MartinLogan Depth powered subwoofer
My wife's comments arose during my audition of the Depth ($1795), a new, baby servo subwoofer from MartinLogan. Its diminutive height of 16.25" belies the Depth's 65-lb, solid, bricklike build. The configuration of the Depth's three drivers, each angled at exactly 120 degrees to the other two, creates a hexagonal footprint for its enclosure. The aluminum trim around the Depth's top reminds me of the company's logo.
Around back is the service panel, which contains all of the Depth's controls and input and output jacks. At the top is a large rotary knob for output level. This is flanked on one side by the Auto/On/Standby switch, and on the other by a LED that reads yellow in power-saving mode, green in play mode, and red in safety mode (ie, when the internal amplifier has overheated). Just below these are a phase switch with positions of 0 degrees, 90 degrees, 180 degrees, and 270 degrees, and a six-position, low-pass filter switch for filter points of 30, 35, 45, 55, 65, and 80Hz. A control marked "25Hz" boosts or attenuates the Depth's output by 12dB at that frequency.
Below the controls are the inputs and outputs, as well as a three-position (40Hz/70Hz/None) toggle switch for the High Pass Filter function. Three pairs of RCA jacks provide left and right inputs, high-pass filtered outputs, and 0.1/LFE In and Sub Out.
The Depth uses the company's patented "TriLinear" technology, where three small drivers are used to move the same amount of air as a single driver three times the size. The three 8" long-throw drivers are so positioned that vibrational forces inside the cabinet operate in exact opposition to one another, minimizing spurious cabinet vibrations. This technology, which MartinLogan calls BalancedForce bass, which is claimed to reduce cabinet colorations by as much as 25dB, was originally used in ML's cost-no-object Statement E2 system. In addition, the Depth uses an analog servo-control circuit to reduce distortion.
I unpacked the MartinLogan Depth and shoved it across the living room to one of the corners behind my Quad ESL-989 loudspeakers. The Quads were positioned 5' from the rear wall, 8' apart, 3' 9" from the side walls, and toed-in slightly. My listening chair sat facing this short wall of my lightly damped listening room, which is 26' long, 13' wide, and 12' high, opening into a 25' by 15' kitchen behind the listening chair.
The next step was to connect the Depth to my system. I choose two setup options for connecting the Depth: "2-channel mode," with the Quad speakers run full-range; and "2-channel mode with 2-channel output," which used the Depth's built-in high-pass filter to gently roll off the Quads' bass response. For this review, I ran the Depth both ways. For connections in "2-channel mode," I ran a main stereo interconnect from my preamp's main outputs to the power amp's inputs, and a second stereo interconnect to the line-level Right In and Left In RCA jacks on the Depth's rear panel. For "2-channel mode with 2-channel output," I connected a single stereo interconnect between the preamp's Main Out and the sub's Right In and Left In jacks, set the high-pass filter to "40Hz," and ran interconnects from the Depth's Right Out and Left Out RCA jacks to the ML-2 power amps.
These options are clearly explained in the Depth's instruction booklet, which is easily the best setup manual I've seen from a subwoofer manufacturer. It explains why MartinLogan advises against making the Low Pass Filter and High Pass Filter (out) settings identical: "For a seamless crossover of these two slopes, the summation of their two values, at any given frequency, should add to maintain a level output." This message should be stamped on the rear panel of every aftermarket subwoofer!
Keeping this explanation in mind, I used the Velodyne DD-18 subwoofer's built-in signal generator, microphone, and virtual spectrum analyzer for the next step: calibrating the Depth for my room. The DD-18's technology (see June 2004, p.133) is not part of the Depth's standard installation package, of course, but it sure came in handy for this stage of setup (footnote 1).
I set up the calibration microphone on the back of my listening chair at my ear level, 37" from the floor, and set the DD-18's volume control to "0" so that it would put out no signal. I then keyed the Velodyne's remote to display its internal System Response screen on my TV monitor. This automatically initiates a repeated tone from the DD-18's signal generator, the tone is then fed into a tape input of my preamplifier to drive the Quad ESL-989s and the Depth. I could then see the effect of the Depth's controls on the measured response almost in real time (the frequency-response graph is refreshed every three seconds).
Without the Depth, the Quads' frequency response showed a dip at 60Hz and a peak at 40Hz, the response falling off rapidly below 40Hz (fig.1; note that this and the following two graphs cover the region from 20Hz to 200Hz). Adding the Depth, without any adjustment of its controls, extended the response in my room to 20Hz, but with the peak at 40Hz and dip at 60Hz still evident (not shown).
Fig.1 Quad ESL-989, no subwoofer, in-room response (25dB vertical range).
I first tried using the Quads in the MartinLogan's "2-channel mode," without any high-pass filter. The flattest response was obtained with the Depth's low-pass filter set to 30Hz, the phase to 90 degrees, the 25Hz boost to +6, and the subwoofer output level to 2.6 (fig.2). Changing to "2-channel mode with 2-channel output," I set the Depth's high-pass filter to 40Hz, the low-pass filter to 30Hz, the phase to 90 degrees, the 25Hz boost to 3.1, and the level control to 3.0, which produced the room response shown in fig.3. As you might guess, only the 25Hz boost and the gain control needed touching up when I changed from "2-channel" to "2-channel with 2-channel output" mode, maintaining the same ±4dB between 20Hz and 200Hz. From these graphs, it looked as if I could obtain a smoother overall system response with the system in "2-channel mode with 2-channel output."
Fig.2 Quad ESL-989 with MartinLogan Depth, in-room response in "2-channel mode" without a high-pass filter. (25dB vertical range).
Fig.3 Quad ESL-989 with MartinLogan Depth, in-room response in "2-channel mode with 2-channel output," with high-pass filter. (25dB vertical range).
I verified the Depth's setup with the 1/3-octave warble tones at -20dB from Stereophile's Test CD 3 (STPH006-2). The overall system's volume was such that the 100Hz tone registered 0dB on my RadioShack SPL meter. The meter variation never exceeded ±4dB.