JBL PS120 Subwoofer Review

The PS 120 is the top model in JBL’s new series of powered subwoofers designed to add one or more octaves to the lower bass range of most high-fidelity speaker systems. The PS120 contains a 100-watt amplifier and a single 12-inch driver with a 2-inch-diameter voice coil operating in a sealed enclosure. It is normally driven by the sum of the left-channel and right-channel signals, producing a mono output (a practical approach, since there is virtually no directional information in the subwoofer’s range). The summing is achieved electronically.

The PS120 has inputs for both speaker-level and line-level signals, enabling it to be driven by the output of a preamplifier or from the speaker outputs of the main system amplifier. It can be driven by either channel alone or by both.

The PS120’s rear panel contains a number of connectors and controls. Spring-loaded connectors (which accept only stripped wire ends) are used for the speaker-level inputs, and a second pair relays those input signals (unmodified) out to the stereo speakers. A pair of phono jacks are provided for line-level inputs, and a small switch reverses the subwoofer’s polarity to match the polarity of the main speakers.

A small knob varies the crossover frequency between 50 and 200 Hz.

Since the main system speakers operate over their full range when used with the PS 120, this setting is important (though fortunately not unduly critical) for achieving a smooth crossover to the subwoofer. A second knob adjusts the level of the subwoofer output, from zero to maximum, to match the level of the main speakers in the crossover region.

The subwoofer’s grille, which is not removable, occupies about two-thirds of the front panel and is angled slightly to the left (there is no indication that the driver itself is angled, nor would there be any advantage to angling it). A small LED indicator next to the grille lights in red when the speaker is plugged into a power source but not in use. When a signal is applied, its amplifier switches on and the light changes to green. After a few minutes of no signal, the amplifier shuts off automatically and the status light returns to red.

JBL says that the PS120’s lower response limit is 23 Hz and that the upper -6-dB point is variable between 62 and 250 Hz. No other performance specifications are given.

We made close-miked measurements of the JBL PS120’s frequency response at three settings of the crossover control: the limits of 50 and 200 Hz and the midpoint of the adjustment range. At the 50-Hz setting, the response variation was only about ±2 dB from 20 to 60 Hz, and the crossover slope was 20 dB per octave. At the maximum crossover setting of 200 Hz, the response varied about ± 3 dB from 20 to 200 Hz. The midpoint setting gave the flattest response in the range below 100 Hz, varying only ±1 dB from 100 to 20 Hz. The crossover’s slope was constant through the range of the frequency adjustment.

Using the 200-Hz crossover setting, we measured the distortion in the subwoofer output at the 100-Hz input level required to produce a 90-dB sound-pressure level (SPL) 1 meter from the speaker. The distortion remained less than 1 percent from about 280 Hz down to 62 Hz, increasing to 3 percent at 50 Hz, 7 percent at 40 Hz, and 20 percent at 30 Hz. At reduced outputs, distortion remained low down to 20 Hz, but at listening levels where realism would be substantially enhanced by a subwoofer, the PS120’s practical lower limit is probably in the vicinity of 30 Hz.

The PS120 switched on instantly with a signal input of less than 5 millivolts. After removal of the signal, the speaker remained active for about 4 or 5 minutes before shutting down. Extended operation at fairly high levels resulted in only a moderately warm back panel (the amp’s heat sink).

The PS 120 presumably was designed to deliver the essential qualities of a true subwoofer for people using moderately priced components. If that was JBL’s target, the designers hit it squarely. The PS 120 is a complete package, requiring no external amplifiers or crossover networks. Regardless of the sensitivity or frequency range of your existing speakers (assuming they are good ones that, like most, roll off below 50 or 60 Hz), the PS 120 should complement their characteristics very nicely and give you another octave of frequency coverage. Furthermore, its single, compact black box will not interfere with room decor or add a bunch of extra wires and cables to your installation. And best of all, perhaps, it is an affordable way to accomplish those ends without investing in another power amplifier.

In our listening tests, we teamed the PS 120 with a pair of high-quality, modestly priced speakers, which themselves were by no means lacking in bass. The added impact of that extra octave provided by the PS 120 gave its own answer to the question of whether a single powered subwoofer can be a worthwhile addition to a good, but not extravagant, music system. In the case of the JBL PS 120, the answer is a definite yes!

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