AUDIOCLINIC (Nov. 1985)

Cogging

Q. What is meant by "cogging," with reference to turntables?

-Robert C. Watson, Dover, Del.

A. The armatures in some electric motors rotate in a series of stepped movements rather than one smooth, continuous motion. This resembles the jerky transitions created as some gear cogs mesh, disengage and mesh again, and is therefore referred to as "cogging." If such a motor is used in a turntable or tape machine, it can cause jerky motion. Most or all of this jerkiness will be filtered out by the flywheel action of a turntable's platter or a tape recorder's flywheel. Cogging is most notice able while a motor is coming up to its full speed.

Broadcast Dynamics

Q. It sounds as though many radio stations are using dbx encoding in their broadcasts. The dynamic range is low. Would a dbx decoder expand the range to what it should be?

-Steve Herrick, Swartz Creek, Mich.

A. I understand where you have gotten the feeling that radio stations are using dbx encoding-soft pas sages are elevated and you hear cymbals being "sucked down" every time a bass note is heard. What is really happening is that the broadcasters are employing very heavy limiting, which does indeed remove dynamic range.

(They do it so they can cover a greater area without background noise being audible.)

A range expander might help, but only in those instances where the limiting is not so complete as to remove all traces of dynamics. These expanders require at least some variations in audio level on which to build. Thus, if dynamics are lost completely, the ex pander will be rendered ineffective.

Broken Anti-Skating Mechanism

Q. The anti-skating mechanism on my turntable is broken, and the "fix" would be to replace the complete tone-arm assembly at a cost of $63. Simply put, how important is the anti-skating compensation?

-L. C. Davis, Jr., Corona, N.Y.

A. The need for anti-skating on a given turntable system depends on the cartridge and its compliance, as well as the amount of drift that is present without compensation.

I suggest you place the blank side of an Eva-Tone Soundsheet (one of those thin sheet-plastic discs sometimes bound into magazines) on your turntable. With the table rotating, place the tonearm on this disc and observe the amount of drift. If the drift is rapid, you may well need anti-skating compensation. If the drift is very slow, you may be able to get away without such compensation-especially if the stylus is a medium- to low-compliance type. (Editor's Note: An Eva-Tone Soundsheet was bound into our August 1984 issue.)

Multi-Speaker Impedance Formula

Q. What is the mathematical formula for calculating the impedance of a number of loudspeakers connected together? What information is needed other than the impedance rating of the individual loudspeakers?

-Jim Hajeski, Bricktown, N.J.

A. To determine the combined impedance "seen" by a power amplifier, you need to know the nominal impedance of the various loudspeaker systems with which you are concerned. In the case of woofers whose minimum impedance differs markedly from their nominal impedance, you should know the minimum impedance and perhaps use this value rather than the nominal impedance. Even here, if the minimum impedance occurs at a frequency above 100 or 200 Hz, then you can neglect it and simply use the nominal impedance of the woofer.

The total impedance of speakers wired in series is the sum of their combined impedances. For example, the impedance of an 8-ohm and a 4-ohm speaker in series would be 12 ohms.

The total impedance of speakers wired in parallel is found by adding the reciprocals of their impedances (to make a reciprocal, divide 1 by the value of the impedance), then inverting the result. Thus, two 8-ohm speakers in parallel would have a total impedance of 4 ohms (1/8 plus 1/8 equals 1/4; inverted, that is 4).

Computers and FM Interference

Q. Help! My son's IBM-PC is hashing up my FM listening, sometimes with a continuous varying level (more like white noise), occasionally with crack les and pops and some heterodyning.

The interference is worst when I'm listening to distant stations.

I have tried a power-line filter. I have tried an isolation transformer. I have tried a high-pass interference filter (in the antenna input). None of these devices helped reduce the interference.

My FM antenna is the twin-lead type supplied with my tuner. I have it mounted on a beam which I can manually rotate, and the beam is mounted on the ceiling. Without the computer being on, the antenna provides fine reception. If you can solve my problem, I am your friend for life!

-L. J. D'Antonio, Londonderry, N.H.

A. Your problem cannot be solved by any of the devices you have tried. I suppose that, if the power cord is neatly wound to as short a length as practical, and one or another of your power line filters is placed at the computer, some improvement is possible.

The problem is that computers generate high-frequency digital pulses which, being square rather than sine waves, are rich in harmonics through out the FM and TV broadcast bands.

The FCC recognizes this problem, and has set radiation limits which computers must adhere to. Manufacturers do shield the innards of their computers, but even so, they are certainly not completely "clean." The cables which connect the computer to such peripheral equipment as disk drives, printers and so on can also act as transmitting antennas whose resonant frequencies may be within the FM band. Shielded cables are available, at least for some applications, though they are very hard to find.

If you can't find them, it might be possible to wrap copper screen around the cables and ground it, as a shield.

The interference enters your tuner via its antenna input terminals, just as the desired stations do. Because the interfering signal is composed of components which have the same frequencies as your favorite stations, the tuner, antenna and high-pass filter do not know the difference between what you want to hear and what you don't. If you filter out the interference, you filter out your favorite stations as well.

Now for the cure (though it may not make you my friend for life): Get that antenna away from the computer, the farther away the better. Put an outdoor unit on the roof where it belongs. Use coaxial cable to connect the antenna to the tuner. If you are not permitted to install an outdoor antenna, put your antenna in the attic. Use coaxial cable and appropriate balun transformers to connect to the tuner. If you do not have an attic, place the antenna in the room farthest from the computer and its radiating cables. Again, use coaxial cable to feed the tuner.

What I have told you comes from experience. My Apple (which I am using to write this) generates a lot of "hash," but with a good outdoor antenna, my FM radio reception is totally unaffected by it.

Resistors in Loudspeakers

Q. I recently opened my two-way loudspeaker enclosures and was surprised to see perhaps a half-dozen resistors. What role do these resistors play in sound reproduction?

-Dan Welton, Shelton, Conn.

A. The resistors you have noticed are probably in the crossover network.

They may be used to attenuate the signal feeding the tweeter, or they may be there as a part of a treble-boost network. In this regard, some tweeters are not absolutely flat; it is therefore necessary to trim the high-frequency response with a resistance/capacitance network.

Along these same lines, I have seen some circuits where resistors and capacitors are used to boost the extreme top end of the audio spectrum in order to compensate for high-frequency losses due to room acoustics or to accommodate personal taste.

Insulation in Speaker Systems

Q. I am building a speaker cabinet. Why is foam needed?

-Richard Hamilton, Mt. Vernon, N.Y.

A. There is no hard and fast rule about using foam or any other insulation material inside a loudspeaker en closure. Generally speaking, insulation is installed in order to prevent reflections between the walls of the enclosure and the speaker cones. Sound bouncing off the walls and hitting the cone will force the cone to vibrate. This cone vibration will be heard along with the signal. and will color the sound that you hear.

If you have ever sung in the shower, you know now your voice is reinforced because of the hard, reflective surfaces of the walls. The voice tends to be colored in a pleasant way. It makes the poorest of voices seem better than they really are.

In high-fidelity applications, however, we do not want any added sound. (If we are reproducing a poor voice. then so be it; the sound system is not there to make it better.) So the reflections within a loudspeaker enclosure must be suppressed. This is done by producing "soft" walls, lining them with sound-absorbing materials which do not retransmit the sound to the air within the box.

Phase-Inverting Preamplifiers

Q. Some preamps are phase-inverting. To compensate, does one reverse the leads of both loudspeaker systems or of just one?

-Eugene L. Bershad, Freehold, N.J.

A. When using phase-inverting pre-amplifiers, there is no need to compensate by changing the phase of the loudspeaker systems from what would be used with a preamplifier whose in put/output signals are in phase.

(Source: Audio magazine, Nov. 1985, JOSEPH GIOVANELLI)

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