Velodyne Servo 1200 Subwoofer Review

A subwoofer, as its name implies, is a bass speaker designed to operate below the normal frequency range of a typical woofer. Unfortunately, the term is widely misused to refer to the lowest-frequency driver in a system, regardless of its actual operating-frequency limits.

True subwoofers are usually additions to a system’s main speakers, separate from them and often from a different manufacturer. Because of a subwoofer’s low operating-frequency range, it can be located almost anywhere in a room (frequencies below 100 Hz are not normally localizable by ear). Although some people feel that better results are obtained by using separate subwoofers for each channel, most subwoofers are designed to sum the low frequencies of the left and right channels and radiate them from a single enclosure.

Velodyne Acoustics, Inc. specializes in subwoofers, and the Servo 1200 is a recent addition to the low-price end of its line. The approximately cubical 60-pound box (18-1/4 inches square and 16-1/4 inches high, including its 3-inch feet) houses a 12-inch driver and a 100-watt amplifier. The bass driver is mounted on one side of the box, radiating outward in the conventional manner. The cabinet is finished in black wood-grain vinyl.

The driver has a 56-ounce magnet and a 2-3/8-inch-diameter voice coil with a length of slightly more than 1 inch. A piezoelectric accelerometer mounted on the voice coil senses its motion and supplies a feedback signal to the driving amplifier. This servo action (also known as motional feedback) is said to reduce the acoustic distortion of the subwoofer system and flatten its response.

The Servo 1200 can be driven from an amplifier’s normal speaker outputs or at line level from a preamplifier output. Its rated sensitivity is 1 volt at the speaker inputs or 100 millivolts (mv) at the line inputs, which have a nominal 10,000-ohm impedance. The Servo 1200 has a built-in gain-compression circuit that prevents amplifier clipping without affecting its normal operation. An extreme overload or a power-supply fault will automatically shut down the subwoofer’s power supply for 5 seconds. A built-in passive, high-pass crossover network channels frequencies above 85 Hz (with a 6-dB-per-octave rolloff) to the satellite speakers (normally the system’s regular stereo pair).

The Servo 1200 also has an adjustable active, low-pass crossover system that supplies low frequencies to its internal amplifier and driver. The crossover frequency is adjustable from 40 to 100 Hz, and its initial 6-dB-per-octave slope ultimately steepens to 24 dB per octave.

The back of the cabinet has spring-loaded input connectors for the normal speaker outputs of the system's amplifier and output connectors that deliver frequencies above 85 Hz to the satellite (main) speakers. There are also input and output phono jacks for line-level signals, with the outputs going to the satellites through an external power amplifier. The back panel also contains the knob to adjust the crossover frequency and the line cord. All of these are mounted on a metal plate that serves as a heat sink for the internal power amplifier.

The front of the cabinet has a removable cloth grille retained by plastic fittings, a pushbutton power switch and pilot light, and a knob that adjusts the subwoofer’s level relative to the satellite speaker level. In normal use, the Servo 1200 can be left powered at all times. Price: $895.

Lab Tests

We measured the frequency response of the Velodyne Servo 1200 by driving its line inputs directly from the output of our Audio Precision System One. At the 100-Hz setting of the crossover control on the speaker, the close-miked frequency response reached a broad maximum in the 35- to 55-Hz range and was down 6 dB at 20 and 85 Hz. There was almost no difference in the response curve using the 80-Hz setting, but with a 60-Hz cutoff the response was down 4 dB at 20 Hz and 6 dB at 78 Hz. The 40-hz setting produced a maximum response at 25 to 30 Hz, and the response was down 2 dB at 20 Hz and 6 dB at 65 Hz.

We measured the distortion in the acoustic output of the speaker with close microphone spacing, driving the speaker with a 50-Hz input that produced a 90-dB sound-pressure level (SPL) at 1 meter. The crossover control was set at 100 Hz. The high-resolution spectrum analysis in the Digital Domain mode of our System One showed a large number of low-level harmonics (up to the twentieth order) on a 1,000-hz display width. When the eighteen harmonics whose level exceeded -90 dB were combined in an RMS fashion, the equivalent total harmonic distortion (THD) was 0.425 percent.

A conventional measurement of THD plus noise versus frequency at the same input level showed a minimum distortion of just under 0.3 percent at 60 Hz and less than 0.5 percent from 45 to 100 Hz. A 1-percent distortion level was reached at 32 Hz and 2 percent at 25 Hz.

Comments

The measured performance of the Velodyne Servo 1200 indicated that it should be able to add at least an octave of useful bass to most home speaker systems, including some fairly expensive ones.

We put it to the test, using it both with a pair of inexpensive minispeakers, Triad System Fives, and a pair of high-quality compact speakers, Celestion SL-12i’s. The Triad speakers, not surprisingly, demonstrated the capabilities of the Servo 1200 most dramatically. The combination produced floor-shaking bass that almost seemed out of keeping with the diminutive size of the satellites.

In a sense, however, the Celestion speakers seemed to benefit even more from the addition of the subwoofer. Despite having a combined volume less than that of the subwoofer, they are capable of delivering excellent sound unaided. Yet the addition of the Servo 1200 converted a very good-sounding system into an even better one.

The setup and adjustment of the Servo 1200 subwoofer is easy and takes only a few seconds. The setting of the crossover frequency is not critical, although I obtained the best results by setting it fairly low (60 Hz with the Celestions and 80 Hz with the Triads). The most important part of the process is setting the level of the subwoofer output, since too much bass, while it can generate some interesting effects, imparts an unnatural quality (especially to voices). The speaker has lots of reserve level-adjustment range, and I usually preferred to set the level at or just below the center of its range.

The Servo 1200 is one of the more affordable general-purpose powered subwoofers currently on the market. While it is not likely to see much use in low-price systems (where, ironically, its benefits are probably most needed), it would make a worthwhile improvement in almost any system. A full-range speaker system with the bass response of this unit would be very large and very expensive. There is probably no easier way to get a system with satisfying deep-bass performance that will fit into a modest-size room (and budget) than to combine a pair of good medium-price speakers with this subwoofer. Viewed in that light, it is a good value indeed.

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