AUDIOCLINIC (Feb. 1997)

Getting More Volume

Q. How can I get more volume from my power amplifier and sub/sat system? If I buy another power amplifier and biamp the speakers, would that help? I want them to handle extremely loud rock and still sound good with quiet acoustic guitar music.-Phil Wall, Renton, Wash.

A. A speaker can produce only so much sound level without being damaged by excessive input power. Speakers tend to sound coarse and gritty as they near their output limit, or, on a sudden bass transient, the voice coils may produce a sharp snap as they hit the magnet structure (which may damage the coils). If you hear either of these symptoms, lower the volume; you cannot obtain more output.

Is the sub powered or passive? If the latter, then biamping the system-driving the subwoofer and the satellites with separate amplifiers and high-pass filtering the satellites-should result in higher sound levels because the satellites will be relieved of low bass duties and will play louder and cleaner.

But if you are already using a powered sub that has built-in low- and high-pass filters, there will be limits on how loud the system will play.

Perhaps your amplifier is too low-powered and you are pushing it into clipping, which may also result in audible distortion.

The amplifier should have significant re serves of power to handle sudden dynamic peaks.

Achieving higher sound pressure levels may require larger, multi-driver main speakers, a more powerful amplifier, or a second subwoofer. You should also consider trying horn-loaded speakers whose rated sensitivity is very high, 96 to 100 dB SPL. These can produce very high sound levels-albeit, perhaps, at some loss of musical accuracy-with very low distortion.

Foreign FM Connectors

Q. The plug on my 75-ohm FM antenna does not match the 75-ohm input jack on my receiver. The antenna plug has an internally threaded barrel with a hexagonal rim; the jack has a smooth barrel, like an oversize RCA plug, and a thin tube going down the middle. How can I connect them?-G. F. McCarthy, Astoria, N.Y.

A. Your receiver apparently has a European-style antenna jack. Luckily, Radio Shack has an adaptor (Catalog No. 278 265) that will solve your problem for $1.99.

Proper Storage of CDs

Q. Because of space limitations, I put two CDs in one jewel box, one on top of the other. If the upper CD's playing side is touching the label side of the CD below, is this bad? Storing discs "label to label" is not as convenient, but is it preferable? Can I extend this to storing three or four discs in one box?

-Anthony Hudaverdi, Santa Monica, Cal.

A. I do not like the surfaces of CDs to touch, however they're stored-label to label or any other way. Stacking three or four CDs in a single jewel box is even worse:

The holding spindles are not designed to secure that many discs, and scuffing can occur when such boxes are transported.

Packaging designers and record companies have become more creative lately, producing jewel boxes no thicker than a single case. Yet these boxes can accommodate two CDs that don't touch each other because of a hinged plastic panel that folds out to re veal the second disc. Two-disc sets from Ryko, Delos, and Time/Life Records are no table in this regard. I have not been successful in locating a source for spare two-disc boxes, but if sufficient demand arises, I'm sure they will be sold by themselves.

Distortion of Choral Passages

Q. I have a CD transport with a coaxial digital output and a CD changer with an optical (Toslink) digital output. These are connected to a Theta Digital jitter-reduction box and the latter to a mid-priced D/A converter. The system sounds fine except for audible distortion during loud choral passages (especially with female voices) and on some loud instrumental passages. The distortion sounds like a highly modulated LP played with an old noncompliant phono cartridge from the '50s. Cures have been suggested by everyone from a friend, who recommends AC power conditioners for the digital gear, to a high-end dealer who wishes me to part with most of my savings. The distortion is present through speakers as well as electrostatic head phones and is more noticeable with the CD changer's optical output than the transport's coaxial source.

-Eduardo A. Benet, Key Biscayne, Fla.

A. The most likely explanations are that there is something wrong with your D/A converter that is causing it to distort (which should be reparable) or that the in put circuits ahead of your preamplifier's volume control are overloading on high level passages. Choral works are tricky to reproduce at best, and a little bit of overload will cause them to sound like mistracked LPs. DB Systems ( Main St., Rindge Center, N.H. 03461; 603/899-5121) has output attenuators that can be inserted between your CD player or D/A converter and your pre amplifier's input. You will likely need to attenuate the signal by 6 dB or more to solve this problem. You can prove that the distortion is the result of overload if your D/A converter has a headphone output: Listen through dynamic 'phones, not electrostatics, to see if you still hear the distortion.

If you determine that preamp overload is not the problem, try bypassing the jitter-reduction box and the outboard D/A converter entirely. Use the CD changer's internal converter, plugging it into your system directly. If you don't hear the distortion, then at least you have isolated the problem to the transport, the jitter reducer, or the out board D/A converter.

Laserdisc Interference

Q. When playing laserdiscs, I often hear two guys talking with each other on ham radio; it comes through my center-channel speaker loud and clear. I never hear this interference when I'm playing other program sources. What can I do to eliminate it?

-Name withheld

A. Since the RF interference is present only when you play laserdiscs, it is entering your system via the player itself, the video or audio interconnects, or the AC line cord. The RF could even penetrate the player directly if it is not well shielded.

Disconnect the audio and video cables from the laserdisc player. If the player has a headphone jack, put on headphones and play a laserdisc at a time when you know the RF interference is present. If you hear the interference, you know the AC cord or the player itself is responsible. Assuming the AC cord is the culprit, coil up the excess and tuck it close to the player's chassis. That alone might solve the problem. If the RF persists, try adding ferrite beads or collars made for such a purpose to the line cord.

Keep the cord as short as possible so that it becomes less of an antenna.

If no interference is heard through head phones, then the player and its AC cord are not at fault. Therefore, the problem must be the A/V cables. Reconnect them, and play a disc when the interference is present. Disconnect the video cable and see if you can still hear the interference. If you can't, you have isolated the source of the problem. You must use the shortest, best-shielded video cable possible. If there is any excess, coil it up and tuck it close to the player. You might have to place some ferrite beads on the cable, near the player. If you still hear the interference, then the audio cables are at fault.

Again, use cables with the best shielding possible-real braid, not spirally wound shield. Because you hear the RF only in the center channel, it may be entering your sys tem in equal amounts through the left and right audio cables; the Dolby Pro Logic de coder would then interpret it as a mono signal and route it to the center.

If the interference still persists after the aforementioned fixes, you may have to modify your amplifier or receiver by inserting RF chokes or ferrite beads. An RF choke can be placed at each input, wired right to the connector. The other end of the choke is connected to the point that was originally wired to the input connector. You will need a choke for each channel. Try a value of per haps 0.5 millihenry as a start. I hope you won't have to use bypass capacitors because of possible high-frequency losses.

Before you embark on this, try moving the laserdisc player and receiver to a different location in the room or elsewhere in your home. Although this is inconvenient, you may find that the interference disappears. Last, try contacting the ham operators you're hearing; they likely live nearby (or you wouldn't be getting the interference). Most hams are very knowledgeable about RF troubles and will be more than happy to help you solve the problem.

Balanced vs. Unbalanced Circuitry

Q. What are the differences between balanced and unbalanced input circuits in power amps? What are they used for?

-Derek Collins, New Port Richey, Fla.

A. Balanced circuits, found in some con sumer audio components and most pro audio gear, will reduce hum that is picked up via the cables connected to the inputs. However, I don't believe balanced circuits are necessary in most consumer installations because interconnects are usually kept short and hum is low.

In an unbalanced circuit, one signal in put connection is "hot" while the other is at ground potential. To hook up components having unbalanced connectors, you use a cable consisting of a hot center conductor surrounded by a shield, which connects to the chassis ground.

An unbalanced circuit's signal voltage alternates above and below ground potential in terms of its electrical polarity. Any hum that might penetrate the shield can induce hum voltage in the hot lead; that hum will be passed to the amplifier and speakers, where it may be audible.

In a balanced input circuit, the amplifier again has two leads that will accept signal, but in this case neither connection is grounded. Each connection is opposite in electrical polarity from its counterpart. A shielded cable is used, but with two center conductors, each connected to one of the input leads. The shield is again grounded, but it carries no signal. Any hum that penetrates the shield will induce a hum voltage into both center conductors, but the hum will have the same polarity in each conductor. However, in order for the hum to be passed to the amplifier, it would have to produce voltages with opposite polarities in the two center conductors. Inasmuch as this is not the case, the hum signals cancel at the amplifier input and no hum is heard.

Center-Channel Hiss

Q. I drive my center-channel speaker with a mono power amp because my Dolby Pro Logic receiver has only a preamp-level center-channel output. When I turn the volume fully down, I hear a "shhh" in the center speaker; if I increase the volume of the pro gram, the hiss disappears. (I've verified that the connections are correct.) Friends tell me this occurs because my speaker (rated for 15 to 175 watts) is very efficient.

-Ferdi Mariano, via e-mail

A. If your speaker's sensitivity is high, your friends are right. Your speaker is not causing the hiss, but you probably wouldn't hear it if the speaker weren't so efficient at translating the noise's presumably low voltage into sound. Check the speaker's manual (or, if it's still made, its listing in our Annual Equipment Directory in the October issue) for its sensitivity rating; a figure somewhat above 90 dB is high.

The hiss could be coming from your power amp, your receiver, or both. In any case, it's generated after your receiver's volume control, or you would not hear it with the volume turned down.

If your mono amp is an early solid-state design or a tube model whose tubes have gone blue and gassy, it's probably the culprit. Turn off your system and disconnect the receiver from the amp's input. If you still hear the hiss when you turn the system back on, it must be coming from the amp. If your receiver has preamp-out and main-in jacks, you can double-check this by routing the center signal through one of the receiver's amplifier channels to see if the hiss disappears.

If the hiss disappears when the receiver is disconnected from the amp, then it must come from the receiver. You may be able to reduce or eliminate it by turning the receiver's center-channel output up and your mono amp's input level control down. If there is no input level control, you must place one between the center-channel line output of the receiver and the power amplifier input. (Switchcraft makes such a device: a standard potentiometer circuit, suitably mounted in a box equipped with all necessary input and output connectors.) Alter natively, you can buy a fixed attenuator that drops the signal about 12 dB from DB Systems ( Main Street, Rindge Center, N.H. 03461; 603/899-5121).

Adding Speakers to a Mini–System

Q. Can I connect additional speakers to a shelf system? I'm planning to buy a mini-system to save space and add additional speakers to get better sound.

-Eddie Chin, via e-mail

A. The mini-system's modest amplifier may determine whether you can connect extra speakers. Mini-systems usually cannot handle loads of less than 6 or 8 ohms (check the owner's manual or spec sheet). If the system's existing speakers are rated at 8 ohms and you connect a second 8-ohm pair in parallel, the amplifier will see a combined load of 4 ohms, which could trigger overheating and protection-circuit shutdown.

However, you can connect a second pair of speakers in series with the first pair, which will keep the impedance high and won't stress the amplifier-so long as the amp has adequate power to drive both pairs. Many mini-systems have "mini" amplifiers, typically from 10 to 25 watts per channel, which will be ample for moderate listening levels but inadequate at a high volume level. If you choose speakers that have high rated sensitivity--say, 93 dB SPL or higher--you'll ease the demands on the low-powered amplifier. On the other hand, series connection of loudspeakers that are not identical will result in some (probably undesirable) alteration of frequency response.

Consider simply replacing the mini-system's existing speakers with high-quality compact models if you want to improve sound quality. Just be sure to check that the new speakers' minimum power requirement can be met by your mini-system's amplifier.

Improving Weak AM Reception

Q. I live about 50 or 60 miles from New York City and listen to its AM stations. I receive WABC and WCBS very well on all my radios, but WNYC comes in poorly. I would like to receive this station on a small pocket radio and on my hi-fi system. Is there a relatively inexpensive and simple way to do this?

- Richard Aaron, Sussex, N.J.

A. The last I heard, WABC and WCBS, both flagship stations of large net works, had considerably more transmitter power than the nonprofit WNYC, which is why it does not come through as clearly.

The only device I know of that will boost your AM reception on a wide range of radios is the C. Crane Company's Select-A Tenna, which can provide as much as 30 dB of signal boost. The basic model ($57.95) is a passive tuned loop in a plastic case about 11 inches in diameter. To use the unit, you place it near your radio's antenna loop, tune to the desired station, orient the radio for best reception, and then adjust the Select-A Tenna's tuning knob for the loudest signal.

If your house has a steel frame or metallized insulation that keeps signals out, you can use Crane's Model M ($72.95), which includes a wire to feed out your window or tape to a windowpane.

If your tuner's antenna is not accessible but the tuner has terminals for an external AM antenna, try using the Hardwire Select A-Tenna (also $72.95), which can be connected to those terminals. A special remote model, the Ferrite Select-A-Tenna ($86.95), couples to your tuner or AM radio from up to 6 feet away. For more information on these products, contact the C. Crane Co. ( 558 10th St., Fortuna, Cal. 95540; 800/522 8863; e-mail, ccraneco@aol.com).

(Audio magazine, JOSEPH GIOVANELLI, Feb. 1997)

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