Monitor Audio Studio 50 loudspeaker
By Martin Colloms   •   March, 1996 English loudspeaker manufacturer Monitor Audio has mined a rich vein with their exclusive 6½" metal-cone driver, which covers a range from the bass to the midrange. MA designs using this drive-unit have fared well in these pages, ranging from the Monitor Audio Studio 6 minimonitor (reviewed by JA in February '94, Vol.17 No.2) to the floorstanding Studio 20 (reviewed by RH in December '91, Vol.14 No.12, and by ST in April '92, Vol.15 No.4).

The company's 1" magnesium-alloy dome tweeter was well-established. The decision to go for a 6½"-cone unit to partner it was an astute decision, ultimately leading beyond the first objective, which was the design of an all-metal, pure-piston loudspeaker system.

Astute because in the larger 8" size, metal cones haven't scored well. They failed to reach high enough to the usual crossover point to the tweeter, generally placed in the 2.5–3.5kHz range. In the 6½" frame size, a well-designed drive-unit can have sufficient power capacity to satisfy the requirements of even a medium-size floorstanding speaker, yet provide a sufficient frequency range for a successful crossover in a relatively simple two-way design. Moreover, the modest size allows elegant floorstanding pillar-type speakers to be produced, just right for many domestic environments.

A couple of years ago, Monitor Audio launched the Studio 50. This is an three-way system using the 6½" driver, just a little taller than a '20 but otherwise similarly sized. The Studio 50 is priced at $7995/pair in real wood veneer, black oak, or satin rosewood, but for $8895 you can have a pair finished in hand-polished Grand Piano lacquer—I can testify to the near perfection of its mirror gloss. Though expensive, the Studio 50 takes the pistonic-cone concept to an altogether higher level.

To have a high-power three-way in a system as trimly sized as this one is definitely appealing. Standing 41" tall (plus a couple more for the plinth and steel floor points), the Studio 50 is just 8" wide and 12" deep. A seated listener will find the main axis to be close to that of the midrange driver, with the tweeter set a little above head level, 39" from the floor—not a bad thing in view of the tweeter's finely beamed axial ultrasonic resonance, which is typical of this and other related 1" metal-dome units.

Technology
While the Monitor Audio Studio 50 has clear Studio 20 antecedents, divergence is evident. Both speakers share a common high-frequency unit, the 1" "gold" alloy dome, ferrofluid-cooled and fitted with a simple magnet system lacking a rear chamber. The custom faceplate is made of reinforced plastic, and the driver is built to MA's specification by SEAS in Norway.

Both the Studio 20 and '50 use a 6½" metal-cone unit for bass and midrange duty. This is manufactured by MA and benefits from a high-powered ventilated motor-coil working on a 1½" pole. The alloy cone is deeply hard-anodized, so producing a ceramic layering, while the cone edge is folded back to increase its stiffness. Termination is by a modified nitrile-rubber half-roll. In the case of the two-way Studio 20 design, this driver operates full-range, in a reflex-tuned enclosure. In the '50, however, one unit—the uppermost drive-unit, below the tweeter—is used with both its excursion and operating range limited by a sealed-box loading of some 10-liters internal volume (61 cubic inches). This gives this unit a natural, critically damped, acoustic rolloff of 12dB/octave below 100Hz. There's no additional electrical rolloff for this midrange unit. Note that the DC resistance of the midrange unit's voice-coil remains in circuit; ie, in parallel with the twin woofers. Consequently, all the bass voltage input, down to the limit set by the source material, is still applied to this midrange unit, even if it doesn't reproduce significant acoustic power in the bass range.

In the lower section of this design there are two more 6½" drivers, built much like the mid units, but with mass loading applied to their cones to lower their free-air resonance. Higher-impedance coil windings are specified to allow for paralleled electrical connection. Again these drivers operate in a sealed box, the so-called "infinite-baffle" loading. Their response is tapered by an overdamped alignment so as to give good boundary matching up to a rated limit of 25Hz, –3dB.

The speaker may be bi-wired via gold-plated five-way binding posts; these have rather small endcaps, making it more difficult to tighten the connections. The bi-wire transition is from the midrange/bass to the treble. All internal wiring and connections are hard-soldered for consistent electrical performance. Set at 100Hz and 3.2kHz, the electrical crossovers are essentially 12dB/octave, with additional damping to optimize the acoustic slopes and phase matching. The treble feed is via a single 3.3;uF capacitor augmented by the natural treble rolloff; only the high-pass filter to the midrange is purely acoustic. High-quality components are used in the crossover, with polypropylene-dielectric film capacitors used at critical points for both the mid and treble sections. However, no correction appears to have been made for the inevitable metal-cone resonance that appears in the 4–6kHz region.

Constructed of ¾" MDF throughout, and excellently veneered, the enclosure achieves resonance control via a mixture of high-stiffness bituminous damping pads and multi-section, multi-compartment bracing. There are four braces in all. It may not be the heaviest box in its class, but the results show that the technique is effective.

Acoustic foam lines the midrange chamber, this sealed off from the treble section by a full compartmental brace. As the frequency response is restricted to below the nominal 100Hz crossover point, little damping is required in the bass enclosure, which is consequently almost empty. A heavy, stabilizing, black-painted MDF plinth is bolted to the enclosure's underside. This takes strong 8mm floor-locking spikes and provides a larger footprint for the spikes. The improvement in stability must be tried to be believed.

Since I looked at an early production sample for the UK's Hi-Fi News & Record Review magazine a couple of years ago (footnote 1), the Studio 50 has benefited from build-quality improvements including a better grille, the replacement of some electrolytic capacitors in the crossover network with polypropylene-film types, and the addition of the all-important plinth.

One aspect of the Studio series of speakers (and, for that matter, designs from some other manufacturers), is a dependency on ambient temperature. If too cold, both the Studio 20 and the Studio 50 will sound too dry in the deep bass and show a loss of speed and timing precision in the low range—for want of a better expression, the speaker will sound "uptight." As the speaker warms beyond 19°C (66°F), the sound opens out. The bass gains pace and depth, the result sounding significantly more engaging than before. The ideal operating temperature is 21°C(70°F). Beware of a chilly demonstration!

Setting up
These speakers proved to be a breeze to set up. Their tight, quick bass didn't easily tend to boomy excess and allowed for a range of locations. I was happiest with the enclosures nearer the back than the sidewalls—around 2' distant compared with 3½' to the sides.

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Footnote 1: HFN/RR, July 1994, p.48.