AUDIOCLINIC (Jan. 1993)

--Subwoofer Crossover Considerations--

Q. I have read that a weakness in a subwoofer/satellite system could be a gap between the upper frequency limit of the woofer and the lower frequency limit of the satellite speakers.

I have considered adding a 12-inch sub- woofer to my existing loudspeakers. The frequency response of the speakers that would then become the satellite systems extends down to 60 Hz. The subwoofer’s response is 29 to 180 Hz. Because its 180 Hz overlaps the frequencies of the satellites, would these speaker systems blend all right? Is there a gap?

A. If your subwoofer could not handle frequencies above, say, 40 Hz, there would be a gap between its upper cutoff and the 60-Hz cutoff of your smaller speakers. But you have an overlap, the opposite of a gap.

That’s not too good, either. The frequencies in the overlapped region, 60 to 180 Hz, may be overemphasized, because both speakers are producing them. Or you may get cancellations, due to phase differences between the two speakers’ outputs in this frequency range.

Luckily, overlaps are easier to cure than gaps. Just add a crossover to channel low frequencies to the woofer and everything else to your satellites. In your case, the crossover should be set to something like 100 Hz. This should permit both the satellites and the subwoofer to operate over the flattest portions of their response curves.

--Playing Records Monophonically--

Q. I have a large collection of LPs dating back to 1953. My new receiver, unlike previous units, does not have a button for mono. I can’t play my monophonic discs monophonically, and I don’t get as good sound because of this. Is there anything I can do to restore this option?

A. Monophonic records usually do sound better when played monophonically. Any vertical information picked up by a stereo cartridge playing mono records will include only rumble (some of which may be on the record itself), some distortion caused by pinch effect (which drives the stylus upward, especially at high frequencies), and noise. Since this information shows up as a difference between the channels, you need only cross-connect the channels to eliminate it.

The easy way is to connect two shielded Y adaptors in series between your turntable and your preamp. You need one Y adaptor with two female RCA jacks and one male RCA plug (such as Radio Shack #42-2436) and one with a female and two males (such as Radio Shack #42-2435). If this hookup gives you hum problems, try detaching your turntable’s ground lead from your preamp. If the problem persists, carefully trim back the insulation from one of the two RCA plugs that goes into your preamp and then sever the shield surrounding its center conductor—just be careful not to cut into that conductor or the insulator surrounding it.

A more elegant way, which would save you the trouble of plugging and unplugging things, would be to install four RCA jacks and a single-pole, single-throw switch in a small metal box. Wire the left-channel jack of one pair to the left jack of the other, and do the same for the two right-channel jacks. Wire the switch between the center terminals of one pair’s left and right jacks. With the switch open, signals pass through the box unchanged. When you close the switch, the two channels are shorted together, and the signal becomes monophonic. Connect your turntable leads to one pair of jacks, and run a cable from the other to your preamp. It’s a good idea to leave the box in plain sight when you set it to mono to remind you to switch it back again for normal stereo listening.

--CD Timing Accuracy--

Q. I recently purchased a new CD player and have a question about its timing accuracy. Considering all of my experiences with open-reel tape decks and cassette decks, I have noticed that few home units approach professional recording and broadcast standards for timing accuracy: 0.1% speed accuracy, or 3.6 S per hour.

It was natural for me, therefore, to continue the practice of checking the accuracy of CD players. Most of the players I have owned have run just slightly under the times shown on the CDs. You can imagine my surprise when I checked the accuracy of my new player and found it ran about 12 S per hour faster than the expected timings!

You may not think this is a matter of any concern, but this was an expensive purchase, and I would think that it should offer better timing accuracy. I would like your opinion in this matter while I await a response from the manufacturer.

A. The timing of the data stream in a CD player is controlled by its internal clock circuits. If you can obtain a service manual for your player, you may find a trimmer that can be used to adjust the frequency of the clock oscillator. If the player is still under warranty, the maker should be willing to adjust the clock.

(After I sent this advice to Mr. Bailey, he replied: My faith is renewed! The manufacturer replaced the clock module with a high-accuracy unit, and the player now times out perfectly.)

--Are My Loudspeakers Any Good?--

Q. I recently purchased a pair of loud speakers based on how they sounded at my dealer. I have no stereo components as such, so I connected these speakers to my wife’s rack system.

At first I was very pleased with the sound, but then the right speaker sounded very “bass heavy.” I took the loudspeakers back to my dealer, and he tested the loudspeakers by connecting them to a high-powered amplifier.

The dealer could not detect any problems with the speakers and told me that my amplifier was probably not powerful enough to handle the dynamics of the music.

So, now what? What should I do if I invest in a high-quality, high-power amplifier and still have this bass problem?

A. It is difficult to answer your question without having been with you when you were evaluating your loudspeakers. About all I can say is that if these speakers sounded as you expected at your dealer and if they now sound poor at home, there may be some problem in your wife’s rack system. As I understand it, the sound was fine at first, and then deteriorated. This may indicate that something has gone wrong with the rack system, though it’s difficult to imagine what could go wrong that would result in over-accentuated bass.

I really suspect that the bass problem was there all along but that until you became accustomed to the equipment, you failed to notice it.

This bass heaviness may well be the result of room acoustics. Experiment by placing the loudspeakers in different parts of the room and at various distances from walls; you may well find a location that solves the problem.

If you have not resolved this problem, see if you can borrow a friend’s system and connect it to your loudspeakers. If you notice a marked improvement in audio quality, you will know that you need better components. If nothing helps, let’s hope your dealer will exchange your loudspeakers or refund your money.

--More Hiss on CDs than LPs?--

Q. My CDs of rock music from the 1960s and early ‘70s contain much more tape hiss than those same recordings on the original LPs. This problem does not appear on my jazz CDs, and many of these are from the late ‘50s and early ‘60s.

Can you explain why this should be so? Also, at what frequency is this hiss occurring? Would knowing this allow me to use my equalizer to cut it down?

A. I don’t have a definite explanation. Possibly, the rock material was so rich in treble that the engineers had to roll the treble off a bit when mastering the LPs, so as not to cause tracing distortion. This would also roll off any hiss in the master tape. As CDs have no treble saturation problems, they’d require no such roll-off.

Many of the master tapes of these recordings were doubtless made before Dolby noise reduction became popular, so they’ll have audible tape hiss. Overdubbing through multiple generations would raise the hiss still higher. Such overdubbing was and is more common with rock than jazz, which might account for the difference. Also, acoustical instruments are more used in jazz than rock, and such instruments don’t have enough highs to force the engineer to cut back the treble when cutting the LP master. Hiss is usually not a single frequency but a wide band of frequencies that overlap the music, so it’s impossible to equalize it out without losing some musical overtones as well.

HISS IS NOT A SINGLE FREQUENCY, SO YOU CAN’T EQUALIZE IT OUT WITHOUT LOSING MUSICAL OVERTONES.

I’d suggest you experiment by rolling off frequencies in the range from 3 to 5 kHz, where the ear is most sensitive to hiss. Attenuate these frequencies as little as possible, because the sonic quality of the music will also be affected. Try listening to an equalized signal from the CD player and an unequalized signal from LPs, and adjust your equalizer until the two sounds match.

Editor’s Note: Extra hiss can also occur on CDs that are made from later tape generations than the master used for the LP, or from tapes that have been improperly stored in the decades between LP and CD mastering. When trying to get rid of hiss, I’ve found it helpful to cut the top most equalizer band or two to reduce severe hiss; this works best on recordings that don’t have much tonal content above 10 or 12 kHz. And the Auto-correlator circuit on some older Carver preamplifiers (it’s not on the current ones) does a decent job of reducing hiss but has minimal effect on the music.

( Audio magazine, JOSEPH GIOVANELLI, Jan. 1993)

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