PMC EB1i Transmission Line Loudspeaker

Big PMC speakers are always great fun. Big speakers have an intrinsic advantage when it comes to moving air and, all other things being equal, can do this in more entertaining fashion than small ones. Small speakers can seem more fi, but personally I’m in this game for the joy of music rather than the minutiae of soundstaging. So it was with some enthusiasm that I greeted PMC’s artist in residence Keith Tonge when he arrived on my doorstep with these magnificent beasts filling up his Passat. It wasn’t long before we’d lugged them in and hooked them up so that the fun could begin, fun that we managed to spin out for the rest of the day. Sometimes this isn’t such a bad job!

PMC has been busy over the winter. Not for founder Pete Thomas the luxury of putting your feet up by the fire; he was beavering away in the listening room refining pretty much the company’s full range of domestic loudspeakers, prompted by the arrival of a new tweeter from SEAS called Sonolex, a drive unit that PMC had helped develop.

This resulted in the i series, which has swept through the PMC range like wildfire though without the usual destructive results. In fact, if the blurb is to be believed the tweeter has wrought big improvements. I reviewed the EB1, the largest domestic floorstander that PMC makes, only last year so it was a surprise to see it turn up again so soon. But the tweeter is now green and the finish that little bit smoother, while the process of integrating the tweeter has also brought about other changes.

The EB1i is still a substantial loudspeaker of course. Standing well over a metre high and weighing in at a consternating 48 kilos you wouldn’t want to bump into it in a dark alley – or anywhere else for that matter (might scuff the lacquer!). It features a teninch bass driver with a flat carbon fibre and Nomex diapragm, backed up by PMC’s trademark transmission line (TL) loading, although the EB1i deviates slightly in this department. It has an extra chamber at the end of the line, above the place where it vents. The purpose of the TL is to augment the speaker’s output at very low frequencies, by channelling output from the rear of the bass driver down a long, heavily damped “pipe” that’s designed to remove all but the lowest frequencies. In effect it’s a mechanical low pass filter that has the benefit of loading the bass driver resistively and which is said to produce the lowest octaves in phase with the direct output.

The purpose of the additional chamber in the i series is to give greater control over the response produced by the TL, effectively rolling it off more steeply and smoothing out ripples that affected the original. This change is also claimed to make the midrange sound clearer which is immediately apparent when you play something familiar through the EB1i. PMC’s have always had a more obviously open balance than average but this is even more noticeable here. The source of all that mid-band detail is a three-inch fabric dome, with its own plastic enclosure. PMC goes to great lengths to make sure that this enclosure is well damped, using bitumastic sheets and carefully selected foam, which is said to absorb the rearward energy from the driver. The size of the housing also damps the dome acoustically and allows it to operate cleanly down to the 380Hz crossover point.

Other revisions for the i series also include veneer on all faces of the slightly higher density MDF cabinet, plinths that you no longer have to bolt on yourself and a rather nice chrome and enamel automotive style badge.

I wondered why it had taken so long to come up with the secondary chamber idea. According to designer Pete Thomas: “One of things I have a bee in my bonnet about is distortion in the bottom-end. All speakers are actually appalling if you measure the distortion at lowfrequencies. One of the reasons I like transmission lines was not the amount of bass or how low they go but how low the distortion is. The only other speaker I ever liked were electrostatics, which don’t have the punch and power but do have very low distortion. The lovely thing about the line is that it absorbs the rear radiation from the woofer, acting as an acoustic crossover, and the steeper the roll-off the more accurately we can match the driver to the TL. The extra chamber increases the roll-off of the line.

“One of the problems with TLs is that they are complex to analyse mathematically as well as sonically, so our speakers have evolved over 20-years. This extra chamber is quite a jump and we did consider filing a patent for it, but using a chamber as another filter was really part of the slow evolution of our designs. We have had products with smaller chambers in but they didn’t have the effect that the one in the EB1 does. I have probably reduced the distortion by about 50%.”