AUDIOCLINIC (Dec. 1992)

Relays Get Dirty, Too

A few years back, you told a reader with intermittent amplifier output that his problem might be dirty switch contacts. I fully agree with your diagnosis, but I'd like to suggest that the problem might be due to dirty speaker relays, if his receiver has any. A friend recently had the same problem as your reader:

He had to raise the volume to a high level to get one or both channels to come on. This suggested to me that whatever was blocking the signal (such as oxidation on contacts) was being "punched through" when the potential across the barrier reached a sufficient amplitude. Although cleaning the amplifier's output relays solved the problem for a while, it returned several months later. Repeated cleanings continued to offer relief for a few months after each cleaning. However, the in convenience of tearing the system down every few months became tiring. (The amplifier weighs nearly 100 pounds.) It was finally agreed that I would replace all four relays in this amplifier. The problem is gone! I would suggest this repair to anyone experiencing this problem if the equipment is 10 years old or more.

In all my years of servicing electronic equipment, over 90% of the service problems in older equipment are con tact-related. I spoke to an expert in the field of relays. He confirmed what I have said: If a relay is acting up, replace it.

-Brian Hefner, Newport News, Va.

Dubbing the Long Way Around

Q. I have many LPs that I really en joy. Is it possible to dub them onto my open-reel deck? I suspect this could be done by dubbing the disc onto a DAT cassette, and copying that onto the open-reel deck. But the instructions for my friend's DAT deck say, in part, "You cannot make a second-generation tape via 'digital input/output.' "Does this, in effect, mean I can dub from a CD player to the DAT recorder but cannot make another recording from that DAT copy?

-Albert Z. Skelding, Lynchburg, Va.

A. You can copy your LP directly onto your open-reel deck. You could also dub from LP to open-reel via DAT, but there's no point in it. When you copy an LP or any other program source onto an open-reel recorder, some noise and distortion will be added, and the frequency balance may be slightly altered. (The noise may go unnoticed, however, if the original LP is noisy.) Recording onto DAT and then copying the DAT to open-reel tape won't do anything to alleviate this DAT recording adds no significant amount of noise (though it does add some), but it does nothing to clean up the original signal, either. All this inter mediate step will do is cost you time.

On the other hand, if you want to keep all your copies on DAT instead of open-reel tape, you'll have more faithful (if expensive) copies of your LPs.

Your friend's DAT deck obviously has the Serial Copy Management Sys tem (SCMS), found in virtually all home digital recorders. If you copy a CD by connecting your CD player's digital outputs to your DAT recorder's digital inputs, the tape copy will be marked with an SCMS code. If you then connect your DAT machine's digital out puts to another DAT recorder's digital inputs and try copying the SCMS-en coded tape, the second recorder will recognize the code and refuse to re cord the signal.

But SCMS does not apply to analog input and output signals. When you record from an analog source, such as LP, to the DAT recorder's analog in puts, no SCMS code is added to the tape. Even if the SCMS code was present, it wouldn't be included in the analog output fed to your open-reel deck. And open-reel decks have no SCMS recording-lockout circuits, anyway.

You could even copy an SCMS-encoded DAT recording to another DAT recorder, so long as you used only analog connections between them.

Hum from FM Antenna

Q. I am plagued with a 60-Hz hum every time I use either the TV set or the VCR--each connected to my receiver.

The hum is not heard during tuner or tape mode.

I shielded the receiver from the TV set. I've added an a.c. line filter. I have checked the house wiring. I have re versed electrical line plugs. The only thing that kills the hum is disconnecting the FM antenna. What causes the hum? Must I continue to disconnect the antenna when I use the TV set or my VCR?

-Stuart Leman, Portage, Pa.

A. I think it's safe to say that most of the hum is caused by at least one ground loop. That is, your FM antenna and your TV, VCR, or receiver are connected to ground by different paths.

The difference in these paths produces a voltage difference, which causes the hum.

If you have grounded any of this equipment, disconnecting the ground may end your problem. If any of these components has a three-wire, grounded a.c. plug, temporarily plug that into a three-to-two-prong adaptor, the kind you use when the wall outlet is not a three-wire type. Do not ground the terminal. If the hum disappears, remove the adaptor and plug the component back into the wall as you had it originally. Yes, the hum will return, but at least you've proven that your problem is a ground loop.

You can't cure the problem by disconnecting the FM antenna's ground, because you need that for lightning protection, but you can provide d.c. isolation between the receiver and the antenna while letting the r.f. signal through. This requires an isolating balun transformer, such as the Gemini CV89. If you cannot locate an isolating balun, you can make one by connecting two ordinary 300-to-75-ohm baluns back to back. Connect the 75-ohm side of one balun to your antenna and the other's 75-ohm side to your receiver, and use 50-µF capacitors to connect the transformer's 300-ohm sides to each other. This will permit r.f. to flow, but block d.c., and should cure the hum. To avoid future problems, you might use a second isolated balun be tween your cable box and the rest of your equipment.

Setting Subwoofer Levels

Q. What is the proper way to set subwoofer level controls to obtain correct, or flat, bass response, assuming that both my satellites and my sub-woofers have been positioned properly in the listening room? The advice that I get is that the levels should be set by ear, but that does not help because I'm not sure what to listen for or what recording to use when calibrating the system. I have spent many fruitless hours using a Radio Shack sound level meter on a tripod, at what I think is the correct listening position. I use a CD with test tones at 20, 40, 100, and 1,000 Hz. The measurements I make are not consistent from day to day, despite careful siting of the meter. What do I do now?

-Chris L. Walker, King of Prussia, Pa.

A. For my own part, I go along with the advice you have been given, but let me add something that could help.

Locate a piano recording that sounds good on a system you know to be good. When I say "sounds good," I mean that there should be body to the piano when bass tones are struck.

Take that recording to your listening room. Adjust the levels of your subwoofers until the piano sounds slightly too "warm," and then back the settings off just slightly.

Because you are using two subwoofers, you have a slight problem.

Theoretically you would want equal loudness from each one. As a practical matter, it is probably not important be cause bass tends to be omnidirectional, so you won't notice that the bass is slightly out of balance if levels are not right. So, adjust levels as I instructed.

Then feed a tone into your system and make a rough balance with your sound level meter. Chances are the balance will be different with different tones, and this is why you should not, or can not, be really fussy about all of this.

If you were to use your test methods, you would first need more tones; that gap between 100 and 1,000 Hz is a serious one. Next, you must take into account that your sound level meter is not flat. The curve that comes with the meter shows this. The deviation from flat response must be taken into ac count when you interpret measured sound levels.

No matter what you do, there will be discs that sound just great; others won't sound good. You must use a tone control or an equalizer to offset these program differences. In other words, even if your system were perfect, some recordings won't be--at least as far as you are concerned.

(Editor's Note: No curve came with my own Radio Shack sound level meter, but I did have it measured once by a friend at an acoustics laboratory and found that it was quite reasonably flat-within ± 1.5 dB over most of the audio range, as I recall. Mr. Walker's measurement variations may therefore be the result of slight differences in the position of the meter from one measuring session to the next. The higher the frequency, the greater the difference very slight changes in position will have. -I.B.)

(adapted from Audio magazine, JOSEPH GIOVANELLI, Dec. 1992)

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