Genelec HTS4B subwoofer
By Larry Greenhill   •   November, 2005 Home theater has dramatically influenced the design of aftermarket subwoofers. Multichannel processors automatically provide a properly filtered low-frequency signal to drive a subwoofer, relieving the need for the sub to be shipped with a passive crossover network or an active electronic crossover. When Genelec offered one of their subwoofers for review, I decided it was important to try to evaluate such a product, even if it meant I'd have to scramble around to find a quality external electronic crossover.

Genelec's national sales manager, Karen Richardson, confirmed that Genelec was selling several subwoofer models capable of producing solid 20Hz organ-pedal notes in my listening room. Which should I choose? Rather than attempt to work with their top-of-the-line model, the 265-lb HTS6, rated at 124dB at 20Hz, I decided to begin with the more modest but more manageable HTS4B.

Description
The Genelec HTS4B is an internally powered subwoofer with a magnetically shielded 12" driver on the front and one 12" passive radiator on each side panel, all sharing the same inner cavity. The active driver, which sits behind an attractive grille, has a 2" voice-coil and 1" peak–peak excursion. The two passive radiators act as ports, but without any of a reflex port's potential chuffing noise at high levels. The cabinet is made of internally braced, 1"-thick MDF and weighs 81 lbs.

As I said, the HTS4B is shipped with neither the high- nor the low-pass filters necessary for operation with a two-channel, high-end audio system. This became evident when I examined the rear panel, which contains level and phase switches, but no high-pass frequency settings or line-level output jacks to feed the main amplifier for the upper-range speakers. Two XLR connectors, one for input and the other for link out, are paralleled by a single line-level RCA input jack. The link-out XLR permits the coupling of two or more subwoofers. Connectors for remote power On/Standby switches are also provided. At the bottom of the panel are the Power On/Off switch, mains AC voltage selector, and detachable IEC power cord.

A rotary input sensitivity control sits next to the input jacks. Instead of a knob, this control is inset at panel level—you need a flat screwdriver blade to adjust it. The hardest aspect of setup for me was figuring out how to use this control. I first set it to its numerically lowest setting, –6dBu, but that made it play loud. Next I set it to the highest setting, +12dBu, and it played at very low levels. Genelec's Daniel Curran explained that "The sensitivity level is loudest when the level is at –6dB. At this setting you are actually 6dB above 0, or 6dB louder." Right.

The HTS4B's rear panel contains a bank of seven tiny DIP switches. Moving from left to right, I set the first two switches to On to provide a phase setting of 270°. I set the next three switches, which control bass rolloff in 2dB steps, to On-On-Off, for a total bass rolloff of –2dB; and the last two, Autostart and Standby/On, to Off.

The HTS4B's internal class-AB amplifier is mounted on the rear of the front grille, which provides heatsinking. The amp, rated at 400W peak short-term, is equipped with an Autostart function for automatic switching between the Standby and On modes.

The HTS4B's fit'n'finish is professional and neat, and more utilitarian than the more expensive audiophile subs I've reviewed in these pages. The hardware and connections are rugged, easily accessible, and look as if they'll last for years.

Setup
I unpacked the Genelec HTS4B and placed it in a corner, behind and to the right of the rightmost of my pair of Quad ESL-989 full-range electrostatic speakers. My lightly damped listening room has a volume of 4056 cubic feet. (The room is 26' long by 13' wide by 12' high and opens into a 25' by 15' by 8' kitchen.) The '989s were positioned 5' from the front wall, 8' apart, 3' from the sidewalls, and slightly toed in toward my listening chair. I used the Velodyne DD-18 subwoofer's built-in signal generator, microphone, and virtual spectrum analyzer to assist me in installing the Genelec. The DD-18's technology (see my review in the June 2004 Stereophile, p.133) is not part of the Genelec's standard installation package, of course, but it sure was helpful for double-checking the HTS4B's setup procedure (footnote 1).

I placed the Velodyne's 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 audio signal. I then keyed the Velodyne's remote to display its internal System Response screen on my TV monitor. This automatically initiated a repeated sweep tone from the DD-18's signal generator, which is then fed into one of my Krell KCT preamplifier's inputs. First I drove the Quads alone, without the subwoofer, and watched the display. The '989s' frequency response showed a dip at 60Hz and peaks at 70 and 40Hz, the response falling off 10dB by 35Hz (fig.1; note that this and the following graph cover the 20–200Hz region).

Fig.1 Quad ESL-989, no subwoofer, in-room response (25dB vertical range).

Next I connected the HTS4B to my system. This required the use of the Krell KCT's XLR outputs to connect to an outboard Bryston 10B-SUB crossover, which only accepts XLR inputs. I ran a pair of balanced stereo interconnects from the Krell's main outputs to the right and left XLR inputs on the Bryston's rear panel, set the front-panel high-pass filter to 100Hz and 18dB/octave, the left low-pass filter to 70Hz and 12dB/octave, and ran balanced interconnects from the Bryston's right and left XLR out jacks to my Mark Levinson No.431 power amplifier. I set the Bryston's rear-panel slide switch to the common (L+R) subwoofer setup position, so that the crossover's left-channel output was driving the Genelec subwoofer's single mono input with the bass information from both channels.

Adjustments of level and phase were made at the Genelec's rear panel or on the Bryston crossover as I watched the TV monitor, which showed the Velodyne sweep signal. I set the Genelec's sensitivity control to +12 (least sensitive) and its phase to 270°. By iteratively adjusting the Bryston 10B-SUB's high- and low-pass filter frequencies and slopes, I was able to reduce but not eliminate the Quads' 40Hz peak and 60Hz dip (fig.2). The resulting bass response with music was powerful, but the soundstage was slightly shallower and narrower than before.

Fig.2 Quad ESL-989 with Genelec HTS4B, in-room response (25dB vertical range).

I then bypassed the Bryston crossover's high-pass section by driving a Krell FPB-600c stereo amplifier directly from the Krell KCT preamp via separate main outputs and CAST interconnects. I used the Bryston 10B-SUB only as a low-pass filter, set at 70Hz with an 18dB/octave slope. This provided a somewhat wider, deeper soundstage. The correct channel and phase of each hookup arrangement were verified with Stereophile's first Test CD (Stereophile STPH002-2).

Music
Once correctly level-matched and equalized, the Genelec HTS4B generated solid, tight, deep bass with good extension, and delivered dynamics with suddenness and impact. Switching in the HTS4B didn't change the pitch, timbre, or quality of the Quad ESL-989s' midrange response, and the Bryston low-pass filter effectively suppressed any upper harmonics. There was no midbass emphasis, which meant a clean, fast response for bass drum and timpani in orchestral music.

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Footnote 1: Velodyne now makes its subwoofer calibration analyzer/equalizer technology available as a standalone product, the $699 SMS-1. See Kalman Rubinson's "Music in the Round" column elsewhere in this issue.—Ed.