PMC OB1 Loudspeaker

      Date posted: February 24, 2004

PMC OB1 Loudspeaker

Sugg. Retail: $4695 US per pair
Size: 42″H x 8″W x 13″D
Distributor: Bryston Ltd., PO Box 2170
677 Neal Drive, Peterborough,
Ont. K9J 7Y4 (705) 742-5325
FAX (705)742-0882

(Reprinted from the Winter/Spring 2004 Audio Ideas Guide)

      A company best known for its pro monitors, PMC (short for Professional Monitor Company) has been more recently seriously addressing the consumer market. The OB1 is the latest model in this evolution, a 3-way floorstanding design using a transmission line for bass loading with an effective length of 3.3 metres (11 feet). The drivers, according to the literature, are a low-frequency driver that is “doped [paper] with cast alloy chassis”, a “doped, 75mm fabric soft dome” midrange, and a “27mm, silk soft dome, ferrofluid cooled” tweeter. “The bespoke drive units utilised in the OB1 have shown their credentials in both professional and Hi-Fi circles”, according to the brochure. The phrasing makes it clear that this is a British product, Bryston distributing PMC in North America, while PMC handles Bryston in Britain and Europe.

      Another interesting thing about the OB1 is that it is configured for tri-wiring or tri-amping, the first such speaker in our experience since the Energy Veritas v1.8. For listening tests I used a tri-wire cable specially made up for AIG by AudioQuest consisting of 8-foot lengths of Granite for woofers and midrange, with CV-4 for the tweeter, all connected together at the source end. One thing I like about this and other AudioQuest cables is the spring-like banana connectors that fit securely and offer high contact area.

      It’s an unusually attractive speaker that comes in four finishes: Cherry, Black Ash, Oak, and Rosewood. The cabinet is “constructed using CNC precision-machined panels of Medite(TM), which are veneered both inside and out, producing a perfectly matched pair as well as a stable structure. The acoustically treated internal labyrinth braces every panel eradicating the effects of cabinet flex and colouration.”

PMC OB1 Frequency Response

      Proven professional drive units and solid construction should add up to a good speaker, but even that is an understatement with a speaker of this calibre. It is, put simply, one of the best reproducers I’ve encountered in years. That has to start with good measurements, and in our new simplified format, the OB1 can be seen to have very smooth performance at 0 and 15 degrees (virtually identical), with a minor rolloff as frequency increases at 30 degrees, and still very smooth and controlled response at 60 degrees off axis.

      The sum of these measurements, the Summed Axial Response (SAR) is seen at top, an it merely underlines the smooth, extended response; note how the deepest bass shelves between 30 and 20 Hz, ensuring true bottom heft. The speaker is +/-2 dB or better over most of the frequency range, with a gentle rolloff in the deepest bass characteristic of transmission lines; reflex ported speakers tend to roll off more steeply. PMC OB1 Impedence Transmission lines are preferable because their bass is very fast and tight due to the way air is pushed through the line to the port, which is at the bottom front of the enclosure. The very existence of the transmission line “labyrinth” inside the cabinet ensures very high bracing and a very acoustically inert cabinet, as noted above.

     Looking at the exceptional impedance and electrical phase curves we see a slight rise at the upper crossover point between 3 and 4 kHz, with a nominal impedance of 10 ohms, and a low of 7 ohms at 100 Hz. It rises to about 18 ohms in the bass near 50 Hz.

      The phase curve just below is one of the most linear I’ve ever seen in a speaker, a flat perfect line from 200 to 5000 Hz, with a deviation of less than 10 degrees in the upper treble. This has to be one of the best crossover designs I’ve seen, smooth in frequency and accurate in phase.

      This may account for the extremely good imaging both in terms of width and depth, and the clarity and detail I heard in listening. Bass seemed a bit dry, but was well extended, down only 7 dB at 20 Hz; because of the front port, the OB1s can be situated quite close to walls and corners for deep bass reinforcement without much worry about mid-bass bloat.

      There’s a lot to be said about 3-way designs that use midrange domes, with their almost peerless dispersion and power handling. That’s what seduced me about the Veritas, and it’s here in spades with this PMC speaker. The OB1 will play very loud without midrange compression, and this, combined with the effortless, extended bass, makes for a fatigue-free listening experience.

      I thought they were just the slightest bit bright between 6 and 10 kHz, but that’s the only criticism I could make of these speakers, and even this would be ameliorated in a large room with, say, a 12-foot listening distance rather than our 8-foot one. It’s definitely a big room speaker, and quite a modestly sized one at that. Over the past few years PMC has learned how to get professional speaker performance out of reasonably sized cabinets, and it’s the consumer who benefits. I suspect that quite a few studio mixing rooms working in up to 7 channels might also appreciate this new model.

      My listening notes call the OB1 “one of the most natural and lifelike reproducers I’ve heard in years”, and this also comes from phase, as well as frequency accuracy. Phase is time, and if the musical events are happening in their proper order, detail, dynamics, and image width and depth will pop into proper relationships, and this is what is happening here. In the PMC OB1 it all adds up to a speaker system that really does it all.

Andrew Marshall

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