From Hi-Fi World - January 1998 issue
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Round The Horn
Haden Boardman explores the history of Lowther Horn loudspeakers.
Horn loudspeakers are one of those subjects which can raise the heart-beat of dedicated enthusiasts. Problem is, there are so many different kinds of horns, with so many different applications, not all of which are really hi-fi. However, Lowther is a name which has become synonymous with horn loudspeakers, some of which are truly excellent.
Lowther started producing hi-fi amplifiers and tuners in the early 1930s in the large wooden radiogram-style cabinet that was ubiquitous at the time. What was unusual was that the better quality sets did not generally have built-in loudspeakers. The natural partner was the Paul Voigt Corner Horn loudspeaker, and the two items were generally sold together.
This loudspeaker was an amazing unit, with its small, six-inch full-range cone and very large, powerful electromagnet. Later modifications included fitting a 'whizzer' cone to the centre of the main cone to improve high frequencies and produce a response that extended past 13kHz. Remember, we're talking mid-1930s here, and this kind of specification was ground-breaking at the time.
After the Second World War, Paul Voigt left England, having been almost driven out because of his German name, and eventually settled in Canada, leaving his loudspeaker designs in the hands of Lowther. By the early 1950s the range of íspeakers had progressed to a handful of different models, although the original Voigt Corner Horn remained.
Lowther had done a lot of work on permanent magnets as replacements for mains-energised types - the original Voigt mains-energised units required 42watts of power at 200volts DC to work! So the new PM1 driver retained the same performance as the original Voigt but was easier to use and marked a new era for Lowther.
The American designer Stewart Hegeman then became involved and produced a new flagship horn known simply as 'The Hegeman'. This relied on the same basic twin-cone driver allied to the new PM4 magnet unit, which has the most unbelievably powerful gap-flux of 24,000Gauss - stronger than anything else I have ever seen.
The Hegeman has a huge monster of a cabinet, four feet tall, four feet wide and two feet deep. It has a folded bass horn to the rear (or, more correctly, underneath) the driver and a plaster-of-Paris midrange and treble horn. I am lucky enough to have picked up one of these and I cannot tell you how much I would give to make a stereo pair - it is a truly awesome loudspeaker.
By the late 1950s stereo was becoming popular, and most of Lowther's models were huge. As a result, the constant-width rear-horn Acousta, built to work with the PM6 and PM7 units, was born. These two drivers had physically smaller magnets than the earlier PM1 but, through the use of new materials such as Ticonal, these were actually much more powerful, especially in the PM7 model.
The Acousta cabinet is simple enough in terms of construction. It isn't exactly small or attractively-proportioned by modern standards but it isn't so big a pair won't fit in a modern room. One advantage it does have over more modern loudspeakers is that it often works better situated in a corner - it is not the kind of íspeaker that appreciates sitting in open space. This is basically because its horn mouth is designed to work in conjunction with the wall and floor to effectively increase its size and lower its bass cut-off frequency.
I find the sound of the Lowthers beguiling. They are very efficient - in a cabinet, the PM7 hits over 103dB/watt. A good, clean valve amplifier is the best way of driving Acoustas. Lowther make some rather optimistic claims about power handling which, in reality, is about 15watts. Mind you, that translates into more than 111dB in-room, and that's LOUD! A íspeaker of 83dB/watt would require in excess of 1kW per channel to go that loud, if it could handle so much power.
The Lowthers' frequency response is hardly ruler flat, but the older the unit is, the smoother it gets. A new drive unit or one freshly repaired will take around three years to run in. This is because the cone is so very light its sound is heavily influenced by the age of the roll surround and spider.
The treble is very directional so you have to sit fully on axis to get the best out of it. When you do, you'll find its quality is excellent, similar to an electrostatic, only more dynamic.
On first audition, a pair of Acoustas may sound flat, lacking in definition and bass-light. The longer you listen though, the more you become aware of just how much there is going on in the music that lesser moving coil loudspeakers miss out on. I recently made a recording of a drummer for sampling purposes and playing it back over the Lowthers was a scary experience it was so life-like.
Alnico and ceramic magnets are distinguished by their different shapes. The Alnico on the left is small and bulbous while the ceramic on the right is flatter and larger in diameter.
Lowther drivers do have their problems - they are very easily damaged and cost a small fortune to put right. When buying second-hand, unless you have documentary evidence they have been replaced, budget on the foam surrounds being rotten. Even if they look okay, they may still be past their best. Newer units (recognisable for a black chassis) have updated surrounds that do not suffer this but the cones are not as well made in my opinion. Also look out for magnets that have been swapped over - the late ceramic magnets fitted from the early 1980s are not as good as their Alnico counterparts.
Never use Lowther drivers with any transistor amps that have a switch-on 'thud' - the thin, delicate voice-coil wire will fall off the coil former. This can happen more easily than you might think because Lowther wind on the inside of the coil former as well as the outside to obtain maximum efficiency.
Despite their faults, like a slightly boxy sound, the Acoustas are a very enjoyable listen. The fact they're efficient is an added bonus. They are an easy load too (early ones are rated at 15ohms, later black-chassis versions at 8ohms) but have a high average impedance curve. This makes it hard for a low-power amplifier with high output impedance to drive them correctly.
Steer clear of later MDF Acoustas and the narrow dual-position or corner types which have the driver pointing up at 45degrees. Stick to the pre-1970 plywood incarnations and 'A' (Alnico: aluminium/nickel/cobalt) magnets on either PM6s or PM7s. Also check out the rare PM1 and Audiovector models.
Lowther made some impressive valve power amplifiers as well although these are rarer than hens' teeth. Avoid Lowther's early experiments with transistors; like many amps from solid-state's infancy they're not particularly good. A good pair of Acoustas with PM6As or PM7As will command a price of around £300-£500 and that's money well spent.
This review was published in the January 1998 issue of Hi-Fi World. No material may be reproduced from this review without the written permission of the publisher. Copyright Audio Publishing Limited