Subwoofer Reviews

Subwoofer THD will mar the performance of a very-high–quality midrange-treble unit, like Quads, Apogees, or Magneplanars. This was the main reason I did not use my earlier subwoofer (an RH Labs SB-1), despite its otherwise high quality, workmanship, and ability to put out huge amounts of deep bass. Hall's studies led him to believe that 0.5% THD is the maximum allowable distortion tolerable in a subwoofer. His "High Gain Servo" (HGS) circuitry is said to reduce distortion by a factor of 30. The ULD-18 is rated at 104dB SPL at 20Hz at 0.5%, well within this limit.

Design features
The Velodyne subwoofer is part of a system, so one doesn't have the "hidden" expenses of extra amplifiers or crossovers that might or might not work out. 12dB/octave crossover slopes and the 85Hz crossover point for low- and high-pass filters were selected as the best general match to most full-range systems. The ULD-18's enclosure, crossover, and amp have been optimized for one another, and are sold as a package. The ULD-18 is only sold as an optimized system, with its own 400W amplifier, error correction, and crossovers optimized for the particular driver in each unit (serial numbers of subwoofer enclosure and remote unit must match!). This means that the audiophile isn't faced with identifying, purchasing, and trimming in the right crossover with the right slopes, and right high- and low-pass frequencies and right amplifier. It's all done at the factory! In fact, the Velodyne's amplifier will not drive the woofer without the servo mechanism in place.

Hall found that every detail of design and production was critical. Reducing distortion at 20Hz means optimizing cone stiffness (polypropylene proved too flabby), magnetizing the speaker correctly, individually reducing the standing waves in each driver by watching it perform, electronic and mechanical hand-tuning of each system manufactured, and sealing every screw hole with special glue so there is no air leakage. Like balancing a tire, small weights are applied to each loudspeaker and small holes are punched in the cone to break up standing waves. This means each ULD-18 is virtually handmade. Final adjustments are made within each remote unit, so one can't simply pull another untrimmed remote electronics off the shelf to replace one in the field.

But error correction doesn't stop with the physical setup of the subwoofer—it continues during every second of its operation. Velodyne's error-correction accelerometer samples cone movement 3500 times per second. The resulting data on the cone's real-time behavior are further refined by Velodyne's special HGS system. The Velodyne 400 amplifier becomes part of this correction loop, with 30dB negative feedback reducing distortion in the amplifier-speaker system.

Velodyne has refined the concept of motional feedback. Hall mounts an acceleration-sensitive piezoelectric pickup on the voice-coil form. It generates a tiny voltage proportional to the rate of change in velocity of the cone. An additional amplifier, mounted in the subwoofer itself, sends this voltage back to the HGS system. Additional circuitry in the remote box compares this analog of the acoustic output of the woofer with the command signal coming into the Velodyne remote from the preamp. When there is deviation, the controller alters the drive signal to the subwoofer's amplifier. Rather than the subwoofer's acoustical output being distorted, its input signal is pre-distorted in such a way that the acoustic output from the ULD-18 more closely matches the source signal.

Sampling acceleration data is a feature of the Velodyne system. David Clark's review of the Velodyne ULD-15 for Audio (Vol.71 No.11, pp.78–90)) best explained the advantages of acceleration-sensitive pickups. Briefly, the rate of change (acceleration) of a subwoofer coil can vary greatly in the profound bass range; it is most critical for the subwoofer cone to maintain the same speed variation from 20–100Hz ("a flat frequency response for a small radiator...results from equal peak acceleration at all frequencies") than simply to have a capacity to move (large long-throw capacity). The HGS system depends on the accuracy of error data, which becomes the critical factor in keeping the distortion down. Other manufacturers use servos and accelerometer pickups in their price-no-object full-range speaker systems, such as the $50,000 Infinity IRS (Series V). Infinity uses one of the 12" woofers in each of the IRS Beta's two bass towers to derive the servo signals.

The HGS demands a high-gain loop, as as much as 30dB of negative feedback must be applied, and the actual speaker amplifier has to be powerful (400W) to handle any compression being applied to the input signal. A "compensation" circuit must be used to prevent the system from turning into an oscillator, where the servo signal runs away with the system. In fact, the owner is warned that the speaker cable leads from the Velodyne remote box (where the subwoofer's power is generated) must not be inverted when plugged into the subwoofer. Additionally, a gain-compression or limiter circuit is used to prevent the amplifier from going into clipping (or the subwoofer cone from jumping out of the coil gap!) by receiving too much input signal, when one leans too heavily on the volume control.

Obviously, a clipping amplifier doesn't respond to error correction. For example, the bass drum beat on David Wilson's Winds of War and Peace easily drove the Celestion System 6000 into serious, noisy clipping, but not the ULD-15 or ULD-18, the Quad/Velodyne system being able to play this passage louder than the non-servo subwoofer. The gain compressor circuit will signal the comparator circuitry to shut down the subwoofer's power amplifier for 5 seconds, just as it will shut off if it detects that the servo interface cable is disconnected or the power amplifier's power supply has developed problems.

Besides costing more than the Series I, the Series II line added a number of critical advantages, including a smarter, more rugged protection circuit for speaker and amplifier, with automatic shutdown for a short, open circuit or input overload. This reduces the likelihood of woofer damage if one reverses the speaker-cable terminals at the subwoofer, turning the servo into a powerful oscillator. The cable carrying the servo feedback to the system's amplifier was simplified from a DIN cable to a simple phono cable, and the crossover components are now mounted in a plug-in gold-plated carrier, allowing for quick changes of crossover frequencies and slopes. The built-in amplifier is now run in the more efficient Class-G mode to reduce heat and provide the increased voltage swing needed for the signal comparator, gain compressor, and protection circuits.

Description
The original ULD-18 was Velodyne's first product. The enclosure size was modeled on a published design of an enclosure using an 18" JBL woofer, but one inch smaller in each dimension. Philips had designed a servo circuit to make a small woofer sound like a large one; Hall refined the circuit with special signal compression and protection, in order to make a large woofer sound even better. Yet he and his sales representative found the first 18" Velodyne subwoofer unwieldy to take on the road for dealer demonstrations. He went on to design the ULD-15 in a much smaller enclosure. The higher compression of air necessary for the '15 produced more nonlinearities than found in the '18, and this necessitated a more sophisticated HGS system. The ULD-15's servo was then incorporated in later models of the '18.

Today's Series II ULD-18 remains a massive, 105 lb sealed woofer box, finished on all sides in oiled walnut or oak veneer. The woofer faces down, a "floor-loading" position. Even though the cone diameter is only 3" greater than the ULD-15's, the '18's enclosure is more than double the size (5.8 cubic feet vs the ULD-15's 2.6 cubic feet) and almost twice as heavy (105 pounds vs 57 pounds). The '18's 1" wall thickness exceeds the ULD-15's by 0.25". Setup reminded me of JGH's description of the 10 hours of "sweating and cursing" required to find the best location for two 120-pound Nelson-Reed 1204 subwoofers (Vol.11 No.4, p.122).