AUDIOCLINIC (Dec. 1990)

Personal Radios in Cars

Q. I am interested in using a portable Walkman-style CD player/radio in my car. I understand that these de vices typically use headphone leads as the antennas for radio reception.

How is this accomplished? Is there any way that I can insert a lead into the headphone jack to act as an antenna when I use the portable in my car? Can I connect the car's existing antenna to the player in some way to improve reception in the car?

-Donald S. Leitner, Wayne, Pa.

A. First of all, using a headphone portable while driving is not a wise thing to do. Headphones isolate you from the road sounds which provide cues as you drive. Passenger use of headphones is okay, of course.

Many people do use portables such as yours in cars but as an inexpensive way to add a CD player to an existing sound system, to be heard through loudspeakers. Adaptors that let you feed your CD player's signal in through a cassette slot are available from Radio Shack (No. 12-1951, $19.95), Recoton, and others. Other adaptors, available through car stereo dealers, feed CD player signals into car radios via their antenna inputs or even via the car's cigarette lighter. For car stereo systems with separate amplifiers, there are adaptors that feed a portable CD player's signal directly to the amps.

Your question, however, emphasizes uses of your portable's tuner section.

There are technical problems with that beyond the ones you mention. The front-ends of such radios are designed to have high sensitivity at the expense of a tendency toward overload. As a result, any strong station is likely to pop up at several points along the dial, often interfering with other stations.

Portable radios are also hard to tune while driving because of their small dials and controls and because they slide around as you drive. A cheap car radio, if you can still find one, will probably give you better results and be easier and safer to use. Low-cost radio/cassette units can be had, complete with speakers, for under $100.

As to the use of the headphone cord as an antenna, any such wire will pick up some r.f., especially if its length is appropriate for the wavelengths to be received. In small portable radios, the headphone jack is connected to the r.f. input. An r.f. choke in series with the line between this point and the amplifier output keeps r.f. out of the audio circuits but passes audio signals.

To use the radio with an existing antenna and still be able to use head phones, you'd need a Y connector. Chances are that you won't be able to use the shield of the antenna cable, and therefore you will pick up extraneous noise. If the point of all this is to use headphones, it would seem simpler to connect them to a more conventional car radio, adding an attenuator to avoid burning the 'phones out and to minimize hiss when the volume is turned down.

Voice-coil Problem

Q. I have a problem that's driving me nuts! What I hear is a sound some thing like low-frequency thunder. I am familiar with 60-Hz hum, but this sounds lower in frequency and constantly varies in intensity. It is heard on all program sources: CD, tape, phono, TV, and VCR.

I have rearranged wires, hooked separate ground systems, hooked the CD player directly to the power amplifier (bypassing the preamplifier), used two different power amplifiers, and used two different CD players. Nothing changes this sound. What does all this mean?

-A. B. Carter, Apple Valley, Cal.

A. It's really great when a reader gives me lots of information because I can be much more certain of my answer. If I had to bet money on this one, I would tell you that your loudspeakers are at fault. Why do I say this? Well, you eliminated the preamplifier and still heard the annoying sound. You eliminated the CD player by using two different units. You eliminated the power amplifier by substituting a second one.

With tests like those, we can eliminate the TV set, the VCR, phonograph, etc.

Although not so rigorous, I think we can eliminate grounds and wiring lay out as well.

What you are hearing is the sound of the voice-coil of at least one of your drivers rubbing against its pole piece.

You should connect one speaker at a time, run the system and see if you hear the sound. It may be that one loudspeaker will work fine and the other won't. It is possible that both are defective-which could have occurred if you continuously feed more power into the speakers than they can safely handle. The heating of the voice-coils created by that condition would eventually lead to their going out of round. If the loudspeakers are valuable to you, the manufacturer probably can repair them. Of course, if they are under warranty and if the maker finds that the failure was natural rather than caused by abuse, the speakers would be re paired at no cost other than shipping.

Volt-Amperes and A.c. Power

Q. The rear panel of my amplifier says "450 W a.c.". Does this indicate the power at full audio output or at idling? This same panel also shows "520 VA". Is this the output current or voltage at maximum power output?

-Aron Goldberg, Los Angeles, Cal.

A. The 450 watts shown on the rear panel of your amplifier is the power taken from the a.c. line. If this amplifier is a Class-A device, this power re mains constant regardless of the amount of audio power being produced. If the amplifier is Class B or somewhere between Classes A and B, then this power will be drawn when the amplifier is fully driven.

The "520 VA" is the product of the maximum voltage and current draw.

While power in watts is frequently de fined as the product of the volts and amps involved, this actually holds true only for d.c. or for a.c. circuits whose voltage and current are in phase. The discrepancy between the power and VA figures is because they are not in phase here. As with power, the amplifier's class of operation will determine whether the VA figure applies all the time or only at maximum audio output.

CDs Left in Cars

Q. I am considering a CD changer for my car. Can I leave the discs and changer in my car, or must I remove them? In this part of California, if the trunk gets as hot as the car's interior, then temperatures inside the trunk in summer can run between 120° and 140° F.

-Samuel J. Neiditch, Red lands, Cal.

A. I have heard no reports of heat damage to either CDs or changers, and the changers are designed to be left in car trunks. Nevertheless, CDs and many of the parts in today's audio gear are made of plastic. Unless these plastics are of the thermosetting type, they will melt or soften at high temperatures. I think it's better to be safe than sorry, and to remove both the discs and (if possible) the player when the temperature is expected to rise much above 90° F.

I have no experience with this, how ever. Have any readers suffered heat damage to CDs or changers? Or can any of you in hotter climates than mine state from experience that there is no such problem? Please let me know.

Center Hole Damage to CDs

Q. It would appear that, after years of usage, some wear to the hole in the center of a CD should occur from placing into or withdrawing it from its plastic storage container. If this happens, what effect would it have on performance? Was this possibility of damage considered when the packaging for CDs was developed?

-John P. Pavelchak. Forest City. Pa.

A. I agree that, after many insertions and withdrawals of a CD. its center hole might be slightly enlarged. This process would be greater at first but decline as the tension of the box's spindle "fingers" decreased. Also, some of the wear will affect the spindle rather than the disc.

In all events I do not think that change in the center hole diameter will be very great. This fact becomes less important when we realize that tests have been conducted showing that there is room for a certain amount of disc misalignment. A CD player's servo tracking is surprisingly flexible and forgiving. I have no data as to whether or not this matter was considered when the packaging of CDs was developed. Given all the care taken when developing this product. however. I have to believe that this matter was examined. In any case. one can reduce the risk of wear by pressing on the fingers of the spindle while withdrawing the disc.

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

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