James Loudspeaker EMB-1200 subwoofer
By Larry Greenhill   •   September, 2004 Harry Partch (1901-1974), composer and inventor of musical instruments, delighted in generating deep bass. Finding most standard orchestral instruments wanting in that department, he built the huge Marimba Eroica, which he described on his A Glimpse into the World of Harry Partch: 27 Unique Instruments (LP, Columbia MS-20576):

"In the right room acoustically, the Eroica is felt through the feet, against the belly, and, if one sits on the floor, it ripples through his bottom. It is very difficult to put on tape, and especially on records, with any fidelity. Adequate playback equipment is absolutely essential, which means that the poorer generally are not privileged to experience a rippling though their backsides by an art form."

Powerful deep-bass response means more than just pure tones. Rather, it requires raw power, tactile surges of air pressure, and a mix of the senses of hearing and touch. To move the amount of air required, subwoofer devotees have opted for large enclosures housing big drivers. When Mark Levinson set about creating the HQD—his ultimate, $80,000 two-channel audiophile system—he built two 24" Hartley drivers into huge credenzas. In my listening room, I've reviewed subwoofers whose large, dense, cubical cabinets weigh more than 150 lbs.

More recently, much smaller powered subwoofers have become popular for home theater systems. These use powerful internal amplifiers, lots of equalization, and servo correction to generate deep bass. When James Loudspeaker offered me a chance to review one of their bandpass subwoofers, I eagerly said yes.

Compact bandpass subwoofer
Although I began reviewing the smaller James EMB-1000 ($1595, see Sidebar, "The James Loudspeaker EMB-1000 subwoofer"), the EMB-1200 was made available to me in the middle of my auditioning. Because the larger sub seemed better at moving air in my large listening room, I decided to make it the subject of this report.

Measuring just over a foot wide and tall and a foot-and-a-half deep and weighing 75 lbs, the EMB-1200 ($2195) is about half the size and weight of the Velodyne DD-18 subwoofer, with which I compared it and which I reviewed in June. The EMB-1200's build quality is satisfyingly robust, and it's finished in a shiny black Piano Gloss or in Black Granite. A 12" passive-radiator diaphragm of spun aluminum sits in front, set off by 18k-gold hardware accents at each corner. The heavy passive diaphragm and the carefully sealed enclosure rule out the chuffing and wheezing noises that can be heard from some ported subs.

The interior of the EMB-1200 is divided into three chambers; between two of them is a long-throw 12" woofer with a 15-lb magnet. The central chamber creates critical damping of the powered internal driver, to deliver fast rise and decay times. The front chamber couples the woofer's output to the external aluminum diaphragm. The EMB-1200's 1200W amplifier has twice the output required for it to reach full volume. Internal equalization allows the system to reach down to about 20Hz.

The rear panel holds the line-level RCA input and output jacks, as well as high-level speaker binding posts when using the output of a power amplifier. A switch allows the use of a low-pass filter rotary control to diminish the subwoofer's upper-frequency output, or bypasses the crossover altogether. Other features on the rear panel include a phase switch (0 degrees or 180 degrees), and an IEC socket for the power cord.

Clever design
James Loudspeaker's goal for the EMB-1200 was to design a small, powered subwoofer that could handle, without huge amounts of distortion, the deep-bass pulses found on DVD soundtracks. Whether reproducing the thunderous avalanche in Ice Age, the hull rumblings in U-571, or the pulses of the torpedo motors in Pearl Harbor, today's subwoofers are challenged well beyond what they might encounter from a two-channel music system. Cost-effective servo systems will work up within their dynamic-range limitations, but peaks can be compressed. In addition, when the typical servo used in a small sub reaches its correction limit, this can produce odd noises that are much more disturbing than the usual types of distortion. James Loudspeaker set out to provide acoustic solutions that would minimize distortion while still allowing the subwoofer to clip gracefully when hit with an unmanageable sound effect.

Distortion in the EMB-1200 is controlled by the sub's bandpass design, which is similar to those of the Isophon Europa or KEF Reference 107 loudspeakers. The active woofer is mounted completely inside the enclosure. The diaphragm that communicates with the outside world is actually a passive radiator.

The passive woofer is not driven by an electrical signal but is air-coupled to the active woofer. The passive radiator's resonance frequency is designed to be in the middle of the system's passband, not at the bottom, as might be found in a bass-reflex system. For the EMB-1200, this resonance point is 45Hz, about the middle of its flat frequency-response range of 25-120Hz.

At the resonance frequency, the "lump of air" between the two diaphragms presents a stiff barrier to the active woofer, which will therefore appear to be standing still while the radiatior moves vigorously. This resonant point effect, which James describes as "energy multiplication," keeps the active woofer's excursion to a minimum in this region, limiting distortion not electrically but acoustically. At the resonant frequency, the driven woofer moves only about a tenth as far between 35 and 45Hz as it would at other frequencies. The result, says James, is lower distortion, greater output in the band where bass energy is concentrated, and increased dynamic headroom.

Setup
Wiring up the EMB-1200 was simple. No high-pass filter was involved, which meant that inserting the EMB-1200 in my system involved simply running an interconnect from my preamp's secondary Main Out jacks to the EMB-1200's line-level RCA inputs. Placement was also simple: I put the EMB-1200 where the Velodyne DD-18 had done best—in the right corner of my room, behind the two Quad ESL-989 loudspeakers I use for most of my music listening.

The Quads were positioned 5' from the rear wall, 8' apart, 3' 9" from the side walls, and slightly toed-in. My listening chair was 10' away, facing the Quads. This system sat near the short wall of my lightly damped, rectangular (26' long, 13' wide, and 12' high) listening room. Behind the listening chair, the other end of the room opens into a 25' by 15' kitchen. To match levels among the speakers and to choose filter frequencies that would give the smoothest overall response, I set the Velodyne DD-18 subwoofer's calibration microphone on the back of my listening chair at my ear level: 37" from the floor.

I adjusted the EMB-1200 using the Velodyne DD-18's built-in signal generator, microphone, and onscreen spectrum analyzer. (The DD-18's woofer was shut off by setting its volume to "0.") I fed a small TV set from the DD-18's Video Out jack to display the DD-18's System Response screen. The DD-18's signal generator repeatedly sweeps a test tone from 20Hz to 200Hz; I fed this into my preamp's tape input to drive my overall audio system. The sweep signal reproduced by the James subwoofer was picked up by a microphone placed on the back of my listening chair.

With the sweep signal running the full 20-200Hz spectrum every three seconds, I set the EMB-1200's gain control to provide the best match with the Quads. This was found at about +4, or 10 o'clock on the EMB-1200's gain dial. I then adjusted the phase and low-pass filter points to achieve the flattest frequency response at my listening chair. Without the subwoofer, the Quads' frequency response showed a dip at 60Hz and a peak at 40Hz, with rapidly declining response below 40Hz (fig.1). I found the flattest response by setting the EMB-1200's Phase control to 180 degrees and its low-pass filter to 85Hz. The overall system frequency response was then 20-200Hz, ±4dB (fig.2).

Fig.1 Quad ESL-989, in-room response without subwoofer.

Fig.2 Quad ESL-989, in-room response with James EMB-1200 subwoofer.

Music
The EMB-1200 blended well with the Quad ESL-989s, yielding a seamless integration of the midbass and lower bass. For example, the bass notes on Mary Gauthier's Filth and Fire (CD, SIG 1273) were more taut and solid with the EMB-1200 in the system. The dreaded midbass emphasis produced by poor subwoofers was totally absent with the EMB-1200, which rendered a cleaner, bettre defined impression of the midbass and up. In addition, the EMB-1200 had the muscle to move lots of air in my large listening room when the occasion demanded it.