AUDIOCLINIC (Apr. 1996)

Midrange Output from a Subwoofer

Q. Even though my subwoofer is fed frequencies only below 125 Hz, I can still hear most of the information above this point, albeit at much lower levels. I tested this by passing a 1-kHz tone from a test CD through the sub alone, and I was able to hear it clearly. Do I need to use an electronic crossover network rather than the passive networks I now use in order to prevent these frequencies from being heard?

-Paul Hanley, Jersey City, N.J.

A. Crossovers don't chop off frequencies above or below the crossover point; they roll those frequencies off. If your crossover is a first-order type, which rolls off at 6 dB per octave, the signal fed to your subwoofer would be only 18 dB lower at 1 kHz than it is at 125 Hz (though the woofer may have some additional rolloff of its own). With a fourth-order crossover, which rolls off at 24 dB per octave, the level at 1 kHz would be down 72 dB, but even that might be audible if the original signal were loud enough. If you feel the need for a steeper rolloff than you're getting, you'll need a steeper crossover; you can use either a passive or an electronic type. On the other hand, if the leakage through the subwoofer is apparent only when the main speakers are disconnected, there may be no real advantage to charging.

Shielding TV Sets

Q. In the past, you've discussed how to shield a TV set from nearby speakers by using thin sheet iron or sheet steel. I bolted four pieces of 16th-inch sheet metal to the underside of the shelf that holds my center-channel speaker. That gave me an undistorted picture, but only if I moved the speaker nearly all the way to the back of the shelf I wanted to put the speaker as far forward as possible.

When I wrote to you about this, you suggested magnetizing these sheets with a permanent magnet (first moving the shield and the TV apart, of course). Your advice was right on target. I magnetized the bottom plate with a magnet from a hefty 10-inch woofer, which cleared up 80% of the problem; I still got a bit of picture distortion when I moved the speaker back and forth on the shelf But tilting the rear of the loudspeaker up, to aim it at the listening position, eliminated the pic ture distortion and put the speaker at the front edge of the shelf right where I wanted it.

Thank you for your advice.

-Wayne A. Pflughaupt, Katy, Tex.

A. The reason this works is that magnetizing the sheets increases their permeability to magnetic fields, which improves the shielding.

High-Output Cartridge into MC Input

Q. I want to add a second turntable and use a mono phono cartridge whose rated output is 22 millivolts for a 1-kHz recording at 10 cm/sec. My preamp's moving-mag net phono input is already in use. I have a moving-coil input available, but its sensitivity is 100 microvolts. The MC input's impedance is 50 ohms; the cartridge's required load depends on the type of equalization needed. What kind of network could I use to match the cartridge to this MC input?

-Name withheld

A. I'm not at all convinced you can do this without serious overload, but here's a possibility: Use a Y connector to feed your cartridge's mono output into both the left and right MC inputs. Then put a 47-kilohm resistor in series with the "hot" lead from the cartridge. This should form a voltage divider with the preamp's 50-ohm input, reducing the signal going to the pre amp. It should also satisfy the cartridge's load requirements, flattening its response so you can use your preamplifier's RIAA equalization.

If this doesn't work, you'll have to make a shielded switchbox that lets you select either your regular stereo cartridge or the mono cartridge to feed to your MM input.

If you like, you can wire the mono cartridge so that its output feeds both channels. Paralleling the two channels this way will alter the frequency response a bit, giving you a peak followed by a rather fast rolloff; if you're playing 78s, this could be desirable, however. If you want flat response from the cartridge, put a series resistor of about 22 kilohms between the cartridge's hot lead and the switch. You'll lose about 6 dB of signal, but chances are that will also be to your advantage.

Equalizing 78-RPM Records

Q. I'd like to transfer my 78-rpm records to tape. I think I could really do a good job if I had information about the various recording curves used by record companies when they were producing these discs. (Most of my records are from the '40s and '50s, with a sprinkling of discs from the '30s.) Where can I find this information, and can I use my stereo third-octave equalizer some how?

-Harry R. Porter, Louisville, Ky.

A. I don't know of any books that include the information you're seeking. But I can tell you from experience that published curves won't be very useful, because you can tell just by listening that many companies' curves varied from record to record within the same time frame. On the other hand, once you have a setting for a given record company, you will need to de part from it only occasionally.

When I transferred my 78s to tape, I started with my preamp's standard RIAA curve, which was designed for LPs. I fed my preamp's output to a graphic equalizer and, listening for the most lifelike sound, made adjustments by ear for each record. If you try this, you might want to have a friend listen with you, to get a second opinion.

Since there's probably no bass below 40 Hz on any of your discs, you can turn your equalizer's lowest band all the way down, which will reduce rumble quite a bit. If this low-frequency cut intrudes into the next octave, you might need to boost that octave just a bit. In many cases, you may need to reduce bass between 100 and 200 Hz in order to avoid boominess. Keep in mind that you will also be starting with the RIAA curve, which boosts bass-often more than is needed for 78s.

I roll off highs above the highest frequency on the discs (usually 10 or 12 kHz), to minimize background noise. You may wish to boost highs somewhere below this cutoff point.

You may find that you'll be reducing frequencies in the region of 2 to 3 kHz, to compensate for the record producer's idea of what sounded good on phonographs of the day.

Experiment! It's amazing what a small change in settings, sometimes just 2 to 3 dB, can make.

This technique of equalizing for the difference between a record's correct playback EQ and the RIAA curve is also the basis of a commercial product from Esoteric Sound ( 4813 Wallbank Ave., Downers Grove, Ill. 60515). The Re-Equalizer ($310) comes with data on suggested settings for various companies' recordings (and was reviewed in the November 1985 issue).

Slow Preamp Warm-Up

Q. When I first turn my preamp on, the left and right channels fade in and out but not in unison. It takes approximately 5 minutes for the unit to operate properly.

There is no reliable repair shop in my area, and I am reasonably good with electronics; should I repair the preamp myself?

-Grant W. Prokop, Winnipeg, Manitoba, Canada

A. This sounds to me like a real challenge, because your problem has many possible causes. I would not proceed without first getting your preamp's service manual.

Dirty controls or poor solder connections can cause this problem, as could a defective IC. If your amplifier has a volume or gain control, you may be able to use it as a signal tracer. Connect a test lead to the amp's in put through a capacitor of about 10 microfarads. Use this lead to check various stages of the preamp, working from input to out put, until you find the one that's acting up.

You might also want to measure voltages, to see if they change during warm-up. Per haps the power supply is slow in coming up to voltage on one channel. If you get really frustrated, you might want to replace ICs without regard to which one is causing the problem. But this is easier said than done if the ICs are soldered to the board rather than socketed, and it is often difficult to obtain the proper ICs.

Turntable Safety

Q. While I admire the sonic qualities of belt-drive turntables, I worry that those thin little belts will let go, allowing the heavy platter to spin across the room at a wicked 33.3 rpm. Do you know of a turntable I can rely on not to do this?

-B. Wildered, Fanwood, N.J.

A. I have recently been informed that Lirpa Laboratories is developing a turntable that will offer the utmost in security, thanks to a revolutionary belt-and-suspenders drive.

Static "Pops"

Q. On cold winter days when the air is dry, I have noticed static buildup that causes a popping noise when I touch my components. Is this harming any circuits?

-Sal Rosselli, Leominster, Mass.

A. I don't think you have anything to worry about. And if you touch a grounded object just before you touch your components, the problem will go away.

Improving Car-Speaker Gaskets

The gaskets I've seen supplied with car speakers are thin, hard, and cardboard-like. These gaskets don't readily conform to the often irregular surfaces surrounding typical loudspeaker cutouts and can't keep speaker vibrations from being transmitted to the mounting surface. Worse, some drop-in speakers have no gasket at all.

When I heard considerable buzzing from my rear-shelf car speaker, I removed it and applied a liberal thickness of silicone rubber around its periphery, to form a resilient gasket. I used a layer about 1/4 to 3/8 inch thick and about 3/4 inch wide, mounted on top of the original gasket. This eliminated the vibration. It also created a really air-tight seal, which is important for good bass response.

The same approach could be applied to drop-in car speakers by forming a silicone gasket on the underside of the speaker rim. I wouldn't be surprised if this same technique might improve the performance of some home loud speaker installations. It seems to me that manufacturers could supply better gaskets, maybe even just as add-ons.

-Ken Massey, Indianapolis, Ind.

(Audio magazine, JOSEPH GIOVANELLI, Apr. 1996)

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