JBL LSR 28P and LSR 12P: POWERED 5.1 MONITORING SYSTEM

Mar 1, 1999 12:00 PM, George Petersen

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Look back 20 years and you'll find that one name ruled supreme in the field of studio monitors: The JBL 4311 was king, and every studio had a pair. In the years that followed, JBL continued to introduce innovations in studio speakers-most notably the Bi-Radial horn-but as JBL's focus shifted to developing touring and P.A. systems (such as the Eon and HLA Series), the company began losing market share in studio monitors to a host of competitive compact systems designed for near-field listening applications. Now with its LSR series, JBL is poised to regain the throne with a superb series of studio monitors designed for stereo or 5.1 applications.

The newest entries in the LSR (Linear Spatial Reference) line are the $999 LSR 28P two-way monitor and the $1,099 LSR 12P subwoofer. Both are powered systems with onboard Class A-B amplification: 260 watts driving the LSR 12P, and a bi-amped 200+70 watts pushing the 8-inch woofer and 1-inch dome tweeter in the LSR 28P. The system is equally at home in a 5.1 (LCRSS+sub), LCRS, left/right stereo or left/right stereo+ sub(s) environment.

DYNAMIC BRAKING The Lab Analysis sidebar has more details about the drivers, amps, active crossovers, system design and specs, but there are a few points worth emphasizing. For one, all of the drivers in the LSR Series are designed from the ground up for these monitors-there are no "off the shelf" drivers used, and all components in the LSR line are specifically intended for monitoring applications. The LSR 28P's front panel features an Elliptical Oblate Spheroidal (EOS) tweeter waveguide that shapes the HF dispersion into a wide, smooth 120degreesx60degrees (HxV) pattern. The woof-ers in the full-range cabinets and sub are based on JBL's Differential Drive(r) technology, employing dual voice coils for driving the cone. A third coil acts as an electromagnetic dynamic brake, actively slowing the cone action in extreme SPL situations where overexcursion would possibly destroy the woofer. Besides protecting the driver under such radical conditions, the dynamic braking allows the user to push the monitors to the absolute brink without worrying about damaging the system.

From the standpoint of connectivity, the LSR 28Ps can handle just about anything. The balanced inputs are Neutrik Combo 11/44-inch TRS/XLR, with +4 dBu at the XLR and -10 dBv at the TRS; the TRS jack also accepts an unbalanced input with no problem. A recessed pot for continuously variable attenuation (up to -12 dB of cut) is provided. The pot has no detents or click stops (and I hate these on monitors, as they make exact level matching between monitors impossible without using test gear). An 8-switch DIP next to the pot provided all the flexibility I needed. The first switch defeated the rotary pot (whoopee!), while the next two (marked -4 and -8 dB) provide a choice of -4, -8 or -12 dB of attenuation. Other DIP switches allow LF tailoring at 150 Hz for bass alignment (0 dB at 24 dB/octave, or 0 dB/-2 dB/+2 dB at 36 dB/octave). These allow the user to compensate for minor room anomalies and nearby reflective boundaries, such as walls, corners, etc. In a semi-free-field environment (on stands behind the console) the LSR 28Ps sounded fine with the DIP switches in the "flat" no-roll-off position.

Other DIP switches are provided for cutting or boosting the HF output by +/-2 dB. Again, I preferred the monitors in the flat position, but if you work in an extremely reflective or absorptive environment, these cut/boost switches would be useful. I liked the fact that they tailor in gentle +/-2dB steps (on other monitors, the +/-3dB steps often seem too much) and operate as smooth, wide-shelving filters beginning at 1.8 kHz.

The LSR 12P subwoofer has a similar input gain adjustment system to the LSR 28P's and offers XLR connections for the left/center/right LSR 28Ps. This configuration offers numerous possibilities and allows the user to set up full-range LCR front speakers or bandpassed fronts with the subwoofer providing the dividing network. The subwoofer may also be driven directly via a +4 "sub direct" XLR input. A clever 11/44-inch footswitch jack allows the user to disable the sub for monitoring check purposes or to switch the system between modes: from stereo say, to Dolby Pro Logic playback. The subwoofer also has -2/-4dB DIP switch attenuators that can be used separately or together to modify bass response to meet room needs.

Despite its small (16x13x12.75-inch) dimensions, the LSR 28P weighs in at a hefty 50 pounds, so watch your meter bridge if you plan console-top mounting. A safer alternative is JBL's optional SS2-BK monitor stands, which are adjustable from 44 to 79 inches and have a 110-pound capacity.

NATURAL REPRODUCTION I began testing the system by listening to stereo mixes of favorite CDs (mixed by myself and others) on a simple setup using two LSR 28Ps in a near-field position, about five feet apart, in an equilateral triangle arrangement without the sub. I was immediately struck by the natural character of the reproduction-there was no edginess at either frequency extreme, and the 1.7kHz crossover region was smooth as glass. Unlike many other monitors I have used over the years, the LSR 28Ps do not require any adjustment, period-you simply need to plug these in and go.

Next I added the sub, and the transition from the LSR 28Ps to the sub was natural as well-no boominess, and the bass was well-damped and solid without sounding artificial or over-blown. I should add, however, that the LSR 28P's bass response on their own is respectable, only rolling off below 50 to 60 Hz, so anyone who is looking for a powered two-speaker system would be right on track starting off with two LSR 28Ps and then adding a sub or extra channels as needs (or funds) expand.

To check out long-term power handling, I fed the system from a gnarly rock CD, turned the volume up full and left it on with the looped CD playing for three hours. I later returned and found, to my surprise, that not only was the system still functional, the amp heat sinks on the back of the speakers were only slightly warm! A quick measurement confirmed that the speakers are indeed capable of a REAL 108+dB SPL long-term reproduction, although you'll have to supply your own ear protection. The system provides tons of headroom, and there was no compression or dynamic shifting when it was pushed.

When I set up the system in surround mode, a few things became clear: The imaging was rock-solid and was reminiscent of that reach-out-and-touch realism of a system like the UREI 809s. However, the LSR 28Ps sounded very natural and uncolored, even off-axis, creating an almost seamless surround mix environment. With the L/R front speakers set up 60degrees apart, the center on-axis and the two surrounds set back 110degrees from center, I really felt in control of my panning moves, and playbacks were handled with pinpoint accuracy.

If you think I seem enthusiastic about the LSR 28P/LSR 12P system, you're right. Here's a combination of power, accuracy, imaging and natural reproduction that I've never heard from a JBL monitoring system, and at a retail of $999 each ($1,099 for the sub) this package is equally affordable to the project room or large studio. So, ask yourself: Have you driven a JBL lately?

JBL Professional, 8500 Balboa Blvd., Northridge, CA 19329; 818/894-8850; fax 818/830-7802. Web site: www.jblpro.com.

PHYSICAL CHARACTERISTICS The LSR 28P cabinet is a rear-ported, bass reflex design. The baffle is a massive 151/48-inch-thick composite consisting of a woven carbon fiber front skin, foam core and a Fiberglas rear skin. The remaining five sides are black painted MDF. Both the woofer and tweeter are recessed, and all mounting bolts are mated to T-nuts. The rear port is oval in cross-section, and both ends are flared. Inside, the cabinet is stuffed with real Fiberglas.

The 8-inch woofer is a ceramic version of the Differential Drive. The drivers in this series incorporate two voice coils on a single bobbin-not coils wound on top of each other, as on some automotive subwoofers, but two voice coils (actually three-there is a shorted winding centered between the two active coils) separated by a length of bobbin.

Two coils require two magnetic gaps. Imagine a typical loudspeaker motor cross-section. If you cut a ring into the back plate that was the exact size of the top magnetic gap, you would have two identical gaps. The trick is to keep everything concentric and centered. The LSR 28P's cast alloy motor case does just that and has heat sink fins on the back side to assist in cooling the coil. The basket is cast alloy, and a sculpted cast alloy trim ring sits on top of the basket rim to reduce diffractions. The cone is a slight curvilinear composite of polypropylene-graphite terminated by a half-roll rubber surround on the outside and a 1.5-inch voice coil. The 5-inch diameter spider is flat, and terminations on this unique driver are 0.205- and 0.250-inch male tabs.

The tweeter is a 1-inch hard dome loaded by a cast alloy EOS (Elliptical Oblate Spheroidal) wave guide-in layman's terms, an oval-shaped faceplate with wider dispersion in the horizontal plane. JBL states that the dome is a titanium composite; it appears to be a formed titanium foil with a dampening, stiffening, grainy black coating on the front side. The diaphragm assembly looks like a scaled-down version of the company's compression drivers. The dome and diamond rhombic patterned surround is one continuous piece. A round wire coil is wound on a Nomex former. This diaphragm assembly is mounted onto a butterfly, which also serves as a heat sink; this butterfly is then mounted onto the backside of the faceplate/wave guide. The pole piece is slightly extended above the surface of the top plate. It is also quasi-vented; there is a through-hole in the pole piece, but it's sealed off by a decal on the back plate, creating a chamber that is damped with foam. Oddly enough, the foam expands outward toward the diaphragm, applying a continuous force to the back side of the diaphragm.

The self-powered bi-amplified design delivers more than 200 watts to the woofer and 70 watts to the tweeter. The amplifier features a toroidal transformer, double-sided PCB with plated through-holes and a heat sink that occupies more than half of the rear baffle. The Neutrik Combo input accepts both XLR and 11/44-inch connectors. Next to the input are a series of DIP switches that allow the user to attenuate the input, bass alignment and high-frequency level. All of our measurements were made with the controls defeated or in the "flat" position.

ACOUSTICAL CHARACTERISTICS The JBL LSR 28P is a true reference monitor. The acoustical characteristics of this powered monitor are nothing less than astonishing, and this speaker exhibits the best frequency response of any monitor we have tested. Over its entire bandwidth, the LSR 28P is flat to within +/-1.5 dB. Only after 18 kHz does the on-axis response begin to fade. Off-axis response is tightly controlled, and after 2.5 kHz it only drops -3 dB out to 18 kHz, and that roll-off is exceptionally smooth.

Time response is classic. If you were to draw the perfect transient, it would look a lot like the LSR 28P's. After proportioned initial impact spikes, the overshoot resembles textbook decay-after that there is nothing, revealing that the alignment of the two drivers is perfect.

The LSR 28P's distortion measurements are also very good. Only for one data point right around the edge hole frequency of the woofer, where the surround (edge) and the cone are operating out of phase, does the THD go beyond 1%. For the rest of the audio spectrum, the THD is maintained at around 0.5%. The self-noise ("spectral contamination") of this monitor is very low. When noise is more than -50 dB down from the input signal, the figures are considered pretty good. At some points, the LSR 28P's noise floor approaches -60 dB down.






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