Velodyne Acoustics Downward-Firing 12" Active Subwoofer in Sealed Cabinet

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 had not been very successful as full-range audio amplifiers, at the time when the ULD-12 was designed. 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.

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.

This Velodyne ULD-12 is finished in a satin black with rounded edges. The size and shape of the speaker module allow its use as a small end table or plant stand.

List price when new was $950 USD. Asking just $395 for this well thought out downward-firing, closed-box design 12" Subwoofer from Velodyne.

NOW REDUCED TO JUST $295

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