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PMC FB1

Floor-standing Monitor Loudspeakers By Hugh Robjohns
Published February 2000

PMC FB1

Developing their TB1 nearfield into a floor‑standing loudspeaker, PMC have come up with a full‑range monitor which nicely balances studio performance with the cosmetics of quality domestic hi‑fi.

Monitoring loudspeakers are not what they used to be. There was a time when they were the size of small brick outbuildings and looked as if they were made out of rough‑hewn railway sleepers. But these days tonal accuracy isn't the only consideration — engineers also demand compact dimensions with the aesthetic appeal of nice wood veneer!

Although large soffit‑mounted monitors (installed to be flush with the wall) are still popular in the biggest studios, for most smaller rooms, more modest systems supported on stands behind the console are preferred. Apparently there is also a trend towards floor‑standing units, which are often seen as being aesthetically better than heavy metal stands, particularly in some mastering and post‑production suites where the high chairs at the console have been discarded in favour of comfy sofas and an altogether more relaxed atmosphere.

It is with this background that PMC were persuaded to develop a new budget floor‑standing monitor speaker to satisfy the needs of the post‑production, home‑studio, quality hi‑fi and home‑theatre markets — the relatively high sales volume in this last sector helping to justify the development costs. It should be said, though, that PMC are no strangers to floor‑standing monitors, as their top‑of‑the‑range BB5 system, found in mastering rooms and recording studios around the world, can be configured as a gargantuan floor‑stander of around seven feet in height!

The FB1 Investigator

The FB1 (Floor Box 1) project started out purely as an experimental modification of the highly successful TB1 (Tiny Box 1) nearfield monitor. Designer Pete Thomas hadn't been able to try such a small bass driver in a true transmission‑line system before, because the TB1 used a hybrid arrangement which he calls 'Transflex' — a combination of reflex and short transmission‑line elements (see 'The Transmission Line' box for an explanation of these designs). However, the additional cabinet volume gained through redesigning the TB1 as a floor‑stander has enabled a proper transmission line to be constructed.

The FB1's cabinet shares the TB1's width and depth, but is obviously much taller, with overall dimensions of 1000 x 200 x 300mm (hwd). Although the FB1 looks quite large it remains sufficiently well proportioned for domestic approval, but is big enough to allow some typical PMC magic at the bottom end!

Independent driver connections on the rear panel of the FB1 allow it to be bi‑wired or bi‑amplified should you require.Independent driver connections on the rear panel of the FB1 allow it to be bi‑wired or bi‑amplified should you require.Most floor‑standers (most speakers, in fact) are reflex designs and, because of the physics of reflex‑loading, the full volume of the box is often more a hindrance than a blessing due to unwanted standing waves and resonances. However, in the case of the FB1 the additional volume has enabled the inclusion of a transmission line with an effective length of around three metres. The bass driver is positioned around a third of the way along the line and, through skilful balance of the numerous parameters involved, the FB1 has been blessed with a frequency response which extends between 22Hz and 20kHz at the ‑3dB points. In fact, it is substantially flat above 27Hz, and I am not aware of any other full‑range monitor speaker of this size and at this price point which shares the same degree of bass extension.

The cabinet is a little more 'designed' than those of many professional loudspeakers, featuring rounded front corners and an optional black plinth base. It is also available in a selection of real wood veneers including Light Oak, Walnut, Cherry, Black Ash and Rosewood (this last at extra cost). Such detailing is mainly intended to gain the FB1 acceptance within the domestic hi‑fi marketplace, but also helps to make the speaker stand out amongst its professional monitor peers.

The optional plinth is a worthwhile accessory as it not only increases the stability of the tall cabinet, but also provides a visual 'full stop' to the design. Fitting it is simply a case of unscrewing the spikes from the base of the cabinet, securing the plinth using the supplied Allen bolts in the old spike threads, and then refitting the spikes to the newly installed plinth.

The end of the transmission line vents to the front of the cabinet through black acoustic foam, which is visually balanced by a removable black fabric grille over the drive units at the top of the baffle. However, I'm sure many users will prefer to take these covers off to reveal the shiny metal tweeter.

Vifa 2000

The drive units employed in the FB1 are the same Vifa components as those in its diminutive sibling. A 170mm doped‑paper‑cone mid/bass unit on a cast magnesium‑alloy chassis is coupled with a phase‑corrected, aluminium‑alloy‑dome tweeter with Ferrofluid cooling.

Not surprisingly, the crossover is very similar to that in the TB1 and retains its gentle 12dB/octave slopes, integrating the two drive units at 3kHz. However, it has been tweaked slightly to rebalance the mid and top end of the frequency spectrum to take into account the extended bass response.

The rear connection panel is equipped with a pair of binding posts and gold‑plated linking bars. Removing the links allows the speakers to be bi‑wired or bi‑amplified, while four threaded inserts above the connection panel provide mounting points for a Bryston 'Powerpac' amplifier module to be fitted if required. Available in either 60 or 120W versions, the Powerpac has been designed to turn the FB1 into a powered monitor which uses the internal passive crossover.

The passive crossover design presents a very easy 8Ω nominal load and the speaker has a sensitivity of 90dB per Watt at 1 metre, permitting quite modest amplifiers to generate impressive volumes. The peak SPL output is quoted at greater than 110dB at 1 metre, which is more than sufficient for most mastering, post‑production and home studio applications.

Finding Your Sweet Spot

The FB1s were delivered for this review by Pete Thomas himself and, after moving my own PMC AB1 monitors out of the room, he set about finding the best position for the new boxes. Pete's approach is to concentrate initially on the bass, moving the speakers a few inches at a time to find the position in the room where the bass response is as smooth and uniform as possible. Once satisfied that the best compromise had been found, he adjusted the relative angle of the speakers — the toe‑in — to maximise the precision of the stereo image.

Whether you are spending a few hundred or several thousand pounds on your monitors, you really should take the time and effort to experiment in this way and not just plonk the monitors where they look good! It is surprising what a difference just a few inches of movement can make to the balance and imaging of the speakers. It was also interesting to note that although we started with the FB1s in the positions occupied previously by the AB1s, they ended up quite a bit closer together, slightly further forward, and much more toed‑in — so just because a position works for one speaker it doesn't mean it will be right for another!

For this review the FB1s were driven by a Bryston 4B amplifier, although at 250W per channel they were, perhaps, a little overpowered — PMC recommend around 100W for general monitoring duties with reasonable headroom. However, I am very familiar with both the big AB1s and the tiny TB1s driven from the 4B, so it allowed fair comparison to be made.

Listening

My first point of reference were the compact TB1s which provide almost an extra octave of bass extension compared to most equivalently sized nearfield monitors. They also possess excellent levels of neutrality, detail and imaging for the asking price, so I was not surprised to find similar qualities in the FB1s. However, I was completely bowled over by the sheer scale of sound these modest boxes created.

I was simply astonished that, using exactly the same drivers, the FB1s could sound so much bigger than their siblings and I would not have believed it possible to generate powerful, trouser‑flapping bass from a six‑inch driver! These relatively compact speakers were actually standing closer comparison with the much larger AB1s — which is very impressive indeed given the price differential.

As might be expected from the metal tweeter, the high end is crisp, detailed and revealing, but it was not fatiguing at all. I was able to listen for many hours at a time without feeling the need to take a break, which is always the sign of a good low‑distortion monitor. The usual recording anomalies, such as clicks and dropouts, were exposed clearly but without being unduly distracting. The metal‑dome tweeter is recognisable, but doesn't seem to suffer any obvious resonances and works well in this design.

The mid‑range is just as neutral and transparent as the TB1 which shares its excellent Vifa paper‑coned driver. I find mid‑range quality is quickly revealed with well‑recorded voices and the FB1, whilst not matching the best three‑way designs, certainly stands up extremely well against any equivalent two‑way monitor. I also found the integration with the tweeter to be natural, both on‑axis and off‑axis — the phase response and dispersion seemed to have been blended very well.

The bottom end, as I have already mentioned, is solid and weighty, but without any hint of boominess or monotonicity. Complex bass lines come across with both melody and dynamics perfectly intact and remain distinct from bass drums. The system, as a whole, is also very fast, with the transients and LF thud of a kick drum arriving together as a single sonic event.

Combining a narrow baffle with small, wide‑dispersion drivers provides stereo imaging which is razor‑sharp on appropriate material. It is also extremely stable, although I found that the 'sweet spot' is at its widest if the axes of the two speakers are arranged to cross slightly behind the listening position — if they cross in front, the image tends to collapse into the nearer speaker as you move from side to side.

I was simply astonished that the FB1s could sound so big, and I would not have believed it possible to generate powerful, trouser‑flapping bass from a six‑inch driver!

Unlike reflex‑loaded speakers, the transmission‑line design tends to retain its frequency balance irrespective of listening volume, and the FB1 demonstrated this quality very well. Even with the level turned right down, the low end was still present and correct, with bass guitars and kick drums retaining their fundamental components rather then being relegated to a few weedy harmonics! I particularly appreciate this aspect of the transmission‑line system, as I'm sure would many who end up mixing late at night or in close proximity to neighbours.

For those involved in mixing in surround sound, the FB1 would also make an ideal component — its role depending on the application and on the size of room. For example, it can be used as a full‑range rear speaker in larger systems, or for the main left and right monitors in more modest installations. A complementary subwoofer (the XB1) and matching centre‑channel versions of the FB1 are also available.

The bottom line is that a pair of FB1s are playing in the same ball park as the ATC SCM10, Mackie HR824, or Genelec 1029s with the 1091 subwoofer. You can't put them on the meter bridge as you can with the ATCs, Mackies or Genelecs, but they will produce a wider window on the audio signal than all of these contenders, produce similar levels of resolution and sound a lot more 'together' than the separate subwoofer approach. As always, there are horses for courses, but if domestically acceptable styling and installation in the midfield is appropriate, I would strongly recommend the FB1s.

The Professionals

Like all of the remaining British professional monitor loudspeaker manufacturers, PMC is a very small company. Originally known as the Professional Monitor Company, the business, based in Welwyn Garden City, employs a total of just 16 staff in the main premises, with a further 20 at the cabinet‑making factory.

In the early years of the company, cabinet construction was subcontracted but last year PMC acquired a cabinet‑making business to bring that element of the manufacturing process in‑house. This has allowed PMC much greater freedom in the mechanical design of their cabinets, both structurally and aesthetically (for example, the rounded front edges on the FB1, which are the result of an innovative vacuum forming process which bonds the veneer to the curved frame).

Many of the larger PMC systems are active designs using Bryston amplifiers which PMC have distributed since 1994. Bryston reciprocate the arrangement by distributing the PMC product line in the USA and Canada, where PMC speakers have become common in mastering, post‑production and film‑scoring applications.

The product range starts with the TB1 nearfield at under £500 with the LB1, FB1 and AB1 all being two‑way designs of increasing size. The IB1, MB1 and BB5 are all three‑way systems, the latter topping out around £25,000!

The Transmission Line

There are several fundamental loudspeaker designs, but for systems employing moving‑coil drive units the most common arrangements are the 'Infinite Baffle' and the 'Reflex'. The infinite‑baffle design is usually employed in small nearfield speakers and has a completely sealed enclosure, which produces a very gentle low‑frequency roll‑off. The bass‑reflex approach uses a tuned vent in the enclosure for bass extension, but with the side effect of a much steeper bass roll‑off and often a peaky bass response.

The transmission‑line system places the rear of the bass driver in a very long 'pipe' lined with acoustic absorber (foam in this case). The absorber is carefully specified to soak up mid and high frequencies but to allow the lowest wanted frequencies to emerge from the end of the line, acting just like a second driver. One of the technical advantages of the transmission line is that the excursion of the bass driver is controlled properly (which is not the case in reflex designs) resulting in extremely low levels of LF distortion. The upper bass and mid‑range frequencies benefit as they are no longer masked by LF distortion harmonics, which is a common problem in many inferior reflex designs. The design also allows greater SPL from the given driver size, and shares the gentle 12dB/octave roll‑off with the infinite‑baffle design. Moreover, the frequency response of a transmission‑line speaker remains well balanced even at low levels — you don't have to crank the volume up to get the bass end to work properly!

The inevitable drawback is that the transmission line is difficult to design properly, because there are a large number of variables involved — line length, driver position, line diameter, flaring, absorption material, density, positioning and so on. Transmission‑line speakers also require relatively large cabinets in order to work well. Unfortunately, many of the early attempts in hi‑fi circles to manufacture this kind of loudspeaker tended to favour bass extension at the exclusion of accuracy and speed, creating the myth that transmission‑line speakers are not suited for quality monitoring.

PMC have spent many years developing and optimising their transmission‑line designs and have shown that these kinds of loudspeakers can match reflex cabinet designs in every respect, but with an extra octave of bass, lower LF distortion and a frequency balance which is more independent of listening level.

Pros

  • Compact dimensions.
  • Genuine 20Hz‑20kHz range.
  • Highly accurate and very neutral.
  • Easy to drive.

Cons

  • The bass extension often reveals problems in source material!

Summary

A floor‑standing full‑range monitor worthy of the name. Its compact dimensions belie an astounding bass extension combined with the neutrality and accuracy required of a true monitor loudspeaker.

information

FB1 (standard finish) £1100 per pair; FB1 (rosewood finish) £1569 per pair; plinths £65 per pair. All prices include VAT.

https://bryston.com/pmc/

Published February 2000