Audioclinic (Sept. 1980)

Tweeter Phasing

Q. In wiring together three tweeters of different makes, how can I positive ly ascertain the correct phasing since it is not possible to see the cones move when I pass a current through the voice-coils?

- A. J. Rizzo, West Palm Beach, Fla.

A. Using tweeters of different makes can be troublesome, not because of phasing considerations but because of differences in efficiency. If one tweeter happens to be far more or far less efficient than the rest, the system is usually operating with the most efficient tweeter(s).

As for determination of phase, connect one tweeter. Feed a test tone into the amplifier at a frequency which is covered by that tweeter, somewhere just above the crossover frequency, and then connect one of the remaining tweeters. If the volume increases, the wiring of the second tweeter is correct as to phase. (It just could hap pen that the volume increased be cause the second tweeter is much more efficient than the first one. To check, reverse the connections to the second tweeter and note if the volume decreases. If the efficiency of the second tweeter is far greater than that of the first tweeter, reversing the connections won't make an audible volume difference. What you must do is to pad down the most efficient tweeters to match the efficiency of the least efficient unit. Then the reversal of connections will have meaning.) Next, disconnect the second tweeter and wire in the third one. If the sound produced by the combination of tweeters one and two is louder, this third tweeter is wired correctly also. If it isn't, reverse the connections so that it does become louder.

Once all three tweeters are phased with respect to one another, the entire tweeter array must be phased with the woofer or midrange. As for how this is to be accomplished, well, this depends on the crossover networks used and their phase shifts. It may be necessary to put the tweeters out of phase with respect to the midrange at the cross over frequency. You might have to listen to program sources and check the phase this way. Another method is to take the completed speaker system out of doors and set up phasing with a sound level meter. The outdoor approach is useful because it tends to re duce reflected sound; the ear or the sound level meter has a much better chance of telling you the truth.

Noise From a Timer

Q. Having separate components connected to a preamplifier which is, in turn, connected to a programmable timer, I experience a "pop" emanating from the speakers whenever the timer turns the equipment on or off. This sound is noticeably loud; I fear it may eventually damage the speakers.

Would it help to connect the power amplifier directly into the a.c. line, bypassing the timer? I am not sure if the power amplifier should be left on continuously, as would be the case under these conditions. Is it harmful to do so?

- Edward J. DeLauter, APO N.Y.

A. Chances are that leaving your power amplifier running all the time would not solve the "pop" problem. The transients involved with turning on and off the timer would be fed from the output of the preamplifier into the input of the power amplifier, and still result in the "pops" you now hear. The sound might even be louder than it presently is.

If your timer has mechanical contacts (and I presume it does), locate them and shunt them with a series network consisting of a 200-ohm half-watt resistor and a 0.02 pF capacitor at 600Vd.c.

Minispeaker Cone Excursions

Q. My minispeakers display a large amount of passive radiator cone movement (approx. one-half inch) during deep bass passages at normal listening levels. Is this amount of excursion normal?

-SFC Gary M. Jartman, APO N.Y.

A. Minispeakers have relatively small cones. If these small cones are to be successful in producing bass, they must make up by motion what they cannot do by size, in terms of the amount of air they move. It is there fore to be expected that your cones will exhibit a large amount of motion when reproducing bass.

(Audio magazine, Sept. 1980; Joseph Giovanelli )

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