AUDIOCLINIC (Jan. 1995)

Phone Turns CD Off

Q. I have a peculiar situation that even my CD player's manufacturer cannot solve. If the phone rings while the player is operating, the player stops. I have a couple of telephones in the room as well as an answering machine. Could the ringing of all this equipment at once somehow affect the CD player?

-Charles Yagerman, Brooklyn, N.Y.

A. If one of the telephones in your room is a cordless model, its base unit sends out an r.f. signal whenever a call comes in. It could be that this r.f. is finding its way into the player and causing its microprocessor to "mess up."

Whether this guess is right or not, I suggest that you remove all but one of the telephones. Arrange for a call to come in while you are playing a CD. If the machine does not malfunction, remove that phone, substitute the next one, and try again.

Note from Charles Yagerman: Thank you for your reply. You certainly hit the nail on the head. My cordless telephone was the culprit; when it rang, the player shut off.

J.G.: I appreciate hearing from you. So often I deal with a somewhat obscure problem and never do learn how it all came out.

When To Use Dolby NR

Q. When recording audio and using Dolby noise reduction, is it best to use it during the playback process as well? On a related matter, many people say that Dolby noise reduction dulls the highs. Is this true?

-Steven Matthews, Louisville, Ky.

A. Tapes recorded with Dolby noise reduction should be played back with the same type of NR (Dolby B, C, or S) for the best frequency match to the original signal. If you are copying a tape made with Dolby NR, you should have the appropriate noise-reduction switched in on both the playback and recording decks.

If the Dolby NR circuits are not properly calibrated for your tape, both in recording and in playback, the high-frequency balance will be thrown off-but this is as likely to boost the highs as cut them. This problem most often arises when tapes recorded on one deck are played back on another.

Power-Line Transients

Q. My new receiver works great-except that when I turn it on or when a light is turned on or off I hear a "pop" from my loudspeakers. Is this normal? Will it do any damage to my system? How can I remedy the condition?-Name withheld AI rather doubt that the sound you hear, or the condition causing it, will damage any components in your system.

"Pops" are common and result from sudden, or transient, voltage changes on the power line when the load on the line changes. Many audio dealers sell line filters that plug in between your equipment's line cord and the a.c. outlet, to keep such pops from getting into your equipment.

These noises can also be caused by the quick charging or discharging of your equipment's power-supply capacitors and the accompanying change of voltage in the output circuit. The voltage change will be passed along to the loudspeaker as a "pop." Some components have delay circuits that mute the output until the equipment settles down.

Sometimes the equipment maker can offer help in eliminating these problems.

Check with the company about this.

TV Interference from an Amp

Q. I have a Yamaha DSP-2070 digital sound-field processing amplifier, an excellent unit that has changed the way I listen to music. But when it's running, a herringbone pattern appears on the lower VHF television channels. In my situation, it is not practical to have cable TV or an outdoor antenna. I have tried moving the antenna (which has a coaxial lead-in) to various locations but got no improvement. I cannot place the Yamaha in another room. I have tried an a.c. line interference filter; again, there was no improvement. Can you help me with this problem?

-Richard Townsend, Jr., Chicago, Ill.

A. I can only offer suggestions and hope that they will solve your problem. Obviously, an outdoor antenna would likely cure your problem: It would provide stronger TV signals, while any interference it might pick up from your amp would be weakened by distance. Unfortunately, you say that you can't install one.

It is possible that some clock or processing signals are being radiated from your amplifier via the loudspeaker cables, power cord, or input cables. Try mounting some ferrite beads on these wires or cables, as close to the amplifier as possible. It is also possible that the TV set is picking up the interference via its power cord. Try some beads on it as well.

These beads serve as r.f. chokes, presenting a high-impedance path for the interference. There are various ferrite "mixes," each designed to be effective on specific parts of the r.f. spectrum. What you will need are beads that are most effective over the frequency range between 50 and 200 MHz.

(Editor's Note: There are some commercial clamp-on filters from Audio Advisor and AudioQuest that may solve your problem, and the MAGIC ground isolator from Mondial has, I am told, been effective when other methods didn't do the necessary job. -E.P.)

Bypass each amplifier output terminal to its chassis, using disc ceramic capacitors of about 50 to 100 pF. Keep the leads as short as you can make them.

Perhaps the amplifier's circuit boards are not well shielded. Some judiciously placed aluminum foil can sometimes create good shielding; be sure to ground the foil. Also, be very careful that the foil doesn't touch and short out any wiring or components.

Subwoofer Crossover Networks

Q. My satellite loudspeakers are fed as full-range systems. Because I love organ music, I also use a subwoofer. I feed this with two active crossover networks in cascade; the first is 18 dB/octave, followed by a 24 dB/octave unit. The crossover frequency is 50 Hz. I use two networks to obtain a steep slope to avoid interference with the mid and upper bass, such as is evident with a deep male voice. The sound from all this is quite good, but after using an audio-frequency generator and an oscilloscope, I found there was a complete polarity reversal between 20 and 50 Hz. I also found that, when feeding a square wave into the system, an almost perfect sine wave appeared at the output.

What suggestions can you make to help improve the performance of what is, even now, a good system?

-Ronald N. Leutwyler, Orleans, Mass.

AI much prefer using a crossover net work having a smaller rate of roll-off than you presently employ. Even with active networks, the extreme amount of roll-off you are using almost certainly must produce some ringing. I am not convinced that the extreme phase shifts in your sys tem won't produce problems in the region of 50 Hz.

If you can manage it, try as little as 6 dB/octave. You won't have nearly as great a phase shift as you now have; you also should have less tendency to "ringing." For this gentle slope to work properly, I suggest you use a high-pass crossover to your satellite speakers. This should remove any "boominess" in the mid-bass. If that condition persists, try crossing the satellites (but not the subwoofer) at a somewhat higher frequency than 50 Hz. If you are still unhappy with the integration between the subwoofer and the satellites, try 12 dB/octave.

I am not too concerned about the square waves you mentioned. Any transient at tacks found in bass instruments will be carried by the satellite speakers. After all, if the purpose of a subwoofer crossover is to roll off upper frequencies, that crossover will reduce harmonics--including those which make a square wave square. Remember that your system employs a very large amount of roll-off. You will find that a net work having a gentler crossover slope will also produce output that is closer to a square wave when fed a square-wave input.

(Editor's Note: There are some commercial crossovers using the classic Linkwitz Riley alignment which might improve your performance. However, if you're happy with the audible results, stick with what you've got.-E.P.)

Measuring and Matching Signal Levels

Q. Some manufacturers of audio equipment measure input or output signal levels in dB; other manufacturers use volts.

How do I use these figures to be sure I don't damage any of my components because of some mismatching of levels?

-Gerald E. Fields, Morrow, Ga.

AI can appreciate your concern, but you are very unlikely to damage any electronic components, even if you happen to overdrive them. If you do put too much signal into a given piece of gear, you will likely hear audible distortion. You will also find it hard to control sound level with the volume control (if any) in the overdriven component, because the control's action will be crowded into the lower quarter of its range. Reduce the signal before it gets into the next device in line, until the distortion clears up or the action of the volume control is more normal. For components which have no means to reduce output signal level, such as some CD players, there are commercial attenuators you can wire between that component and the next piece of gear in the line.

Burning In

Q. What is meant by the term "burn in"?

-Name withheld

A. You may have noticed that when electronic equipment breaks down, it usually does so rather soon after you start using it. To make sure you won't have this experience, some manufacturers will "burn in" their equipment before shipping it out, by running it for several hours or even for several days.

Some people feel that a few days of burning in equipment at home, under actual signal conditions, makes it sound better. This is most widely accepted when it comes to amps and their capacitors, but it is also done for speakers, whose materials can often be a bit stiff at first and can limber up after some use.

Mono VCRs and Stereo Sound

Q. If I connect the output of a mono VCR into the inputs of a stereo system, can the VHS Hi-Fi track be decoded into stereo?

-Steven Matthews, Louisville, Ky.

A. No. A stereo system can't transform a mono signal into a stereo one.

(Audio magazine, JOSEPH GIOVANELLI, Jan. 1995)

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