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JBL LSR6328 & LSR6312

Active Monitors & Subwoofer
Published May 2005
By Hugh Robjohns

JBL LSR6328 & LSR6312Photo: Mark Ewing

JBL's new monitors incorporate Room Mode Correction technology which claims to be able to reduce the bass problems caused by standing waves at the listening position. But does it really work in practice?

Under review here are two members of JBL's new LSR6300 series of monitors: the LSR6328, a two-way midfield system, and the LSR6312 subwoofer. Completing the range are the compact two-way nearfield LSR6325, and the three-way LSR6332, either of which can also be used in concert with the aforementioned subwoofer. The LSR6328 is of a size I associate with midfield monitors, but JBL suggest a working range of one to three metres, the short end of this range conforming with nearfield conditions. Nevertheless, I'd recommend placing these monitors on tall stands behind the desk rather than on the meterbridge.

LSR6328 Active Monitor

The LSR6328's two-way design has cabinet dimensions of 330 x 406 x 325mm (H x W x D) and weighs 18kg. It is constructed from 19mm MDF, finished in a dark-grey paint, and has convenient recessed lifting handles built into each side panel. The integral amplifier's heat sink is on the rear panel, along with the connections, setup controls and a rear-facing Linear Dynamics Aperture (LDA) port. (For more information on LDA and other proprietary terms, take a look at the 'JBL Transducer Technologies' box elsewhere in this article.)

The driver complement comprises an eight-inch bass unit with an unusually low 2Ω nominal impedance, and a 4Ω one-inch titanium/composite tweeter mounted in an Elliptical Oblate Spheroidal (EOS) waveguide. An LED between the two drivers illuminates when the system is powered. The amplifier chassis is a two-channel Class-AB design with a 120W monolithic (chip) amplifier for the high-frequency driver and an all-discrete 250W circuit for the low-frequency driver.

The rear control panel is surprisingly complex at first sight, but the handbook provides clear setup instructions and a Room Mode Calibration Kit is supplied to help configure the system (see 'Room Mode Correction' box for more details). The audio input is hooked up via a combi jack/XLR connector with an associated and recessed input-level trim control. Mains is provided via the usual IEC socket with an adjacent power button. A set of eight DIP switches, three recessed rotary switches, a bypass button, and remote control socket complete the configuration facilities.

Three of the DIP switches configure the input, enabling the input level trimmer or presetting the input level to +4dBu, +8dBu, or +12dBu sensitivity. The LSR6328's input trimmer is aligned at the factory such that a -10dBV input provides 96dBSPL at one metre (in an anechoic environment). The fourth switch activates low-frequency protection circuitry which replaces the default 24dB/octave high-pass filter with a 36dB/octave filter. The next pair of switches adjust the high-frequency level, introducing a subtle shelf to the response above 2kHz, amounting to a very modest 1dB up or down. The final pair provide low-frequency boundary compensation, with a shelf cut of -1.5dB, -3dB, or -4.5dB below about 250Hz.

The specifications claim a nominal response of 50Hz-20kHz (+1/-1.5dB), extending to 46Hz at -3dB and 36Hz at -10dB. For an SPL of 96dB at one metre, distortion is claimed to be better than two percent below 120Hz and 0.6 percent above. Maximum peak SPL is 111dB/1m and self noise is less than 10dBA.

JBL Transducer Technologies

A number of innovative proprietary technologies have been used to improve the performance of the LSR monitors. The first, Differential Drive, is an arrangement of two separate voice coils on the bass driver, providing twice the surface area of conventional speakers and allowing greater heat dissipation. This gives 3dB more power handling with less power compression. The two coils are wound with opposite phase to reduce their mutual inductance, which provides a more consistent impedance relative to frequency and makes the driver an easier amplifier load. Another advantage is that less iron is required in the magnetic path than with most designs, making it lighter.

A third, shorted coil within the motor assembly does absolutely nothing most of the time! However, during extreme cone excursions this central coil enters the magnetic fields of the two driving coils and a current is induced within it. This sets up an opposing magnetic force which acts against the motion of the cone, acting as a brake. JBL call this Dynamic Braking and it is a clever protection device. Apparently, the introduction of the third coil into the magnetic gap of the main coils also helps to cancel the inherent distortion which builds as the main voice coils reach the outer edges of their magnetic fields.

Reflex cabinet loudspeakers can suffer from turbulence within the port when monitoring at high levels, simply because of the large volume of air moving around. The LSRs use a Linear Dynamics Aperture (LDA) for their ports, which smoothes the contour of the port exit to minimise this turbulence and thus improve low-frequency accuracy at high listening levels.

Common to the tweeters of all the LSR range is the use of titanium and composite materials, which are claimed to provide a better transient response with low distortion, and thus reduced 'ear fatigue'. Furthermore, the tweeters are combined with an Elliptical Oblate Spheroidal (EOS) waveguide, which helps to shape the high-frequency dispersion, directing sound across a listening area of ±30 degrees horizontally and ±15 degrees vertically, and maintaining an even frequency response within 1.5dB across the entire zone.

LSR6312 Powered Subwoofer

The LSR6312 powered subwoofer measures 394 x 635 x 292mm (hwd) and weighs 23kg. The cabinet features a front-facing port and contains a single 12-inch driver powered by a 260W discrete Class-AB amplifier. The frequency response is given as 28-80Hz (-6dB points), and maximum SPL is 112dB/1m. The 6312 incorporates bass management for a three-channel satellite/subwoofer system, as well as RMC facilities. Combi jack/XLR inputs and associated outputs are provided for three channels (left, centre, and right) plus a fourth Sub Direct input (with a 10dB gain-boost button). An eighth XLR socket provides a summed bass output for additional subwoofers.

The three-channel input signals are summed together and then low-pass filtered at 80Hz before being combined with the direct input (normally used for a dedicated LFE signal. At the same time the input signals are individually high-pass filtered at 80Hz and presented to the corresponding outputs to feed the satellite speakers. An LCR Bypass mode (remote-controlled with a footswitch) disables the high-pass filtering to the outputs, providing a direct loop-through — only the dedicated Sub Direct input feeds the subwoofer, so this essentially bypasses the sub as far as the satellite monitors are concerned.

A recessed input trimmer and six DIP switches provide similar input-level options as those on LSR6328 monitor, supplemented with a polarity inversion option, a boundary compensation mode (a -4dB shelf below 50Hz), and an RMC bypass mode. The RMC facilities are the same as before.

Room Mode Correction

The LSR-series Room Mode Correction (RMC) is a grand name for what is, at the end of the day, a simple single-band parametric equaliser. This can be used to tame a single low-frequency response peak, typically formed by the interaction of standing waves within an untreated room. It is no substitute for properly designed acoustic treatment with effective bass trapping, but it can help damp the effect of a boomy resonance in a room. If your mixes often come out bass-light (because a boomy room mode fools you into thinking the bass is more prominent than it really is) then this could be a simple, if partial, solution.

The LSR6328's rear panel Room Mode Correction controls.The LSR6328's rear panel Room Mode Correction controls.Photo: Mark Ewing

The RMC controls incorporated into the LSR6328 monitors and the LSR6312 subwoofer all provide a single parametric equaliser, with adjustable frequency, bandwidth, and attenuation. The control ranges are 26-96Hz, Q of 0.5-0.05, and up to 14dB of cut. The difficult thing is to adjust each of these controls appropriately, but that's where the supplied RMC Calibration Kit comes in. A simple sound level meter and dedicated calibration CD, both provided, are used to chart the low-frequency response of your room, and a series of charts in the manual can then be used to reposition speakers as necessary and adjust their rear-panel RMC controls. The system is simple and elegant, if a little time-consuming — the whole setup process for all three speakers took me about 40 minutes.

The rear panel of the LSR6312 subwoofer houses the main XLR and combo audio connections (left) plus the Room Mode Correction controls at the right.The rear panel of the LSR6312 subwoofer houses the main XLR and combo audio connections (left) plus the Room Mode Correction controls at the right.Photo: Mark Ewing

Listening Tests

The LSR6328 is a powerful nearfield monitor with excellent dynamics and transient handling. Not only can it play very loud indeed, it can do so effortlessly and with 'rifle shot' attacks when called for! The mid-range is very clean and neutral, and reproduced the critical vocal range with commendable clarity and precision. I found it easy to listen into a complex mix, and heavy bass instruments didn't mask the mid-range detail at sensible listening levels. Stereo imaging was portrayed equally well, with a wide and stable sound stage over a relatively large sweet spot.

With all the configuration controls in their default positions I found the tonal balance to be a tad on the hard side, but in a more heavily damped control room I suspect it would sound pretty neutral from the off. In my listening room I found setting the high-frequency shelf to the -1dB position was all that was needed to tame the top end — something which I find typical of most monitors tested here. Recording flaws and imperfect mixes are revealed clearly, but, while many monitors with this ability quickly become fatiguing, I found I was happy to listen for extended periods without any problems.

The bottom end of the monitor is very well controlled and has a reasonable extension for the size of cabinet. In a modest control room the LSR6328s do not demand the use of a subwoofer on grounds of either extension or headroom — although both obviously improve with the dedicated subwoofer in service. Hooking up the LSR6312 subwoofer is as simple as re-plugging a couple of XLRs. By default the input signals are high-pass filtered at 80Hz for the satellites, and the subwoofer handles everything below 80Hz. The integration with the satellites was excellent, and the greater extension was immediately obvious on suitable material, with the bass becoming more powerful and solid. On bass-heavy material the system really came alive, and if you are looking for a system that can really move some air, this combination would be a good place to start your auditioning.

The Final Word

As straightforward monitors, with or without the dedicated subwoofer, this new LSR6300-series system performs impressively. When the music calls for it, these monitors will sound smooth, delicate, and very revealing. However, they can also deliver a really solid, weighty, and dynamic wall of sound when asked, with huge sound pressure levels that can be sustained all day if your ears can take it. Spoken voice and classical music are portrayed with neutrality and fidelity, and clearly this speaker is a good all-rounder, equally capable regardless of the source material's genre.

The trumpeted Room Mode Correction will be useful in some situations, but I can't help feeling that messing up the speaker's low-frequency response in an effort to correct a physical standing-wave issue is not the right way to go about things! Having said that, the calibration tools are well thought out and easy to use, and when applied carefully they can at least help to tame response peaks at the listening position.

Published May 2005