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JBL LSR28P

Bi-amplified Monitor Loudspeaker By Hugh Robjohns
Published April 1999

JBL LSR28P

JBL's latest active two‑way reference loudspeaker system has been derived from their impressive LSR32 range‑topper.

JBL's recently developed approach to monitor loudspeaker design has had a radical effect on the European perception of the marque. Former JBL monitors shared a family characteristic which typically produced a hard, aggressive, 'American' sound, not generally found appealing to European ears — although there has always been a healthy respect for the almost unbreakable nature of JBL speakers, ensuring their popularity in PA systems and the more raucous of recording studios! However, a change of design emphasis at JBL in the last couple of years has lead to the production of a new line of monitor loudspeakers which compare very favourably indeed with some of the best designs available in Europe. Indeed, the range‑topping LSR32 (reviewed in the October 1998 SOS) has already attracted many positive reviews, including my own, and much of the technology in the flagship model has been handed down to this junior sibling.

Background

The LSR in the title of this monitor stands for 'Linear Spatial Reference' and refers to part of JBL's new‑found design technique. The basic idea revolves around taking accurate measurements of the frequency response in the complete sphere around the loudspeaker, and then optimising the various design parameters to produce as smooth an overall frequency response as possible. The theory is that this approach gives a much more consistent sound than purely on‑axis design techniques, since the direct, reflected and reverberant sound fields created by the speaker (and its cabinet) all tell the same story!

To put the LSR28P in context, it's an active two‑way model at a cost which makes it directly comparable with the Genelec 1031A and Dynaudio BM15A active near/mid‑field monitors. Building in a reasonable amount for amplification, the new JBLs would also be roughly on a level with such well‑known passive loudspeakers as the ATC SCM20SL Pro, Harbeth/BBC LS5/12A, PMC AB1 (currently my own main preferred reference monitors), and the Tannoy System 1000. In other words good quality, mid‑priced professional reference monitors.

Technology

The LSR28P design uses the same titanium composite tweeter as the bigger LSR32s, complete with the Elliptical Oblate Spheroidal (EOS) Waveguide — who dreams up these terms? This waveguide is simply an aid to generating a predictable dispersion characteristic for the treble energy. Coupled to the tweeter is a JBL model 218F 8‑inch bass unit derived from the larger 252G 12‑inch woofer used in the flagship monitors. This smaller bass/mid‑range unit shares the same Differential Drive technology as the 252G, with two driven voice coils plus a central third, shorted, coil which acts as a dynamic brake, and a graphite/polypropylene composite cone.

The cabinet has a 50‑litre volume — the same as the three way LSR32 design — and is ported to the rear via a 'Linear Dynamics Aperture Port' which is a flared design intended to reduce 'chuffing' and port compression by matching the internal and external air impedances correctly. The cabinet resonance is tuned to a low 40Hz. Like that on the flagship model, the 28P's front baffle is made of a carbon‑fibre composite which looks very impressive and is as solid as a rock. The speaker feels as though it's made of stone, too, weighing in at a considerable 20.5kg (45lbs) — much heavier than anything this small (406 x 330 x 324mm) ought to be!

The active crossover splits the audio input into two bands at 1.8kHz, with a sixth‑order Butterworth or fourth‑order Linkwitz‑Riley filter (user selected) for the bass section and a fourth‑order Linkwitz‑Riley filter at 1.8kHz for the top end. The overall frequency response is quoted as 37Hz‑22kHz at the ‑6 dB points, and the bass frequencies are handled by a 200W amplifier whilst a 70W amplifier feeds the treble driver. Audio input connection is via a combi‑jack socket accepting balanced or unbalanced signals on quarter‑inch jack or XLR. The XLR is configured for +4dBu signals, whilst the jack accepts ‑10dBV levels, although a bank of DIP switches hidden away on the rear panel allow a good range of adjustments. A screwdriver‑operated rotary level trim is normally out of circuit, but can be introduced by the first DIP switch if required, for precise and individual level matching over a 12dB range. The second and third switches provide 4, 8 or 12dB of fixed attenuation.

JBL LSR28PThe next three DIP switches affect the bass response of the speaker. With all switches in their off position, the response falls at 36dB/octave and is maximally flat. However, this can be changed to 24dB/octave with switch four, if required, to increase the bass extension slightly (at the expense of reduced power handling). Switches five and six alter the overall 36dB/octave slope by 2dB up or down below about 150Hz, and are provided to compensate for room effects caused by positioning the speakers near a rear wall, for example. Similar provision is available for the tweeter, with +/‑2dB trims above 1.8kHz on switches seven and eight.

Other rear‑panel facilities include a mains voltage change‑over switch (120 or 240V), a mains fuse, a push button to power the unit, and an IEC 3‑pole mains connector. Much of the rear panel is taken up with vertical fins forming a heatsink for the internal amplifier packs, which generally run warm to the touch. The only indicator is on the front panel (obviously) and is normally illuminated green when the unit is powered, but flashes red if the signal level nears the onset of amplifier clipping. When the unit is first powered, there's a switch‑on mute delay that prevents thumps and clicks from reaching the loudspeaker.

In Use

The LSR28Ps are designed to be used vertically, with the tweeters directly above the mid‑range/woofer (to ensure that the sound from both drivers arrives with the listener at the same time), and with the cabinets angled in towards the monitoring position. I used them mainly in a mid‑field monitoring role, about a metre from the rear wall and 2.5 metres from side walls, which allowed me to keep all the spectral balance switches in their normal positions, although I did experiment with them to confirm their effect.

I found the monitors' stereo imaging and presentation of depth to be very fine indeed, giving a very realistic portrayal of stage depth with appropriate material, and revealing the spatial relationships of instruments with great precision in pan‑potted and coincident mic recordings. The LSR28P is certainly capable of generating powerful sound pressure levels (up to 110dB SPL, apparently) without obvious signs of distortion or fatigue — in fact, distortion levels are extremely low throughout the frequency range and even at high levels, which is a very good sign of a competent loudspeaker. These monitors don't sound loud even when they are being driven hard, which can be a problem when you try to have a conversation with someone, and is a sure indication that distortion levels are extremely low!

One of the things that most impressed me about the LSR32s was their superb mid‑range clarity, which could reveal every nuance of a particularly challenging piece of Vivaldi...

The integration between the tweeter and bass/mid‑range unit was excellent, and moving up and down in front of the speaker did not reveal any nasty suck‑outs or peaks in the output — the LSR technique has clearly delivered the goods here! The overall sound balance is commendably flat, and the plots show the frequency response to be extremely accurate, although I detected a slight tendency towards 'pluminess' in speech, particularly on female spoken and singing voices. There's plenty of power at the bottom end, and the LSR28Ps are capable of generating trouser‑flapping bass with the right material, although they could lack clarity and detail, tending to become thumpy in complex mixes. One of the things that most impressed me about the LSR32s was their superb mid‑range clarity, which could reveal every nuance of a particularly challenging piece of Vivaldi I like to use on my speaker tests. The LSR28Ps had a good stab at it, but a two‑way speaker just cannot match the capabilities of a well‑designed three‑way monitor — the 32s remain the champion of this test so far!

In comparison with my stock PMC AB1s and 250W‑per‑channel Bryston amplifier combination, the JBLs were a close match. I felt the PMCs were more tuneful and revealing in the deep bass and sounded a tad more natural in the lower registers of the female voice, but there really was not much to choose between them, and if I was looking for a pair of active reference speakers which I could get in and out of a car easily, the JBLs would be very high on my short‑list. Although I wasn't able to make direct comparisons of the JBL against the ATC or Dynaudio active equivalents, I would imagine choosing between them would depend very much on a personal assessment of the finer nuances of tonal balance rather than on any obvious technical differences. I would happily give these JBLs space in my monitoring system, and that's the second time I've made the same comment about JBLs in the last six months!

The LSR family really does offer something special and is a very welcome direction for JBL to be pursuing. If you are considering upgrading your monitoring system in the near future I would definitely recommend giving the LSR28Ps a careful audition alongside your other favourite active systems. Alternatively, the LSR32s offer the added benefits of a well‑designed three‑way loudspeaker monitor, but at slightly greater expense and with the need to add a decent amplifier.

Differential Drive Technology

The bass/mid‑range unit of the LSR28P shares the same patented DCD technology as the 12‑inch bass driver of the passive three‑way LSR32 model. The most interesting aspect of this design is the unusual use of three voice‑coils — the two outer ones provide the motive force, whilst the middle coil is shorted (ie. its two ends are connected together).The idea is that, as the coil is moved in a magnetic field, induced currents create their own magnetic field which opposes the original, thus acting like a dynamic brake, and limiting the maximum cone excursion. This has the benefits of protecting the driver from abuse without adding to the distortion figures at high sound levels.

Most European loudspeaker designs obtain high linearity between input voltage and cone movement by ensuring that the (single) voice‑coil remains well within a uniform magnetic flux — hence the overwhelming prevalence of the 'short voice‑coil in a long magnetic gap' approach. JBL have chosen to pursue a radically different design theory, employing two driven voice‑coils, each of which is deliberately arranged to rest with half the coil outside the magnetic gap. Clever design of the magnetic core of the drive unit means that the magnetic flux is guided around a neodymium magnet assembly such that it passes through the two driven coils in opposite directions. As the coils are wired in opposite polarities, their mechanical efforts combine to equate to a conventional single coil, moving the speaker cone as required.

Although this approach initially appears unnecessarily complicated, JBL have claimed a number of advantages for it. First, since there is double the surface area of coil, heat dissipation is far better, allowing a 3dB increase in power handling and a reduction in dynamic compression. Second, because the two coils are wired in opposite phases, their mutual inductance is reduced, presenting an easier load to the amplifier. Possibly the biggest advantage is that the entire magnet and voice‑coil assembly can be made far more compact and requires less iron in the magnetic path than conventional designs, making the drive unit lighter.

The additional shorted coil in the middle of the assembly does nothing for most of the time, as it remains well away from the two magnetic circuits. However, during large cone excursions this third coil begins to enter a magnetic field and the induced current sets up an opposing magnetic force acting against the motion of the cone, progressively damping its movement. An added benefit of this ingenious design is that the introduction of the third coil into the magnetic gap also helps to cancel the inherent distortion artifacts caused by the magnetic field instability created as the main voice‑coils reach its outer edges.

Pros

  • Easy interfacing.
  • Good room equalisation options.
  • Excellent stereo imaging and depth presentation.
  • Flat frequency response with good bass extension.
  • Capable of high sound pressure levels with very low distortion.

Cons

  • Perhaps a tad plummy on female voices.

Summary

The LSR28P is a powerful, active, two‑way reference loudspeaker system incorporating JBL's latest thoughts on loudspeaker design. Very low distortion, high sound pressure levels, good spectral balance and sensible room equalisation options make this a worthy mid‑field contender.

Information

£1899.95 per pair including VAT.

https://jblpro.com

Published April 1999