Allison AL 120 Speaker System Review

The AL 120, at the middle of a series of loudspeakers from Allison Acoustics, is a compact two-way, floor-standing system. The cabinet is finished in a light oak-grain veneer, and all four front edges are grooved to retain the curved black plastic grille, which is easily removed and replaced.

The AL 120 uses two 6-inch woofers in an acoustic-suspension enclosure. The upper one, located in the middle of the front panel, faces forward in the conventional manner. The lower one, however, is physically reversed, with its cone facing inward and the magnet facing forward. Both woofers cover the same frequency range (up to 2,000 Hz), but they are connected electrically out of phase to compensate for their opposite orientation, so that their cones operate acoustically in phase. This “push-pull” configuration, which is similar to the one Allison uses in its top-of-the-line IC-20 system, is said to reduce low-frequency, even-harmonic distortion resulting from driver-suspension nonlinearity during large cone excursions. Such distortions tend to cancel out in the combined output of the two drivers, whose cones are at opposite ends of their travel when driven to extremes.

At 2,000 Hz there is a crossover to a 1-inch Allison convex-diaphragm tweeter, cooled and damped with ferrofluid, that is located conventionally at the top of the front panel. Its dome is protected by a metal mesh grille. The AL 120 has no external level adjustments.

The system’s specifications include a 6-ohm nominal, 3.5-ohm minimum impedance, a 90-dB sensitivity, and a low-frequency resonance of 65 Hz. The bass response is rated as — 3 dB at 52 Hz and — 6 dB at 42 Hz. To protect against thermal overload, all the drivers are wired in series with Power Guard bistable resistors, which normally have a very low resistance and do not affect the speaker’s performance. If excessive current flows through a voice coil, however, the protective resistance suddenly increases to hundreds of ohms, reducing the speaker drive to a safe level. When the overload is removed, the system automatically returns to normal operation.

According to Allison, the AL 120 may be used with amplifiers with up to 150 watts output, clipping 10 percent of the time, and over most of the frequency range it will handle at least 350 watts for 0.1 second, 125 watts for 1 second, and 60 watts for 10 seconds.

The input binding posts recessed into the rear of the speaker cabinet can accept single or dual banana plugs. The Allison AL 120 measures 24 inches high, 11-1/8 inches wide, and 10-3/4 inches deep (11-3/4 inches including the curved grills). Weight is about 33 pounds. Price: $600 a pair.

Lab Tests

When we spliced our close-miked woofer-response curve with the room response of the AL 120, the resulting composite curve had a dip of about 4 dB from 650 to 700 Hz and a peak of the same amplitude at 1,600 Hz. The lower-midrange and upper-bass response was within + 1.5, —1 dB from 100 to 550 Hz, with a small rise to +3 dB at 75 Hz and a 12-dB-per-octave rolloff below 70 Hz. The overall response variation was ±4 dB from 48 to 20,000 Hz, with the maximum output occurring at about 1,800 Hz.

The close-miked woofer measurements were made with the microphone midway between the two woofers to measure their combined output. The response curve showed a large dip at 700 Hz, of about 9 dB amplitude, that was apparently the result of interference between the outputs of the two drivers. As the room response showed, however, this dip affected the total output of the system rather than being an artifact of the close-miked measurement. Individual close-miked measurements of the two drivers suggested that the effect was associated with the lower (inward-facing) woofer, although the two had nearly identical response curves up to 400 Hz except that the output level of the lower cone was about 6 to 7 dB lower than that of the upper cone. The close-miked measurements also indicated that the crossover frequency to the tweeter was, as specified, about 2,000 Hz.

Allison convex-diaphragm tweeters are noted for their dispersion, which was amply demonstrated by our measurements. The response curves measured on-axis and 45 degrees off-axis were virtually identical up to 10,000 Hz and then diverged. The system’s overall group-delay variation was within 0.2 millisecond from 2,000 Hz to beyond 20,000 Hz, reaching about 2 milliseconds at 180 Hz.

The sensitivity of the AL 120 was slightly higher than rated, with a measured 92-dB sound-pressure level (SPL) at 1 meter with a 2.83-volt input of pink noise. Its impedance, however, was unusually low, measuring 3 ohms below 40 Hz and between 100 and 300 Hz. At the bass resonance frequency of 65 Hz, the impedance was 5 ohms, and it rose to a maximum of 9 to 10 ohms between 1,600 and 3,000 Hz before dropping to 3.5 to 4 ohms from 6,000 to 20,000 Hz. Although the speaker’s impedance had no effect on its acoustic performance, we do question its “6-ohm” impedance rating. A 4-ohm rating might be justifiable, although it did have a 3-ohm impedance over a wide frequency range.

Because of the system’s unusual woofer configuration, we were especially interested to check the low-frequency distortion. With a constant drive level of 2.25 volts (equivalent to a 90-dB SPL at 1 meter), the distortion was only 8 percent at 20 Hz, 2.5 percent at 40 Hz, and between 0.5 and 0.7 percent from 60 to 130 Hz. This is exceptional performance for a pair of 6-inch drivers and seems to confirm the merit of the push-pull design.

The power-handling ability of these drivers was impressively demonstrated by the way they absorbed a single-pulse input of 330 watts into their 3.2-ohm impedance at 100 Hz before generating an audible rattle. In pulse tests at middle and high frequencies, our amplifier clipped at outputs in the 1,200- to 1,600-watt range before there was any audible sign of distress from the speakers.

Comments

The installation instructions for the Allison AL 120 speakers recommend that they be placed on the floor, several feet from any wall, for optimum performance. Although the instructions do not rule out placement on a table, shelf, or stands, we chose to follow the recommendation of the manufacturer.

On first hearing, perhaps the most obvious characteristic of the AL 120 was its apparent brightness. In comparison, our reference speakers temporarily seemed rather dull or muted. Since measurements did not reveal a rising high-end response, we had to conclude that the AL 120’s emphasized output from 1,000 to 3,000 Hz was responsible for this effect. Experiments with a graphic equalizer soon confirmed that hypothesis.

More extended listening indicated that the brightness we had heard was program dependent. Unless the recording we played had an overabundance of upper-midrange content, the sound was excellent without resorting to external equalization. And at all times the smooth, extended, and nonboomy bass response of the AL 120 set it apart from most other speakers of comparable size, demonstrating its worthiness to bear the Allison name.

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