Velodyne ULD-12 Subwoofer Review

The Velodyne Acoustics ULD-12 powered subwoofer brings the essential characteristics of the company’s previous models, which used 15-and 18-inch drivers, to a smaller, lighter, and less expensive product. The “ULD” nomenclature stands for “ultra-low distortion.” All the Velodyne ULD subwoofers are motional-feedback speakers; that is, an accelerometer attached to the voice coils supplies a negative-feedback signal to the input from the driving amplifier. Negative feedback has the same effect on a speaker’s output as it does in amplifiers—it extends and flattens the frequency response and reduces distortion.

The 12-inch driver of the ULD-12 has an 80-ounce magnet structure and a 2-1/2-inch voice coil capable of a 3/4-inch (peak-to-peak) linear excursion. The downward-facing driver is mounted on the bottom of the sealed cabinet, which measures 21 x 16 x 17 inches, including its 2-3/4-inch feet, and weighs about 50 pounds. The cabinet also contains a 250-watt Class D switching amplifier whose heat-sink fins are on the bottom of the enclosure.

Although Class D amplifiers are extremely efficient and generate little heat even at very high power outputs, they have not been very successful as full-range audio amplifiers. Filtering the ultrasonic frequencies from the output and shielding the amplifier to prevent the radiation of interfering signals add expense and complexity.

A subwoofer, on the other hand, operates only at the low end of the audio range, which allows its switching frequency to be much lower than would be required for full-range operation. In the Velodyne ULD-12, the amplifier does not even have a power transformer, since it is fully contained within the speaker cabinet and the user cannot come into contact with any part of its circuitry. The signal path uses optical coupling to isolate the input jack from potentially dangerous voltages. The system is protected against overdriving or other mishaps by circuits that shut down the amplifier if its chassis temperature exceeds 140° F. or if there is a short or open circuit in the phono cable running between the speaker and its controller unit. To protect the system against excessive drive levels, there is also an automatic-gain-reduction (compression) circuit that limits the speaker-cone excursion to safe values even if the input drive is excessive.

The controller, a vital part of the ULD-12 system, is a black box about 8 inches wide, 6 inches deep, and 1-3/4 inches high. It has phono-jack inputs and outputs that connect to the preamplifier outputs and power-amplifier inputs, respectively. Alternatively, the controller can be connected between the power-amplifier outputs and the speakers through spring-loaded clips that accept stripped wire ends. A single phono-jack output carries the low-frequency signals to the ULD-12 speaker unit through a provided 25-foot cable (its length may be extended as desired).

The controller is essentially a passive unit, requiring no connection to the power line. The speaker module, however, must be plugged into a 120-volt AC outlet and can be left energized at all times. Its internal power suppy shuts off automatically about 5 minutes after input signals stop and switches on automatically when a signal is applied to the controller. The front panel of the controller contains a pushbutton switch that turns the subwoofer on or off (after its power supply has been switched on by an input signal). A knob varies the drive level to the subwoofer module from fully off to maximum.

The input impedance of the controller is rated as 20,000 ohms, and a 300-millivolt signal is sufficient to drive the subwoofer’s amplifier to full power when the controller’s level is set to maximum. When the subwoofer is in use, the low frequencies in the signal going to the main speakers are rolled off below 85 Hz by a 6-dB-per-octave crossover in the controller, which eases the load on the system’s main amplifier. Setting the controller switch to off restores a full-range drive signal to the main speakers.

The Velodyne ULD-12 is available finished in a satin black or in oiled walnut or oak veneer with rounded hardwood edges. The size and shape of the speaker module allow its use as a small end table or plant stand. Price: $950.

Lab Tests

We measured the frequency response of the Velodyne ULD-12 at two different microphone positions: in the gap between the bottom of the cabinet and the floor, and close to the woofer cone (with the cabinet inverted). The two response curves were essentially identical in shape, but the latter was about 4 dB higher than the former. These measurements were made with a drive level well below the speaker’s maximum of 300 millivolts to insure that its output would not be compressed.

The output reached its maximum at 45 Hz and was down 6 dB at 21 and 85 Hz, closely matching the manufacturer’s rating of 20 to 85 Hz ±3 dB. We also measured the frequency response at the controller’s high-pass output (which goes to the main system speakers). It was flat through the middle and high frequencies and rolled off gently below 100 Hz to —6 dB at 30 Hz. The bass response at a maximum drive level (300 millivolts) clearly showed the effect of the amplifier’s protective compression circuit. It was identical to the lower-level response down to its maximum at 70 Hz and rolled off at 18 dB per octave below that frequency.

As might be expected from its motional-feedback design, the distortion of the ULD-12 was extremely low. We drove it at a 300-mv input level from 100 Hz downward in frequency. The sound-pressure level (SPL) at 1 meter from the woofer was very high, with the cone nearing its excursion limits at the lower frequencies, but the distortion remained under 1 percent (typically about 0.6 to 0.8 percent) from 100 to 25 Hz. Even at 20 Hz it was only 1.7 percent. These measurements support Velodyne’s rating of less than 1 percent distortion at half power and 3 percent at full power at any frequency.

During our tests, the speaker’s protection system shut it down a number of times (we were driving it at full power for prolonged periods), but it always returned to service in a few seconds when the input was reduced. An hour or so of this treatment left the speaker’s heat sink only mildly warm. The high efficiency of its Class D amplifier can be inferred from its specified power consumption: less than 5 watts with no signal and a maximum of 275 watts at full power (about 90 percent efficiency). The standby power consumption of the system is only 3 watts, no more than that of an electric clock.

Quasi-anechoic FFT measurements confirmed the steady-state response data. Our group-delay measurement showed excellent phase linearity over the speaker’s operating range. The variation was less than 40 milliseconds overall from 30 to 100 Hz.

Comments

Impressive as the specifications and measurements for the Velodyne ULD-12 are, they are overshadowed by its performance in a music system. We used it with several types of main speakers, none of which could be described as lacking in low bass, and its impact was frequently overwhelming. Even at a moderate level, it could be felt through the entire house on some organ-pedal and bass-drum notes.

The only way to adjust the level of the ULD-12 subwoofer is by ear, and the resulting setting is necessarily a function of the amount of low bass in the program. If it is set too high, even the upper bass may be boosted somewhat, producing the unnaturally heavy or tubby sound that mars so many speaker systems. If it is set correctly, the speaker will not be audible on most program material, which is as it should be. Even so, switching it on or off in our tests often disclosed a bass “floor” that was not audible in the main speaker output.

We had expected to find a major improvement when using the ULD-12 with smaller speakers, ones whose lower limit was perhaps 45 to 60 Hz. What we did not expect was the tremendous improvement it provided even with the largest, most bass-potent systems at our disposal, which went down to 30 or 35 Hz without external help. Possibly the very low distortion of the Velodyne system was responsible for the improved bass we heard, or perhaps the fact that its output could be boosted above normal woofer levels, compensating for their natural rolloff at the lowest frequencies. Whatever the explanation, we are convinced that there are very few speakers whose low-bass performance could not be improved by adding a ULD-12 to the system.

Because of the nondirectionality of the frequencies radiated by the ULD-12, it can be placed almost anywhere in the room without compromising the stereo qualities of the main system. As with any speaker, its output is enhanced by corner or wall placement, but its performance did not suffer noticeably even in a freestanding location. We found only one limitation in the applicability of the ULD-12. If it is used with highly efficient main speakers, it is important that the main power amplifier have level adjustments. When we teamed it up with the Klipsch Chorus (sensitivity of 96 dB), for example, the subwoofer was not able to produce a worthwhile enhancement even at its maximum level setting. With other speakers having more usual sensitivity ratings (less than 88 to 90 dB), however, we had no problems.

The Velodyne ULD-12 is not inexpensive, but it is difficult to imagine any neater or more cost-effective way to enhance the bass end of your system, whatever speakers you may now have. It is an attractive, compact, intelligently designed product. And be warned—it can be habit-forming!

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