PMC IB-1S loudspeaker
By Kalman Rubinson   •   September, 1998 I have a soft spot in my heart (some say my head) for transmission-line designs. I remember being entranced by the authoritative but effortless bass of John Wright's IMF and TDL Monitors, and I have been inspired to experiment by building my own lines in various sizes. Then, as demonstrated by Bryston's Jim Tanner at the 1997 WCES and at HI-FI '97, PMC's IB-1S loudspeakers threw an enormously deep soundstage. (I have a soft spot for that as well.)

The Professional Monitor Company (PMC) was founded eight years ago by two BBC veterans, Adrian Loader (footnote 1) and Pete Thomas. In view of the BBC's more recent divestiture of their speaker-development operations, PMC's BBC heritage and connection with BBC suppliers places them well to supply the demand for a wide range of monitor loudspeakers. From the humongous (the BB5, or Big Box 5) to the petite (the TB1, or Tiny Box 1), all PMC speakers use transmission-line loading for the low frequencies, and all the three-way systems use a large fabric-dome driver to cover as much of the audio spectrum as possible. Originally, PMC used the well-known ATC midrange dome, but they have now developed their own unit, which is used in the IB-1S along with their new flat-panel woofer.

Almost all PMC designs are modular, with interchangeable passive and active/powered crossover operation. Since the company's founding, PMC loudspeakers have enjoyed great success in British studios, and are now making a noise in the US, particularly in film/video post-production monitoring.

Design & technology
The PMC IB-1S uses a proprietary 10" bass driver with a flat front surface, a property it shares with the KEF B139 driver of the classic IMF TL speaker. The mouth of the transmission line, on the front baffle just below the woofer, has an area similar to that of the driver, but the cabinet size suggests that the tunnel cross-sectional area can't be much greater farther up the line. The length of the line is 2.4m, or 8', suggesting that the IB-1S is designed for a low cutoff below about 34Hz.

Although much attention has been paid to the stiffened carbon-fiber/Nomex bass driver and its TL loading, the most prominent feature of the front panel is the large, centrally placed 3" midrange dome. This covers a range of more than three octaves, from the upper bass to the lower treble. The burden falls on this driver to reproduce most of the musical fundamentals. The sharp 24dB/octave crossover allows for little overlap of the woofer and tweeter into the 380Hz–3.8kHz midrange bandwidth. The tweeter, a conventional-looking 1" silk-dome unit, is placed just above the midrange so that all three drivers lie on the panel's vertical line center.

The rear of the cabinet has a large, removable panel that bears the crossover, and individual pairs of binding posts for each of the three drivers. (The panel itself can be replaced with one bearing an active crossover and amplifiers to make the IB-1S into a self-powered speaker.) The 25-element crossover employs air-cored inductors, low-loss capacitors, oxygen-free wiring, and silver-loaded solder at all joints.

The rectangular cabinet is constructed of black MDF (in 25mm and 18mm panels), with radiused edges and engraved lines and logos. Still, some vibrations could be felt on cabinet surfaces (especially the crossover panel) at moderate to high sound levels. Although more domestically desirable veneer finishes are promised, the unobtrusive studio matte black was the only finish available at time of writing. US distributor Bryston also supplies matching steel stands, but these are not particularly graceful for living-room use.

Setup
I tried to place the IB-1Ses in my usual spots, but found the bass a bit lumpy and the midrange unfocused. Remembering the extremely wide spacing used at HI-FI '97, I progressively pushed the IB-1Ses as far apart as I could. At my usual 12' listening distance, the IB-1Ses might have enjoyed an even wider spacing than the 10' I had to settle for. Sitting closer, narrower spacing was acceptable. Indeed, I found that the IB-1Ses were most transparent and balanced when the listening triangle was obtuse; ie, when I sat a bit closer to the speaker plane than the speakers were to each other. Minimal toe-in was needed. A listening distance of about 3.5' with speaker spacing of 5' was extremely exciting and involving, and suggested that placement atop a mixing console would be hospitable for the IB-1S.

The sturdy, spiked stands were required to maintain the bass on my carpeted floor—as, indeed, they would be even on hard surfaces. Moving about the room—sitting, standing, lying on the floor—it seemed to me that the IB-1S was most integrated with the midrange driver at ear level and would benefit from even higher perches than the 11" lift (10" stands plus 1" from the adjustable spikes) the stands afforded. This, too, is consistent with the speaker's use in recording/mixing studios. Lacking taller stands, I often slouched on the couch as I got into the music.

The PMC was easily driven to great loudness by all the amps I had on hand. However, it had some tendency to bass boom, which could make it sound all bark and no bite. Not surprisingly, the mighty Bryston 7B-ST monoblocks were champs at getting the best from their stablemates. A single McCormack DNA-1 was excellent as well, but it lacked the Brystons' absolute authority at very high output levels. With the Brystons, the bass was tightened considerably. The otherwise liquid and transparent Simaudio Moon W-5 amp seemed to have too tight a grip on the IB-1S's bass, sapping it of LF weight. The Sonic Frontiers Power-2 went the other way, offering an overripe bottom end. These observations suggest that, sensitivity apart, the IB-1S presents a power amplifier with a complex load.

Making music
After all that jockeying for position and partnering, the PMC IB-1S proved a quite neutral and revealing transducer. Driver integration was excellent, and despite the steep crossover, all sounds were represented precisely and uniquely in space, even when I listened quite close to the speakers. This integrity probably contributed to the depth of the IB-1Ses' soundstage, which, unless one listened from a great distance, seemed to begin just behind the speaker plane.

As a long-time microscopist, I think of this in terms of an optical analogy: The different drivers with their individual frequency contributions can create an integrated virtual source just behind them in the same way that the parallel light rays emerging from a microscope eyepiece can be focused by the eyes into a sharp virtual image that seems to be a foot or so away. A more forward presentation usually implies the existence of response anomalies—as, in my analogy, aberrations in the optical path always seem closer than the viewed object.

The 3" dome midrange and its associated crossover were responsible for the superbly accurate and detailed reproduction of voices and instruments. Female voices, singing or speaking, had uncanny natural presence, as was demonstrated by Barbara Hendricks in the last movement of Mahler's Symphony 4 (Salonen/LAPO, Sony Classical SK 48380)—she was so present she was almost palpable! From there I went to Eugene Fodor's new recital disc, Witches Brew, on Clarity CCD-1017. (Hey, guys, isn't that title getting tired?) The sound of the violin was sweet and grainless, with just the right undertone of the friction of bow on string, especially in Ravel's Tzigane. The piano accompaniment was rich and resonant, and both instruments sounded as if they were in my room. The only clue to the recording venue was the distantly rearward placement of the bell obbligato played by Mrs. Fodor in an arrangement of Paganini's concerto setting of "La Campanella."

I alluded above to the IB-1S's use as a monitoring speaker; the honest and revealing midrange is what eminently justifies that application. In addition, the IB-1S maintained its superb sound quality and coherence at volume levels beyond any necessary in my home.

The upper treble was almost equally good. The response just off-axis, in the optimal arrangement, seemed smooth and extended, and meshed well with the upper response of the midrange. Even at very high levels, the HF retained its characteristic smoothness. But, unlike my experience with the Artemis EOS and the Ruark Equinox, the IB-1Ses rarely fooled me by disappearing; somehow I always knew where the speakers were. This lack of disappearing ability had no effect on the harmonic structure, but only on the localization of specific instruments. For example, on the Vaughan Brothers' "Hillbillies from Outer Space" (Family Style, Epic ZK 46225), the instruments were well-delineated and placed somewhat behind the speaker plane. While the virtuoso finger-snapper was back there with the instruments, the tissshh of the antiphonal cymbal kept pulling my attention to the tweeter itself. This, I suspect, is related to the tweeter's central placement on the broad front panel, and is the reason that most high-end domestic speakers present a small frontal area in the region of the HF driver.

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Footnote 1: Sadly, Loader passed away in July. See this issue's "Industry Update" for an appreciation by Paul Messenger.—John Atkinson