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TOWARDS PERFECTION

The Lowther-Voigt Legacy by Haden Boardman
Reprinted with kind permission from Sound Practices #8

Getting hot under the collar? Cannot find that 100dB/watt speaker to use with yer flea powered single-ended tube amp? Fed up with power sapping crossovers, misaligned phase fouling multiway speakers? Tired of that metal dome tweeter ting and over Q'd bass? Well, read on.

Most multiway speakers suck_period. 99% have crossovers in the 500-5000 Hz range, the area that your hearing and brain are most sensitive to.

Worse still, most makers add a "phasey" dome tweeter (any dome tweet, not just metal 'uns) to the act. The idea of the dome is to disperse the sound as far as possible. The problem is the sound "bounces" off the tweeter's face plate, the loudspeaker cabinet and nearby furniture. This gives the travelling sound wave a small amount of delay, which permanently "masks" the original fine imaging details of the recording, resulting in only LEFT - CENTRE - RIGHT imaging with no, or at least very little, depth of image. Stereo is supposed to be "3D", you do not need surround sound or quadraphonics to fill your room with "3D" sound.

The final problem with "domes" is the fact that most of the dome stays still, most of the sound comes off the edge of the voice coil rather than the mass of the dome. Imagine for a moment, you have a 16mm diameter voice coil dome tweet. 16mm is the full wave length of a 20,625 Hz wave form. Or more importantly, half a 10,312.5 Hz wave form. So if you sit at an odd axis, the sound from the edge furthest away could be out of phase with the edge closest to you. The result is partial cancellation. In practice this is a bit simplistic, but most seem to have ignored the facts, otherwise I am very sure the devilish things would not be so popular.

If you want good imaging, speakers SHOULD be directional, only then can they convey the original imaging data encoded in the original stereophonic signal.

Take the LS3/5a and Spica TC50s. Both speakers go out of their way to mask the diffraction and create a controlled dispersion pattern that at least tries to be in phase with itself. Both speakers have a good reputation as far as imaging is concerned. One has to wonder how much better these loudspeakers could have been if the designers had used a cone tweeter instead _ presuming that any of the "big boys" in the loudspeaker chassis OEM market made any decent ones.

The Voigt Legacy

Back to the thread, there is one speaker that is not hard to construct, is efficient, has output past 20 Khz, with no bloody treble ting or bass boom. It is based on the work initially of one gentleman, Mr. P. G. A. H. Voigt.

Back in the 1920s Voigt worked on the design of the moving coil loudspeaker, while unknown to him at the time, Rice & Kellogg were working on a very similar design, both using a medium sized 6 1/2 inch cone. R & K beat Voigt to the patent, thus denying Paul Voigt whatever fame and fortune was involved.

At this time, Voigt was working for a small English company, J.E. Hough, Ltd. (which later became Edison Bell), manufacturing radios and records. It was here that Voigt patented an early condenser microphone and all electric recording cutting system, instead of the purely acoustic method.

Voigt then left this outfit and set up his own company, Voigts Patents Ltd, after Edison Bell crashed in the Great Slump of 1933. It was now that he began to produce the famous Voigt Corner Horn, which had a mains energised (200 volt, 42 watt) field coil, a single six inch cone, front mounted tractrix horn for the mids and highs and a quarter wave "bass chamber".

A few years later, Voigt patented the "twin cone" cone. If the guy had earned a penny for every twin cone loudspeaker produced after his patent lapsed, he would easily have been a millionaire several times over. This was added to the Corner Horn, with another clever idea _ a double wound voice coil. This must be a pain in the ass to make. The only way I can see it being done is to wind the first coil onto a "forming" rod, wrap the voice coil card over it and then wrap the second coil over the whole lot, presumably adding lots of glue as you go. Incidentally, the voice coil is wound with aluminum wire.

The cone itself is folded from a flat piece of trimmed-to-shape paper, the whole thing held together by glues, lacquers and varnish. Operation of the Voigt Voice Coil spider is very impressive. The cone was terminated in a soft roll surround.

At the same time in the 1930s, another small company was prospering with its "high quality" wireless and gramophone equipment. This company was called "Lowther", run by a Mr. Peter Lowther. The two names were eventually tied up in a marriage that has lasted until the present day.

I often wondered what was the point in making "high quality" equipment, when you played your records with a pick up that lacked any finesse, and was lucky to play anything past 5 kHz. It would have been lucky if the records contained information much higher than this anyway. No FM remember, only AM. Why did Voigt make such joy that his speaker had good output past 13,000? What was the point?

It is easy to forget that AM radio is capable of a very high quality of sound reproduction. These days, with stations limited to an 8 kHz bandwidth and crammed on your radio's dial like sardines in a tin, AM is not even lo-fi , it's no-fi.

Sixty years ago the waves were not as crowded. A radio station could take up as much bandwidth as it was capable of. Microphones of quality were around, as were electronics capable of decent bandwidth. Imagine our illustrious radio listener circa 1935. He could tune into one of the BBC's live concerts, free from any kind of processing (practically none existed), straight to the transmitter, and onto an empty radio band, virtually free from interference.

In the home, genuine "radiophiles" would have nothing other than a "straight set" (or TRF, if you prefer). These receivers had massive bandwidth giving good frequency response, and no superheterodyne whistle to hide. Sensitivity was down to the amount of aerial you had up. The most complicated straight sets consist of two stages of R.F. amplification and "diode" to remove the sound wave from the RF carrier wave. Amplification via a couple of PX4 triodes, push pull class A, all driving the Corner Horn. I can imagine the whole thing being quite awesome.

I once built (sort of as a joke) a wide bandwidth AM tuner (TRF of course) and matching transmitter. I was amazed at how good the medium actually was. Perhaps we all took to FM a little too early. At any rate, the early quest for great radio sound was where what later came to be known as Hi-Fi has its roots.

Lowther through the Hi-Fi Years

Voigt left England soon after the end of WWII hostilities, bad health meant he had to go to the warmer climate of the North Americas. His designs were left in the capable hands of Lowther, and particularly Mr. Donald Chave, although Mr. Mordant, later of Mordant Loudspeakers, also played a very important role.

The original Voigt Corner Horn was carried back into production virtually unchanged after the war. Voigt and Chave, working independently, introduced the first permanent magnet drive units in the late 1940s.

Chave's PM-1 unit brought about a small improvement in gap flux, and dispensed with the need for 200 volts DC. The PM2 followed, and around the same time Voigt's original spider disappeared, to be replaced with a more modern type. This switch allowed Chave to fit a "phase plug" in the centre of the voice coil, to prevent the very high frequencies from cancelling themselves out in the apex of the cone, thereby extending the frequency response from the previous 13,000 to past 18,000 Hz.

The PM3 was the next drive unit to be offered. This was a custom drive unit for the TP1 enclosure, which offered a front mounted Tractrix horn on the front of the units and an exponential horn for the bass. As with the early corner horns, the TP1 must be used in a corner.

An update of the Voigt Corner Horn, the PW2 followed. It was fitted with the new "baby" PM6 unit, which despite its magnet's physical size, had an in-gap flux higher than the PM1 it effectively replaced. The PW2 was not as good as the original Voight Corner Horn but it was far cheaper.

Later drive units are the PM2 mkII (Lowther's biggest magnet), PM2 mkIII (essentially a mkI with a different mounting bracket), PM4 (a PM2mkII with cobalt pole pieces) and last of the line, the PM7, which despite its smaller size I think is a better unit than the bigger PM2, and much easier to use.

All these units share the same basic cone/ chassis. PM2s and the PM4 have a magnet support frame, the original chassis cannot support the full weight of the magnet. The PM3 looks like it is mounted in a saucepan!

There are two options on phase plugs, one is a small beastie that is used in cabinets where the front of the unit is not coupled to a horn, the larger item is for a unit coupled via a horn.

Lowther's mainstay cabinet range during the 1960s was called the Acousta line. Originally in two forms, an odd looking corner thing, that fired the unit into the room's corner, and a much better model, with the unit and mouth of the horn pointing at you. Most Lowther cabinets were now fitted with constant width rear horn, the exceptions being TP1, Audiovector and the L.I.B. (Lowther Ideal Baffle, which used a "drone cone").

Chave constantly updated the designs. The small Corner Acousta became the Dual Position Acousta, and the standard Acousta was updated to suit the PM6 and PM7 units.

Throughout the 1970s countless different variations on the Acousta theme appeared. Mini Acoustas, Super Acoustas, Twin Acoustas _ you get the picture. Cabinet material changed from ply to MDF. In my humble opinion, this was a BIG mistake. Chave should have listened to Voigt's then thirty year old ideas.

Lowther Today

Later in the early '80s, Lowther were the last to switch over to ceramic magnets from the alnico/ticonal they had previously used, although thankfully they still manufacture the original superior units. The current owners of Lowther are convinced ceramics are much better than the alnicos. You can guess what I think.

The current line includes several models using the "BICOR" principle, wherein two different sized horns are fitted to one cabinet. I make no bones about not liking one bit of these designs, although others swear by them.

In recent times, the design of the basket has been improved to some extent. Over the years, there were four generations of Lowther-Voigt drive units: early mains energized electromagnetic units, the mainstay of the '50s through the early '70s one piece alloy cast chassis, to the latest "black" chassis. During the '70s some production was fitted with a plastic basket.

The 1964 Acoustas

This model will work happily with PM6A, PM7A, or if you are on a tight budget, the PM6C. It is a pretty self explanatory blueprint, an' I am no great woodworker, but I would offer these few tips:

Always build a folded horn from the best grade of ply you can afford. NEVER use MDF. When assembling the cabinet make sure you sand all of the internal joints so they are s-m-o-o-t-h, this is especially important near the "neck" of the horn. If you have rough patches, the bass horn loses its efficiency. Worse still, if it is rough near the neck, soundwaves reflect back through the cone and give the sound a "dirty" complexion.

PM6 ceramics have three problems not associated with their Alnico brothers. First the large ceramic puck reflects sound from the rear of the cone back towards it, the alnico version has carefully shaped magnets to avoid this problem. Unfortunately, it would be impossible to duplicate it in ceramic.

Due to this magnet shape, to use the ceramic speaker you must make a "gasket" of 1/4" ply, because there ain't no way that big ceramic baby is gonna fit in the li'l magnet box given in the 1964 plans.

Final problem with the PM6C is more serious and a touch bizarre. For some strange and unexplained reason, Lowther fit a cone to the ceramics which is embossed with an annular pattern. I call this cone the "ribbed for extra sensation", I suppose it is whatever takes your fancy. I can see what they are trying to do, they are trying to stop "bell mode" cone breakup. Problem is, it does not help the other cone breakup mode, the "concentric" mode. The originals have a much better idea. "Diagonal" patterns are deeply embossed into the paper cone. This helps stop both "concentric" and "bell" modes of cone breakup from occurring.

At the end of the day I must conclude that unless you are totally and utterly strapped for cash, go for a pair of PM6 or PM7 alnicos. 'Tis the best, no doubt.

Sound-wise all three magnet options kick ass. Even the "ribbed for extra sensation" PM6C sounds good. I use a pair of carefully made Acousta fitted with PM7A drivers.

They are totally compelling. The stronger the magnet, the more high frequencies you get. The '6C has a pleasant, almost "soft" sound compared to the more expensive Alnico versions. If you are having hum problems with your setup, the '6C might be the one to go for due to its lower (97 dB) sensitivity. Overall, the '6C is very easy on the ear.

By way of improvement, the Alnico PM6A gives an extra 3 dB sensitivity and a "cleaner" presentation. With the Cs you are aware you are listening to Hi-Fi, but the '6As just disappear, leaving only the music behind. The music seems to come from nowhere.

The PM7s go all the way. It is amazing how much extra detail these babies can pull out of the signal. The PM7 is even higher sensitivity (102 dB/1W), cleaner, sharper, and more focused. If these drive units were cameras, the 6C would be a Leica miniature, the 6A a Bronica, and the 7A without doubt a Hasselblad.

If you have carefully constructed the cabinet, you should have very little coloration, and by the time you have "tuned in" (this process usually takes five minutes) you will not notice any.

At first audition, the Acoustas will seem odd. The bass will appear thin, the treble will lack sparkle, but the sound will grab your attention. All of a sudden, a real bass note will come along- BANG you are blown out of your chair by a pair of 6" woofers. A cymbal is brushed and then tapped, at last you can hear the difference. No more phasey ting or engineered bass bloom, everything very natural.

I guarantee you have not experienced anything as dynamic and true to life as this speaker, regardless of your choice of drive units. A 1 watt per channel amp will make your ears bleed (unless you are in a very large room, and then you may have to insist on at least a 2A3 SE).

The Acoustas are so "correct" you will find everything else "odd". If you try to switch back to your TING-BOOM speakers, you now will hear real coloration, and you won't like it.

Be careful to sit on axis with the Acoustas, off axis the sound takes on a woody coloration. The HF is very directional. Sometimes moving your head an inch can cause the highs to do a disappearing act.

My fave Lowther design is the classic TP-1. This speaker is the best. There is a school of thought in audio that if something is butt-ugly, it must work well. The TP-1 is the most odd looking piece of woodwork I have ever clapped eyes on. Whoever decided to fit this thing with Regency style Queen Anne legs must have had a sense of humor. The damn thing looks alive in the corner of the room, almost like some strange robot from a B movie.

But the sound . . . the front horn gives them more presence and less "spotlight" treble than the smaller Acoustas. The bass is just awesome. Even heavy rock fans will love the TP-1.

Given the choice of either a pair of Tannoy 15" silvers/Lancaster cabinets or Quad ESLs against the Acoustas or TP-1s, Lowther wins hands down. It is far more natural, more lifelike, more "you are there", less from a box, less from a pile o' tubes, capacitors and resistors.

If sourcing drive units second hand, be careful. Lowther were one of the first to go to foam surrounds, which have usually rotted by now. Lowther-Voigt in the UK will repair them, but at a high price. NEVER use a Lowther with any transistor amp that comes on with a thud! Any amount of DC through the coil will see the inner voice coil fall off the former, with disastrous and expensive results.

The gap between pole pieces in the Lowther drivers is very small compared with most designs. This helps to saturate the voice coil in magnetism. The PM4A is the only speaker made that has a totally saturated voice coil gap. 24,000 gauss! Nothing comes close.

Even a lowly PM6C has a good magnet to cone mass ratio. It has to in order to get the high frequencies. If a PM4 were a car, it would have a power to weight ratio of 1,000,000 BHP per ton!

It is interesting to note that mechanically the units cross over twice. Very high frequencies come directly off the edge of the inner voice coil (hence the need for a phase plug), the next frequencies down are handled by the "whizzer" cone, and finally the 6" cone.

Nothing is ever perfect, on a swept frequency response the Lowthers do themselves no favours (especially the PM6A, which has a curious peak at 11 kHz, adding a bit of false presence). But on a pulse test, they are in the electrostatic class.

If you overload them they damage very quickly, a small SE or small PP amp is de rigeur. When forced to handle 15 watts, you can make the voice coil fry. Just remember that 15 watts in a Lowther is in excess of 111dB! That is painful. Most owners will NEVER use any more than 1,000 milliwatts of power.

Prices of Lowther drive units vary from L250 for PM6C up to L900 for a pair of PM4A. Second-hand prices vary wildly from country to country_just watch that foam on vintage units.

TP1 and Audiovector cabinets fetch very high prices, justifiably so. In my estimation, Lowther can offer no new complete loudspeaker to match these underrated classics.

I get the feeling that the factory considers the old top of the line cabinets somewhat old fashioned and dumb, but they sound as good today as they ever did. The TP-1 is certainly not an amateur woodworking project but if you have the skill and the plans, go for it. If you happen to have the plans for the original TP-1, pleeeze send me a copy care of SP, so I can get myself a pair.

Perhaps price has something to do with it. In 1962, a Tannoy 15" dual concentric cost L37 10s, while a Lowther PM4 cost L48! The price of TP-1A was just under L200 a pair. For what you're getting for your money, I consider the present selling price of Lowther drivers to be far too cheap. Buy yours now before they realize and shove the price up!