Boston Acoustics Lynnfield 500L loudspeaker
In early 1991, Boston Acoustics began "The Lynnfield Project," an ambitious effort to develop a line of high-end loudspeakers that would push the edge of current loudspeaker technology. The man hired to head the project was Phil Jones, the acclaimed British designer behind the Acoustic Energy loudspeakers. Jones is a leading proponent of metal-cone drive-units, which were used successfully in the Energy line.
The Lynnfield Project is a from-scratch effort with custom-made drive-units. The products incorporate many new design techniques, one of which is a revolutionary method of attenuating driver resonances. Clearly, a high level of engineering effort was expended on these new loudspeakers. The 500L reviewed here is the flagship of the line. (The smaller 300L is currently being auditioned by JA, for a review.)
A company such as Boston Acoustics, that is dominant in the mass-market, may have several reasons for entering the field of high-end loudspeakers. First, the company may see the High End as a potentially profitable market niche. Second, a high-end flagship product engenders prestige and better name recognition for the company. Finally, design innovations developed for the high-end line can trickle down into the company's budget products.
But does an engineering tour de force necessarily make a great loudspeaker? We'll find out.
The Lynnfield 500L is a floorstanding loudspeaker system comprised of four cabinets. The tweeter and midrange drivers are mounted in a smallish sub-enclosure that rests atop each bass cabinet. No drivers are visible in the bass cabinet: The two 6.5" woofers are mounted inside and connected to the outside through two rear-firing, flared ports, each 2" in diameter. The 500L is unusual-looking, owing to the new resonance-attenuating device covering the midrange and tweeter. I'll discuss this in detail later.
The bass enclosure is coupled to the floor via small spikes that fit into large leveling knobs. Those with an aversion to spikes can remove them, allowing the loudspeaker to rest on the knobs.
A set of rubber plugs fits into holes in the bass enclosure's top surface. Corresponding rubber feet are attached to the midrange/tweeter bottom, providing a compliant interface between the two cabinet portions. This decouples the bass enclosure from the midrange/tweeter cabinet and also prevents scratching. The midrange/tweeter enclosures can be mounted on optional stands, allowing the bass units to be placed in a more convenient location.
The three-way 500L can be single-wired, bi-wired, or tri-wired. Three inputs are provided, allowing great flexibility in connection. When single-wired, two pairs of jumper plates at the bass enclosure bottom are installed, and four jumper wires connect terminals at the bass enclosure's top to the midrange/tweeter module. Internal wiring connects the bass enclosure's bottom terminals to the top terminals. This wiring can be bypassed by bi-wiring or tri-wiring.
A black grille can be mounted to cover the midrange/tweeter enclosure's front. The owner's manual suggests that the grille's effect is small but audible. I auditioned the 500Ls with and without the grilles.
I came to dislike the 500Ls' input terminals. They have a knurled knob over a post with a large hole in the middle for inserting bare wire. The post is so big that spade lugs don't fit over it without spreading the spades. Moreover, the flat contact area between the knob and post was too small to adequately hold spades. The many times I connected and disconnected the 500Ls was an exercise in frustration. Finally, one of the posts came loose and turned in its mounting.
Apart from that gripe, the 500L's build quality and appearance are outstanding. My review samples were finished in exquisite rosewood veneer that would complement any decor. I must also comment on the superb owner's manual. It is clear, complete, well-written, and highly informative.
The design's most striking aspect is the large bar across the midrange unit and the corresponding smaller structure in front of the tweeter. These are called "AMDs," short for Amplitude Modulation Devices. These structures are narrow-band acoustic filters that attenuate unwanted output from the drivers, particularly driver ringing and cone resonances. Specifically, the AMDs are a collection of tubes placed in front of the driver. When certain frequencies pass by the tubes, the amplitude of those frequencies is attenuated. The length of the tubes determines the absorption frequency. By carefully tuning the AMD to specific frequencies, driver anomalies such as cone ringing can be attenuated.
The midrange AMD is machined from an aluminum block and uses columns of air rather than tubes, but the operating principle is identical. These columns are ¼"-wide troughs that run the AMD's length.
Footnote 1: I once worked in a hi-fi store that carried the Boston Acoustics line.—Robert Harley