Finally, run the main speakers full range and roll the subs in underneath. Don’t muddy the midrange waters by running them via the crossover’s high-pass outputs.
From theory to practice…
The number of subwoofers on the market that tick the above boxes is astonishingly small. The best known are probably the Wilson Audio subs – the current Subsonic (designed to partner their large, floorstanding speakers) and the earlier – and sadly discontinued – Thor’s Hammer and WatchDog. Those designs stood resolutely apart from the mediocrity of mainstream, powered and DSP EQ-ed offerings, a clear example of how to do the job properly. But even Wilson lost the faith and the compact, powered Loki and Submerge models have surrender the performance-orientated high ground and followed the herd.
So when the original PureLow LO sub came crashing into my audio consciousness it was a considerable surprise. Not only was it an audio-orientated product, it’s approach and construction were almost otherworldly, the purest of purist designs. Who ever heard of a flat-panel subwoofer? Especially one that boasted extension down to 10Hz, a totally flat, amplifier friendly 4Ω load and a 90dB free-space sensitivity! Originally designed to partner with and deliver the low-frequencies lacking from stacked Quad 57s, the PureLow was setting the integration bar pretty high, both in terms of quality and its high-pass frequency. That it met that goal so successfully speaks volumes. But it was also big and imposing. A frontal aspect that’s a full 125cm square puts even large loudspeakers to shame. If the panel itself is only 120mm (or 5”) thick, there’s no escaping the sheer visual impact of one, let alone two LOs – an acronym that stands for Large One!
Reviewing that product, I described it thus:
It’s not often that you come across something genuinely new in audio. From contra-rotating turntable platters to tangentially correcting headshells it’s all been done (or at least attempted) before. In one sense, the same is true of PureLow’s LO sub-woofer. It is based on the Orthophase driver design, a topology that dates back as far as 1959 – or 1929 if you count its genesis in the wonderfully named Blatthaller PA system. But there’s never been an Orthophase driver quite like this! The originals were 10cm square quasi-ribbon designs. The PureLow LO subwoofer uses just one driver – but it’s a metre square!
Perhaps I should start at the beginning… The Orthophase drive system is perhaps most easily understood with reference to planar magnetics like the Magneplanars. Except that, rather than having a flat ‘voice-coil’ placed in front of a magnetic array (a situation that means that the larger the excursion, the further the voice-coil – and diaphragm – moves from the magnetic field) the Orthophase design places the wires of the voice coil on stand-offs, behind the diaphragm and contained in deep, U-shaped magnetic channels, so that they remain firmly within the field’s grip. Scaled up to a metre square, the result is a diaphragm with a swept area equivalent to eleven 15” drivers, with a maximum excursion that’s greater than ±9mm and that’s driven across its entire area! Throw in a remarkably benign impedance that sits around 4 Ohms, an efficiency of 90dB, a fundamental resonance at 9Hz and phase-linear output to well beyond its pass-band and you’ve got an absolute monster on your hands – and that’s before you physically examine the beast! Anyway you look at it, the PureLow LO sub is as imposing as it is impressive, as potentially potent as it undeniably pricey. This is a serious sub-bass solution for serious listeners with seriously deep pockets. It is also a revelation!
