a woofer designed for an air-suspension cabinet would be the best choice here.

If this does not reduce the severity of the condition, move the cabinet to a new location. (I know this is often easier said than done.) A location near the corner of two walls often reinforces bass, probably making matters worse.

I recall having a problem just like yours many years ago when I designed my first "entertainment center" for a large office complex. The client insisted that the loudspeaker be part of the cabinet housing the rest of the gear. He was decidedly unhappy with the sound. I convinced him to let me remove the single loudspeaker (the system was mono). I had to do some extra woodworking to convert the empty space into additional record storage, but at least the music sounded like music.

I suggest that you do what I did especially if the subwoofer sounded the way it should before you had your entertainment center constructed.

Overloaded Equalizer

Q. When I play CDs and use my equalizer, loud musical passages, such as full orchestra in classical pieces, become a storm of pops and crackles. This happens only when playing CDs and occurs at all listening levels. When I vary my equalizer's boost settings, the distortion does not change. I do not hear the noise when I play CDs through the amplifier alone.

Obviously, I have a problem with my CD player or my equalizer. Now what?

-Robert D. Genung, APO, N.Y.

A. I believe there is nothing wrong with your CD player or your equalizer. If something were wrong with the player, it would sound distorted regardless of what equipment it were fed into. If the equalizer were defective, all pro gram sources would sound bad.

I think that the signal from your CD player is overloading the equalizer, causing it to create the noise you de scribed. If the player has an output level control, turn it down to a point where the overload does not occur.

Most equalizers that I have used have input level controls which can also be turned down to prevent the overload we are talking about.

I realize that all other program sources behave as you would expect. However, many CD players produce a great deal of output signal compared to tuners, tape decks and other components.

No Surround Sound

Q. I have a problem that I can't solve. I've contacted various technicians and I'm still waiting for help. I have a Hi-Fi VCR that loops through a projection TV receiver into a preamplifier. Connected to that is my surround decoder. I also have a CD player connected to my preamplifier. I get very nice surround from most of my CDs. I never hear surround from my video tapes! In fact, they sound more like mono than anything else.

I've had my VCR checked. It's supposed to be fine. I've had a couple of different decoders and all are supposed to be all right. What's wrong? Help!

- Steve Metz, New York, N.Y.

A. I do not believe that your decoder is the problem, because if it was, you could not hear the surround effect when playing CDs.

I have a feeling that the problem is a simple one. The linear tracks on most Hi-Fi VCRs are monophonic. It is at least possible that you have set your machine to play this track rather than the Hi-Fi tracks, which are in stereo.

Also, some VCRs have switches that allow a mono signal from either the right channel or the left channel to feed both outputs; if your VCR has such a switch, make sure it's switched to "Stereo." Can the TV receiver be switched to mono? Maybe this was done and has remained undetected.

I suggest that you check all these. If you still run into trouble, disconnect the VCR from the TV receiver and connect it to the preamplifier's high-level inputs.

That will permit it to operate just as your CD player does. If you do not have stereo then, you'll know that there is a problem with the VCR. If you co have stereo, then there is something amiss with the television receiver and its connections.

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(Source: Audio magazine, Jan. 1991, JOSEPH GIOVANELLI)

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