The MartinLogan Purity Hybrid Electrostatic Loudspeaker

“My, You Git a Purity Mouth…”

In many respects, the Purity is the most radical loudspeaker MartinLogan has every manufactured. Of course the behemoth designs with their fullrange panels and intricate sub-woofer enclosures are the real innovators, but the Purity is the shock product in the MartinLogan range.

It’s the shock product because it’s active. Well, powered – but the crossover per se is limited to controlling the pistonic driver in the bass unit. Which means MartinLogan is setting its sights squarely on audiences that don’t use power amps, and that includes plasma TV users and computer music listeners. Previous MartinLogan designs have had amplifiers driving the bass units, but this one is powered from stem to stern, using a 200-Watt amplifier built into the wooden obelisk just behind the stator panel.

You drive the Purity either by means of the speaker terminals (in which case the amplifier drives the bass unit alone) or through the single phono socket on the back of each unit. In fact, an electrostatic design is perhaps the best design for using as a powered speaker, because you have to have a power lead going in that direction anyway, to power the stator panels. Why not just make the process simpler and put an amp in there too? Controls on the speaker are limited to a three position toggle switch that gives a three decibel boost or cut to the standard bass output, factoring in mild tailoring for the bass to keep it sweet in most domestic settings (in my room, for example, the -3dB position was best, but a larger room might demand a fuller bass response). We are used to big and especially wide electrostatics in the UK, thanks to Quad. So the size of the Purity might come as a bit of a shock to some. It’s a slim, understated design, looking like the typical six-foot tall MartinLogan scaled down to domestically chummy size. The stator panel itself is a 71cm tall sheet of gossamer thin Mylar, stretched over a frame with a gentle convex curve (to give a 30° horizontal dispersion) and kept from harm by a front and back pointillist grille. Or, in native MartinLoganese, the Purity features a Generation 2 Electrostatic Transducer, with CLS™ (Curvilinear Line Source) Wave Launch, ClearSpar™ Spacers, Ultra Rigid AirFrame™ and MicroPerf Stators. There also a set of MiniETC™ (Energy Transfer Coupler) Spikes, to fine tune the angle of the speakers and the crossover uses a Vojtko™ topology. If there was a way of trademarking AC mains, MartinLogan would find it. The panel crosses over at 450Hz to a pair of long-throw 165mm aluminium coned mid-bass units in the asymmetrical obelisk chamber at the bottom of the speaker. A large, rear-firing port vents and tunes the bass output. MartinLogan cites the frequency range as having ±3dB points at 41Hz and 23kHz and the bottom figure at least seems to hold true in listening (20kHz - or beyond - and I parted company at a Hawkwind gig in the mid 1980s).

There’s some deceptively simple yet clever tricks going on here, typified by the wedge-shaped baseplate; turn it one way and the speaker beams for walking round, turn it the other and it fires at a seated listening position. Best of all, turning it this way and that requires the removal of just the one knob: all the sign of a product that’s been really well thought out. This thoroughness of execution extends across the whole speaker concept, not just the cabinet itself. It’s the little touches, like the grey cover that you can use to keep the speaker dust-free during the off times (not good with cats, who consider a large grey bag to be an instant playroom, but electrostatics and cats are not a good combination at the best of times. There’s something about the mix of a combination scratching post/kitty assault course and several hundred volts only a claw away that makes them pretty far from cat-friendly speakers). The manual too is perhaps one of the finest examples of the breed and a perfect demonstration of instruction book, er, clarity.

Nothing’s left to chance: everything from installation and positioning and even running in is highlighted and explained. This is a good thing because the Purity is typical of the electrostatic breed in being very demanding of optimum placement; the manual helps big time. Okay, the packaging does not quite conform to our recyclable sustainable save-the-rainforests Eurogreen rules, but shows how to perfectly package a difficult and fragile shape for transit without making it almost impossible to open. There’s only one downside – each box is large enough to hold both Purity speakers. Between them, they make up an armoire-sized chunk of cardboard.

MartinLogan discovered long ago that hybrid drive may work wonders in Toyotas, but can often suffer criticism in listening rooms. Specifically, the crossover between pistonic drive units and electrostatic panels is all too audible. Strangely, this is where the Purity has an edge over bigger MartinLogan speaker designs; the narrow front baffle limits the bass to multiple smaller, faster drivers instead of one big, slower design. MartinLogan also learned early on that the bass units needed to be extremely stiff and fast if they stand a ghost of a chance of keeping up with the stator panel, so aluminium cones help a lot.