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The Vacuum
Tube Logic Book
David Manley
VTL TITBES AVAILABLE FROM:
VTL CALIFORNIA
4774Murrietta St.
Chino, CA 91710
(714) 627-7204 FAx (714) 627-6988
OOO SMALL AND LARGE QUANTITIES AVA'LABLE § O O
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The Vacuum
Tube Logic Book
Second Edition
Paul Mazzarelli, ed.
David Manley
DEDICATION
This littie book is dedicated to all those who love to hear live music, but
who also choose to strive to re-create an illusionary resemblance to it in their
homes. These people wiÌl have the wisdom to understand the difference between
the original and the illusion. The wisest of these disceming audiophiles will listen
to their music through vacuum tube amplifiers, and they shall stand apart among
the rowdy rabble.
David Manley, California. 1989.
-
vtlmaster. inx
TABLE OF CONTENTS
Foreword
Quotes vii
Chapter l: A Little Night History . I
Chapter 2: Measurements and The Sound of VTL 1
Reviews: Double Blind & Triple Dutch 9
Chapter 3: The Human Ear l2
Loudspeakers . l5
Cone-type Moving-Coil or'Dynamic' Drivers 15
Ihe Dome Tweeter t7
Hom and Pure Ribbon Tweeters l8
Quasi-Ribbon Speakers t9
Electrostatic Loudspeakers 20
Room Acoustics 22
Power and Noise 25
Bi-Amping and Tri-Amping . .29
Chapter 4: Cables and Interconnects . 3l
Balanced and Symmetrical Wiring .34
Floating Symmetricai Modc . 36
Custom or Special-Purpose Designs . 38
Overseas/Foreign Importers and Distributors 39
Dealers in Audio Equipment 39
Tweeks 4l
Chapter 5: Preampliflers in General
Cartridges, Loading and RIAA
Preservation of Phase integrity 47
Chapter 6: Tube Power Amplifiers 50
Negative Bias: Why and How . 53
Setting the Bias in a VTL Amplif,er . 56
Output Transformers and Feedback 57
The Ultra-Linear Aspect 6l
The Life and Availability of Tubes 62
Tubes We Use, and Some We Don't 64
Chapter 7: The VTL Manual . 67
Power Requirements . 68
General OperationalNotes . 69
Other Ancilliary Equipment 10
The VTL Range 12
The Maximal 12
The deluxe 72
The Super de Luxe . 74
The Ultimate Preamplifier 76
The Manley Reference Preamplifiers 79
The Stereo Amplifiers 8l
The Compact Monoblocks 8l
The deluxe Monoblocks 83
The 500 Watt and 1000 Watt Monoblocks 86
The Ichiban Class A Triode Monoblocks 81
General Reliability of High End Equipment 90
Warranty 92
Analogue and Digital: Big Discs and Little Discs 94
In Closing 99
PowerlùIains Transformer Color Codes A
Output Transformer Terminations C
Schematic: deluxe Preamplifier E
Schematic: Preamp Power Supplies F
Schematic: Minimal Preamplifler G
Schematic: Inverse RIAA & Crossover H
Schematic: Ultimate Phono-Stage I
Schematic: Ultimate Line-Stage J
Schematic and Notes: Amp Input, Driver, Bias Mixer K
Schemaitc: Amplifler, Typical Output Section M
Schematic: Amplifier, Typical Power Supply Section N
Fourier Analysis: 100 Watt Compact o
Transient Response: 100 Watt Compact P
Spectral Response: 100 Watt Compact a
Sound Level and Musical Frequencies R
Decibel Table S
Oscillograms and Setup Diagram T
Graph and Amplifier Classification U
Standard RIAA Equalization; Resistance
Ohm's Law and Power Formule w
12AT7 Tube Characteristics x
12AU7 Tube Characteristics Y
I 2AX7WA Tube Characteristics Z
l2BHl Tube Characteristics AA
12827 Tube Characteristics BB
6DJ8, and 5965 Tube Characteristics CC
ECC807 and 5965 Tube Characteristics DD
1027 A Tube Characteristics EE
7t168 Tube Characteristics F'F
68Q5,EL84 Tube Characteristics GG
6L6GC Tube Characteristics HH
6B4G 12 A3 Tube Characteristics II
3008 Tube Characteristics JJ
211 Tube Characteristics KK
807 Tube Characteristics LL
845 Tube Characteristics MM
6CA7 /8L34 Tube Characteristics NN
6550 Tube Characteristics oo
KT88 Tube Characteristics PP
Tube Substitution List aa
Addenda
Reprints of Reviews
Journal Subscriptions
FOREWORD
This book is mainly about using the well-proven vacuum tube in today's High
End audiophile systems. Unashamedly, we declare our preference for the tube
over its solid-state counterpart. With genuine pride (though others may deem it to
be downright immodesty) we sincerely believe that VTL builds the best vacuum
tube equipment on the market today. This book clearly spells out these feelings,
though we have tried to counterbalance our pride (or immodesty) with other
meaningful information on tube technology; most of this is very hard to come by
in modem bookstores. However, lest anyone presume or assume that the main
reason for publishing this book is to promote and sell more tube equipment, may
we please disabuse them straightaway of that premise. For it is a stated policy of
ours that we do not try to make 'converts'; neither at the level of the layman nor
the dealer. We love supplying people who know that they want tube equipment
and why; we do not like supplying those who are uncertain, or who may have
been subjected to sales-pressure. Few seem to know about tubes today, and we
feel that that is a pity; for history teaches more than is obviously apparent.
We have endeavoured to keep the book clear and understandable to those
who do not have a technical background, without boring those who do. Only
time will tell us whether we succeeded.
QUOTATIONS
Reverent and lrreverent
"All amplifiers sound the same." PETER WALKER
-
"Amplifìers do sound different." MARTTN COLLOMS
-
"My amplifier sounds like straight wire." DAVID HAFLEFì
-
"But not like MY wire." GEORGE CARDAS
-
"l can make my amplifìer sound like your (any) amplifier." ROBERT CARVER
-
"Maybe but definitely not like zy
amplifier."
- DAVTD MANLEY
-
"My Dad is a better designer than your Dad." LUKE MANLEY
-
"'Views' which cannot be disproved are often perpetuated by the industry and
Press as facts." EDWARD JORDAN
-
"I lìnd the concept of peak current useful in the case of transistor amplifiers."
MARTIN COLLOMS
-
"Wipe itM lt sounds boxy." - ERNEST FLETSCHMANN
"God is in the details." MIES vd ROHE
-
"If you can hear it, I can measure it." ANON. SCIENT|ST
-
"The audio industry as a whole must leam to rely more on the evidence of
experienced ears and less on expensive lab equipment." MARTIN COLLOfilS
-
"Who stole the bass?" ANTHONY CORDESMAN
-
"No one stole the bass." MARTTN COLLOMS
-
"It wasn't there to be stolen." DAVID MANLEY
-
"Kindly desist from pouring molten lead down my back." ANON. PLUMBER
-
"The dream of every musician who loves his art is to involve listeners every-
where." AARON COPLAND
-
"Sound must come Iìrst and specification second."
- MARTIN COLLOMS
"The virtues of vacuum tubes have a lot to do with music." WILLIAM CONRAD
-
"It is sad to say that the hi-fi industry has become a cult in itself, and very
few of its devotees have even taken the trouble to listen to live, non-electric,
non-amplified music." EDWARD JORDAN
-
"I think everybody now agrees that solid-state gear was execrable when it first
appeared [but] recently it has begun to sound almost as good as tube equipment,
but at far higher cost." JAMES BOYK
-
.,Successful amplifier designers talk more of an intuitive feel for the subject rather
than a reliance on a cast-iron set of rules." - MARTIN COLLOMS
"85o/o of our annual output of over four million tubes is sold...for military equip-
ment like rocket guidance and mobile communications." GENE COLTRIN: Plant
-
Supervisor, PhiliPs-ECG
"Fourlayer Reusen-Shielding (in cables carrying less than 500 mV) is required
to isolate electrical interference into the one mega-cycle range; frequency re-
sponse should easily extend to half that." - Standard Studio Cable Requirements:
EMT Wilhelm Franz GMBH, circa 1956
,.But are there some important parameters which are not being measured?"
-
RUSSELL O. HAMM
,.My immitative [solid-state] circuits sound very sweet and warm, with many of
the virtues of tube circuits, though they still don't have the extreme dynamic
range of tubes. I'm amazed by how we continue to hear sonic effects that can't
be scientifically measured in the circuits." - KEIrH JOHNSON
"With few exceptions, the power supplies inside professional amplifiers look like
one big afterthought." BEN DUNoAN
-
"No statislical difference could be found." - AES Spokesman
"That which does not appear to exist is to be regarded as if it did not exist."
Jurisprudence art. 3530
-
"The subject of amplifier damping and its effect on the performance of a loud-
speaker is a controversial one, like pentodes versus triodes... There is now doubt
that many loudspeakers sound better as the damping-factor is increased, because
of peaks in the reproduction, which in some instances rise from 10 to 12 dB
because of impedance variation. . ." Howard M. Tremaine, D.Sc'
"Tracing distortion is the non-linear distortion introduced in the reproduction of
mechanical recording because the curve traced by the motion of the reproducing
stylus is not an exact replica of the modulated groove... (etched by the cutter-
head)." oliver Read, D.Sc.
1
A Little
Night History
HIS IS THE MOST DIFFICULT CIIAPTER TO WRITE: THOUGH CERTAINLY NOT TIIE PUR_
pose of this book, there is quite a lot of 'transistor-bashing' elsewhere
in these pages. It's been thirty years and more down the road and, sadly
to say, solid-state has still not delivered fully in high end audio. More
sadly, the transistor wormed its way into the record-making chain too early in
its developmental process, and there are twenty years' worth of compromised
recordings to prove it. Most sadly of all, so effective was the mass-media sell-in
that it actually constituted a propagandistic aural re-education almost of brain-
washing proportions, literally an "emperor's clothes" story: the public's hearing
was influenced to change. "The new sound is the right sound; the o/d sound was
the wrong soundl"
In case it started to happen when you were too young or you were old
enough but didn't pay too much heed to the flim-flam of the indoctrination tales,
i
we'll mention some (together with our irreverent comments):
:
"These new babies are small like a cigarbox !" [We've never seen
- cigarbox.l
a forklift being used to lift a
"These new babies run coooll" [Yet they need a noisy fan?]
"These new babies have no parts-availability problems; one part fits
alll" [The transistor manuals and cross-refs are bigger than the New
York phone directories.l
"These new babies don't have any nasty plug-in tubes!" [No; every
one has to be soldered in by a technician f he can get the parts and
L
i/ the repair won't cost more than the unit is worth.l
2 I.HE VTL BOOK
"These new babies don't cost so muchl', [pshawl Sixteen Grand ain't
all that much...l
Humour (black) aside, let us agree right off that in the non-audiophile sector,
the price-point promise had the most validity; it still does in rerms of value-for-
money for average consumer music systems. For $300 or so, one can get a
"rack-system" at discount stores which offer manv features: stereo FM, record-
player(?), cassette-deck, 30/40 watt amplifler(?) two loudspeakers plus a whole
bunch of chipboard and glass doors. . . Music it is not; value it is, for average
requirements.
Understandably, the manufacturers loved the low-cost and ease of building
the transistor offered, particularly in countries where a low-priced labour force
was eager to work. So the number of home systems proliferated, requiring even
more records to be pressed, more recordings to be made in more and more
new recording studios with an ever-growing list of artists and repertoire. But
many of these artists were not exactly talents of the Sinatra/Segovia caliber, and
often required days and weeks in the studio with expensive backing musicians.
Monetary efflciency (who said 'greed'?) saw a potential saving in multi-track,
multi-dub, multi-moneyed recording rechnology. The g, 12, 16, 24, 32 and 4g_
track master tape-recorders were neecled to hold an equivalent number of record
amplifiers, playback amplifiers and bias oscillators; ditto the recording mixing
consoles, which had to house crazy configurations like ,,64 IN/4g OUT,,.
Try feeding a tubed Neumann capacitor-microphone placed five feet from
a good tenor straight in without a pad to one of these hunclred-grand solid-state
gems and you'll clip the first transistor to many percent of horrific distortion.
These monsters had to contain hundreds of pre-amps and summing amplifiers.
were they built with tubes, they'are the consoles that stretch twelve feet in width, requiring 2 or 3 operators or (so
much the better for equipment salesmenl) complicated computer-controlled mix-
down consoles requiring more limiring, norz de-essing and compression, norz
graphic etlualization, more delay.channels, ruore noise-suppression. . .but never
more music.
A greater number of master-discs had to be cut at greater speed; master-
tapes had to be dubbed to second and third generation ("what azimuth problem?
That's the master we have from France, and that's the master we'll cutl") multiple
lathes running in tandem driven by rough-sounding solid,state amplifiers, often
operaled by night-shilr part-rimers.
And then came a breakthrough with real potential, though it hasn't always
appeared so (except to the accountants!): Direct-To-Metal disc-mastering offered
to OMIT one whole stage in the cutting-to-pressing process the siÌvering of
-
A LITTLE NIGHT HISTORY
the Iacquer to make an interim 'positive'. (ln any positive to negativc to posilive-
again chain, it is very desirable to leave out a whole generation.)
There have been a few pretty good DMM releases, and there have been
many more that were not 'audiophile-quality' such as can be achieved via the
best lacquers that have remained intact through processing. Then why hasn't
DMM always been 'audiophile perfect'? Well, there are two reasons, both of
which we're addressing.
First, the angle of the cut has to be around 50 degrees, which causes an
oscillation or 'scream' (silent!) at 75180 KHz. This is eliminated (or rather
masked) by a clever 'Black Box' equaliser: we're doing a better one in the
cleverer part of the circuit - the cutter-head's feed-back winding.
Second, the 500 Watt solid-state amplifier it needs that much power
(more actually)
-
to hammer that stylus into the copper... so we've designed a
1000 Watt Manley - amplifier especially for this - not that any audiophile who
also wants 1000 Watts times two J'or stereo will be denied! However, the DMM
lathes in use elsewhere are still driven by the original 500 Watt, crude-sounding
solid-state amplifiers that come supplied with the lathe. . .
And one could easily observe the swing-over from quality to quantity in
the record industry: when the long-playing record was introduced, it was aimed
at people who cared enough to want quality music in their homes, cared enough
to have an investment in the necessary equipment, cared enough to care for the
records. At that time and up to even the mid-seventies, it was standard practice
for record-company 'brass' to have a pretty damn good sound system in their
offìces and listening rooms; maybe not the quality-level of Inflnity Beta's driven
by our top-ranking amplifiers, but surely JBL's driven by Macs in the US, and
Tannoys driven by Leaks in the UK.
And then it started to shift toward the numbers: "I'm not interested in how
it sounds on a five-grand system I'm interested in how it sounds on a flfty-buck
-
car radio." Sooo, in addition to the main monitors in control-rooms, little tiny
4- or 5-inch speakers (mini-monitors) started to appear as the cross-reference.
Then they became the main reference. "Will ya stop worrying, kid? If it sounds
okay on these lil' babies it'll sound okay on anything!"
Long before it hit this all-time low, round about the entry of the multitrack
tape machines, you can readily understand that the love of making the original
recordings lost its joy for some of us. Prior to this period, we were going down
from live-orchestra to two-track (or three or four) tubed tape-recorders, some
with a mechanical transport-system approaching Swiss watchmaking standards
that would make your eyes glisten just looking at them. Often, the tape recorder
was straight-wired from tubed AKG or Neumann capacitor microphones (still
regarded indisputedly as the best and worth ten and twenty times their original
THE VTL BOOK
cost) and suspended/positioned in a BlumÌein or Decca ..tree" variant. or, in the
case of a line-level-only tape machine, the mikes would go via a good 20 to
40 dB tubed preamplifier (ust like ours yes, the circuits were pioneered and
-
proved thuswise) or through a minimal mixer with maybe 6 dB of cut and lift
using gold switches (yeah, we'd heard about gold then too) and, at EMI, with the
most carefully-designed and monitored phase measuring and (passive) correction
circuitry.
In this period of recording, you would have a set-up where the conductor
and/or solo artistes would go to their side of the glass and do their utmost thing,
comfortable in the knowledge that the engineer (often with a producer, though
many times without) would be doing his utmost thing with the largest measure of
good taste available. And when it was "thumbs up", it was committed to glorious
two-track living stereo. After any editing and tidying up, the master-tape would
go to the cutting-room for some trial lacquers to be cut through tube amplifiers
running Class A or A1, and often of powers around only 50 to 100 watts. yes,
we pioneered our amplifier circuits in this mode too. More often than not, the
first lacquers or trial pressings were carefully auditioned in the ,.brass,, offices,
comments noted and the lacquers re-cut with a different pitch or (miniscule) Ee
until they were right really right.
-
Lest you think we're rambling on here about the old days on the Ark,
the answer is "no, my friencl; we're talking about the records standardly being
made in the sixties and even seventies, the ones that you admire most, the ones
you're paying fifty, one hundred and more bucks for pristine copies of." The
point being made here (obliquely, we must admit) is the sad tale that it,s the
audiophile whom the purveyors of recorded music initially address and try to
win over with messages of promised quality, and it is the same audiophile whose
desires for quality matter not one bloody jot when the magic-numbers pointer
starts to swing.
If you're a nostalgia fan with a penchant for the bitter-sweet, let us go
on (with a rueful smile!) to tell that some of us moved a little further away
from pure audio toward the camera-side of motion-pictures, where the stan-
dard drill was breath-taking 35 mm Eastman color Film exposed through Zeiss
or Taylor-Hobson-cooke lenses mounted in pin-registered cameras. And then
came. . .Videotapel Transistorised pictures! Aaarrrggghhht
After passing through a barren desert of depression (not being prepared
to shoot videotape except with a three-eight police Special smack through its
multi-lined glass-facia) we pulled ourselves together and said: "Hang on, mate;
something about quality has been learned in all this and surely there must be other
souls who appreciate quality and won't settle for less than real quality.,, And so
the idea came of putting some of the best circuits and design knowledge from the
A LITTLE NIGHT HISTORY
good recording period into amplifiers and preamplifìers destined for the playback
of music in the homes of quality-conscious music-lovers. Vacuum Tube Logic
was born.
Prototypes were brought to the USA in 198 I to see if anybody cared enough
to order some. Regrettably, charlatans and dishonest souls (of the code "R"
variety) exist in every field where glamour-with-profit is thought to lurk, and we
met one who promised all, one who listened to 'cello sonatas all through the
night, one who was visibly shaken to hear that a thirty watt amplifier could make
a wonderful sound on Dahlquist speakers (not known for hyper-efficiency) one
who ordered a whole raft of goodies and then went ppfftt and underground.
However, we'd survived transistorised sounds and pictures without falling
totally apart, so we did another "pull yourself togethel boy" sequence, put the
ampli{ìers into small but professional production in Britain (where they enjoyed
humble but appreciative results) and marketed them initially in Germany with
great success. (They also care for quality over there, and have proven that with
their motor-cars, to say nothing of Messrs. Bach, Beethoven, Brahms, Mahler,
Mozart, Mendelsohn, Schubert, Schumann, Wagner; Incorporated).
Then in 1985 at the June CES show we gave it another sl.tot in Chicago,
and (not wanting to have another dreadful experience) we elected to distribute
the line ourselves in the capable hands of family member (son of tubes) Luke
St. John Manley. lt was obvious that Americans knew quality and that they
wanted quality; taught ùs that the stuff has to be built extra-strong to withstand
rigorous trans-continental shipping; and made us feel welcome. That was when
we knew that VTL equipment had to be built (indeed, re-born) in the US of A.
First we assembled some models from British and American components
and then we opened our full-production factory in California's "lnland Empire"
two years later with all VTL models on full stream, we launched the Manley
-
range, Manley being our family name (we have continuously used it for custom
professional studio equipment of all types).
Why a second brand at all? Was it really necessary? Yes. We believe VTL
equipment is the finest value for money anywhere, at any pricepoint. We want it
so; we want the customer to pay'x'and feel he got a'2x'value, which is why
we have not asked a price increase and don't plan one.
With VTL units we pour the lion's share of the production cost into the son-
ically audible components; very little is indulged for sheer cosmetic appearance.
(No secret, eh? You have seen joumalists' comments ". . .we're tatking serious
ugly here!") The fact is we /ite the minimalist, functional look of "open" tube
amplifiers: form follows function, no?
So then, are we autocrats, dictators of public taste? No. We were also a
little tired of certain people asserting that we didn't know how to design cosmetics.
THE VTL BOOK
So, for those folk who insist on visual as well as aural beauty, who prefer the
"encased look" and who wish to pay for premium cosmetics, we proudly presented
the Manley range; only we didn't confine the improvements to the visual.
We added electronic embellishments as well: extra power supplies, extra
power, extra facilities
sight. our Manley 350 - watt still with the are still less than two-thirds theout of
but
monoblocks
value-for-money theme never
price
of our competition and sport milled panels and stainless steel finishes. please
pardon the commercial here, but we are proud of our insistence upon value-for-
money and realise that we are somewhat old-fashioned in this view!
Another aspect of the Manley range is that we are supplying more and more
equipment for studio and mastering applications. This gives us great satisfaction:
to get the "good sound" in at the beginning of the whole story the recording
itself. -
Industrial studio usage has another valuable side that won't escape the
reader: as motor-racing experience beneflts street cars in terms of innovation
and toughness, so studio application of our regular production equipment proves
out and helps us monitor reliability under the most stringent and demanding loads.
We have 300-, 350-, and 500-Watt units (VTL and Manley) working rwelve and
sixteen hours per day, every day, at full tilt with nary a problem hardly a tube
change. Don't you agree that's good fbr us, and especially good - for you from a
rel iability standpoinr?
Measurements and The Sound of VTL Equipment
First and foremost, we at VTL are engineers, thoroughly trained in audio
electronics. Our designer has spent a lifetime in the recordin-q industry on three
continents; the circuitry employed in VTL equipment has been tried, proven and
improved in all positions of the recording chain. (The recording chain is almost
exactly a complex stereo system from microphone (cartridge) to cutter-head (loud-
speaker). Laboratory-bound engineers have to work and speak in the language of
measurements ('me asurespeak') but field engineers in music and recording soon
realise that life outside the lab is somewhat different. Phenomena of rnusical
instruments and acoustics seem to constantly remind us that measurements have
to be tempered and balanced with experience and circumstances.
Most dyed-in-the-wool engineers and scientists think that musicians (and
audiophiles in particular) are a bunch of crazies, yet they acknowledge that some
talenl is required is required to create music. When you get them steamed up
enough these ones will resort to "If you can hear it, I can measure it..." at whic
which
point you could say "Then I suggest you begin by measuring a Stradivarius...
In fact one could measure a Strad in terms of output, constancy of volume of
various notes and their respective harmonics; but the measurement would include
a human musician (perish the thought!) and how could you be sure that he/she
played both violins identically? With the same feeling? How does one measure
feeling? At this point the scientist will start muttering obscenities and head for
the bar. . .and you can go back to enjoying your music.
We take the view that while measurements are important vitally so
the human ear has the final vote. We use our ears in the designing
-
- of amplifiers
and our instruments to confirm what our ears tell us. There is no shortage of
equipment of all types that 'trip the light fantastic' on the bench and sound
bloody awful when a dose of music plus the sound of the human voice (speaking
and singing) is applied. And it isn't only equipment: many is the number of times
THE VTL BOOK
that architects and acoustic engineers have designed an auditorium or recording
studio that measures superbly, but when the orchestra plays, when the actor softly
speaks.. . We are often asked how and why VTL equipment sounds the way it
does (truthfully, the question is most often put complimentarily as '...sounds so
good. . .') Our answer starts "Well, tubes, of course", and then when pressed with
"but tubes sound euphonic and, you know, like round and mellow", we have to
go into some detail like explaining that the u:òe itself does not "sound mellow",
that tubes have a linear frequency response to megahertz and beyond; that 'round
and mellow' are kind words for a sloppy bottom and a decaying top.
We say that our circuit design is simplex rather than simple, complex rather
than complicated. We explain about our transformers and power supplies. Then I
we go on to subtle things like component quality and that of wire, solder and
layout. Very, very important are those matters. You may have noticed that the VTL I
products are fairly compact in size (well, for tube gear anyway) and the question
arises as to whether we think that "small is beautiful". While not averse to saving
space in general terms, the main reason is that compact layouts with shortest signal
paths (little or no wire plus short, wide and thick copper traces) do contribute
much to our sound. We also follow some old but oft forgotten rules like vertical
positioning of output tubes (fbr better heat convection) and horizontal positioning
for preamplifier tubes (for less 'shot effect' and better noise characteristics). Yet
some of these design and construction techniques do not in themselves appear in
the measurements. Or more correctly put, it is not obvious how the proximity of
a given component to its important neighbour improves the performance. Besides
the easy measurements like noise and distortion, the "speed" of VTL equipment
amazes those who previously believed that only solid-state could produce speed
and low frequency attack and ovr kind of attack. Our amplifier measurements are
on the average around 22 volts per microsecond which, when taken in isolation,
is not as quick as some of the most exotic solid-state circuitry, but then you have
to look at the whole picture: the musical picture.
In the hi-fi world there are many examples that defy measurement: bi-
wiring is one, fine interconnect cables another, the apparent loudness of tubes
yet another.One ol the most important aural phenomena we design and listen
for is that of imaging. No measurement-unit exists to quantify this or spatial
specificityflocation. Ì-lence our view that we, as audio designers, are akin to
musical instrument makers. There is an an (as distinct from a science) to it and
knowledgeable critics do not dispute this.
Where we side with the 'measure-speak' fiatemity is in the instance of a
piece of equipment which measures downright awful: we fail to be convinced that
such an item could nonctheless "sound good" unless (rernotely) perhaps it could
be placed with other anomalous equipment whose deficiencies counter-balanced
REVIEWS
some of its inherent problems. And of course, the opposite example also exists
in audio where two seemingly acceptable pieces of equipment have individual
aberrations that conflict to highlight each other's faults. So again, measurements
notwithstanding, we get to the art of selecting equipment that interfaces well and,
oh so importantly, suits the room and the persons who occupy that room.
If you are into measurements, or wish to get into measurements of ampli-
fication equipment, we olfer in the appendices some useful diagrams, such as
an inverse RIAA network, plus the set-up connections, as well as a selection of
oscillogram patterns; when fed a square-wave from a quality oscillator, the resul-
tant square-wave on the 'scope often tells as much and sometimes more than row
upon row of very high-tech numerals.
You might have noticed that we do not list specifìcation numbers on our
brochures: this is because we believe that they can mislead just as easily as they
inform. Besides our own lab findings, we have had independent measurements
taken of some of our equipment and have always been highty gratified with the
results. Interestingly (and understandably to u§) "their" measurements do not
always coincide with ours. Our own measurements of our equipment seem to err
on the conservative side, but the reason for the phrase 'understandably for us'
is, as with individual home systems, that the test-bench gear and methodology is
seldom replicable or identically calibrated. We include some ol these findings for
your interest in the appendices: the Fourier analysis of our Compact 100 Watt
amplifier is totally stunning. But we still suggest that you listen for yourself and
draw your own conclusions.
Reviews: Double Blind and Triple Dutch
A great deal of breast-beating, wailing and gnashing of teeth surrounds the whole
topic of audio equipment reviews and the reviewers published in the audio press,
and that's just from the manufacturers/designers! A large number of readers write
in to the magazines complaining that the equipment most often reviewed is either
too expensive or too cheap, that the reviewer is biased, that an impartial panel of
blindfolded folk would give a more useful opinion... On the debate as regards
cost and bias, we can only offer sympathy to the editors and publishers with the
banal comment "well, you can't please all of the people all of the time"' As to the
"blind-panel" altemative, we have to come right out and state that we favour and
believe in the objective (and, yes, subjective) review method every time: win,
lose or draw.
Magazine readers sometimes complain that Reviewer A ìoved a certain
component while Reviewer B liked it not at all, so how are they to reconcile
these differing opinions? The answer here is that both reviews help one gain a
better understanding of that component's good and bad points that may or may
10 THE VTL BOOK
not suit your particular requirement. Like a second or third opinion in law or
medicine (for which you pay dearly) it is up to you to sift the opinions of the
reviewers with those of your dealer, and temper the mix with your own listening
experience. Reviewers are human after all, and obviously will sometimes offer
conflicting opinions: some people do not like the music of Bach, while others
will tremble with joy just walking in the streets of His birthplace.
When one reads the reviews of our products, one cannot help but marvel at
the sheer time in very careful listening sessions the reviewers put in; scrutineering,
one should really term it. No casual plug-up, light-up, and say "yes, very nice.
the best you can buy for $xxxx, etc. No, a very detailed report indeed listening to
old records, new records, pop, jazz, classical orchestra and baroque. Very sincere,
intensive and conscientious work. The adage to bear in mind is "Be guided, not
decided."
Our policy has been (and will remain so) that in the main the 'name'
reviewers are a sincere group of human beings who, though not infallible, are
trying to pass on valid opinions and constructive criticism. If we think he or she
is clearly wrong. we wilì up and say so and know that the publisher will give us
the rebuttal space if our gripes are, in turn, valid and worthwhile. Some you win,
some you lose and some just drive you insane (but only a little).
Let's look at the other side of the coin: that of the "double-blind" panel
or individual listening tests with or without an 'ABX' switch-box, in which tha
panel is played'A','B', and'X'and asked whetehr'X'was in fact'A'or'B'(or
neither; lots of trick questions are posed in double-blind methodology to make
sure nobody is fooling anyone!) The ABX idea was conceived by an eminent
engineer whom we respect highly; what he seeks to porove, and has done so on
numerous occasions to the benefit of many products, is whether or not, when
there really is ro substantial difference between A and B, a panel (like a jury)
can detect minor differences. Often they can't and a point of expense-rationale,
say, is proven.
No doubt this system has great merit. However, we believe it is at its best
in what one could honestly call mass-produced, average price-level, consumer-
oriented products. We feel that true High End audio components fall into a
category where adj udication by experts should be the ruling, as it is in many
forms of endeavour: painting, sculpture, ballet, wine-tasting, gourmet cooking,
book reviews, etc., etc. The list is endless.
Consider a symphony orchestra which proposes to appoint a new principal
hom-player. Should they get the contenders to audition before a random panel?
Rather, they should play fbr their conductor and possibly one or two other guest
conductors whose opinion they value. If a concert-hall wishes to acquire a new
REVIEWS 11
grand piano, should they ask a committee, or ask some name-pianists and known-
quality critics? Please!
Some members of a major broadcasting corporation overheard an "infbrmal
discussion" that was taking place at a convention about this. Outraged at the
double-blind concept, they joined in and it became even more 'informal'! We,
or course, took their side: when a piece of key equipment is being sought by a
responsible body like a broadcasting authority or recording studio, of toarse only
the opinions and ears of known experts (within or outside) will suffice.
We must go on record as stating that in areas of true excellent e conceming
the sound of music, we prefer the opinions of recognized experts. Call them (and
us) "Golden Ears" in mocking terms if you like, but that is where it's at. . .and
that is where it has always been.
The Human Ear
The human auditory system is an extremely sensitive device, able to detect minis-
cule sound pressure levels of air vibration as low as one ten thousand millionth of
a centimetre, far outranking conventional measuring equipment. To compare the
ear's extraordinary capability with the human eye, the equivalent in energy terms
would require the eye to detect a 60 watt lightbulb at a distance of 3,000 miles
in free air-space. The lowest level of audible sound is known as the 'threshold of
hearing', such as the faintest rustle of leaves in an otherwise silent environment
(we'll call that zero dB for these comparative purposes). The highest level hu-
mans can tolerate (before death) is known as the 'threshold of pain', which occurs
at a sound pressure level of about 145 dB the sound of a large jet engine at
ten feet. The 'threshold of discomfort' lies -at about 120 dB. The ratio between
these extremes is on the order of 10,000,000,000,000 to l! Because of rhese
unwieldy numbers, information related to the auditory faculty is expressed in a
logarithmic scale quantified, in decihels abbreviated as dB [see DEcrsEr- TesI-e in
the Appendicesl.
Decibel measurements deal with the intensity or volume of a given sound,
while the pitch, or note, is indicated by the frequency. This is the repetition rate
of alternating air pressure changes in pulses per second, commonly expressed as
'cycles per second', nowadays more popularly called 'Hertz', abbreviated as Hz,
or kHz for thousands of Hertz.
The theoretical frequency response of the human ear is taken as being
20 Hz. to 20 kHz, but this varies considerably with age and sex as well as from
person to person and 'condition' (health) and 'conditioning' to environmental
experience. For example, a jungle inhabitant in his natural surroundings, whose
ears have never been bombarded with jet-engine volumes, might well be able to
clearly hear 20 Hz to 20 kHz, and possibly a little higher. Women seem to retain
their upper-frequency limits longer than men of the same age; dogs have a very
THE HUMAN EAR 13
extended top-frequency hearing ability reputed to reach as high as 35 kHz and
commonly to 25 k[z, which is the nominal frequency of those training-whistles
humans cannot hear. Very probably this is why people have observed their dogs
and cats leaving the room in distress at the sound of digital music. (Our dealers
confirm that they have not been able to make a single digital sale to any of their
canine clients; they prefer analogue but find tumtables a little tricky to handle
with paws.)
In more practical and average terms, human hearing is taken to be from
25 Hz to 15 kHz. The lowest fundamental note on a piano is 26.1667 Hz and
the highest is 4,224 Hz on the Bosendorfer Imperial Grand. However, we have
a 'feeling' of frequencies lower than 25 Hz, as experienced with extremely large
pipe-organs, and also a 'sense' of frequencies above l5 or 16 kHz, as found in the
harmonic structure of, say, a triangle. An intense pressure of about 4 pulses per
second will cause the human sphincter muscle to collapse, and was very seriously
researched in the closing years of World War II as a possible attack-and-disable
weapon; whence cometh some crude expressions we have heard in low quarters.
The 'octave' of any note is double or half the frequency (2:1 ratio) and
simple ratios exist between all the notes as in, say, the C-major scale where the
fifth, G, has a ratio of 2'.3; the fourth, F, a ratio of 3:4; the major third' E, a ratio
of 4:5; and the major sixth, A, a ratio of 3:5. The fundamental notes in music
are combined with the natural harmonics (also called overtones) of the notes as
produced by the instrument/s to give us the pattem of the sounds we listen to. In
nature there exists no pure single-frequency fundamental note; this is reserved for
the test-bench oscillatorwhich appears on the oscilloscope aS a pure sine wave,
or sinusoidal note.
The human ear can detect a harmonic presence of less than one tenth of one
percent of the strength or amplitude of the fundamental note. It is the combined
texture of the pitch, harmonic structure, timbre, attack and duration details that
provide the 'aural colours' in life and music, enabling us to easily distinguish
a single flute note from the identical note played by a clarinet. The 'timbre' is
further deflned by the natural resonance of the instrument (i.e., the propensity to
vibrate more readily at one particular frequency than another) and its 'transient'
characteristics (the speed of the instruments' natural attack and decay).
All of these factors are in tum influenced by the acoustic properties of the
space in which the sounds are being created: larger rooms tend to have longer
reverberation (echo) times than smaller rooms, depending on the reflective (or
non-reflective) properties of the walls, their angle of placement, as well as height,
surface and angle of ceiling. Too, the proportion of height to width to length
will affect room acoustics, as will the room's own resonances often referred
to as 'standing waves' in acoustic engineering. Take a drummer's
- 'rim-shot' or
14 THE VTL BOOK
a wood-block. was it played in a large hall or a small studio? struck hard or
merely close-miked and amplified? we can spot and labeì these subtle variants
instantly simply because we have tvr'o ears (rather than the single microphone
commonly used in measuring setups).
We can readily judge and keep in perspective volume, frequency, harmon-
ics, transients, resonance, distance, direction and environment; be it Church or
dead studio, concrete factory or open air. Traditional measuring equipment, con-
fused by differences in environment, would confirm for example that a seemingly
percussive sound of an approximate frequency at a stated dB level had appeared
for a given duration. No measuring equipment can quantify a stereo system's
imaging and soundstaging ability, or directional and lateral resolving detail. In
the same way that a human ear can tell a stradivarius from a hundred-dollar teach-
ing violin, the experienced ear can judge loudspeaker quality and behaviour in a
given room. However, the human ear is not totally 'linear' or 'flat', not having
the ability to perceive all frequencies at exactly the same volume or intensity.
Measurements taken in an anechoic chamber (a totally 'dead' or absorptive
room) of the loudspeaker will more specifically quantify its performance at given
frequencies, thereby showing major inherent excesses (peaks, caused by reso-
nance and other aberrations) or deficiencies (dips caused by driver and crossover
'suckout'). These read-outs, combined with the over-riding judgement of the
ear, broadly define a loudspeaker's capability. Importantly, they will reveal and
quantrty basic
quantify basic technical parameters such as the impedance variation at different
parameters
frequencies, minimum power requirement and maximum power handring ability
and the unit's overall efficiency factor.
This latter infbrmation will be expressed in decibels (for volume) at one
wan (of drive level) at one metre (the distance between the speaker under test
and the measuring microphone). one watt is chosen mainly because the tweeters
cannot withstand sustained single (sinusoidal) frequencies at more power for any
length of time: the tweeter's voice-coil would overheat and bum out. This is a
standard reference used throughout the world, and is fairly easy to understand. It
is important to remember, though, that a loudspeaker's impedance is expressed
as a 'nominal' value, which is taken at a reference standard of 1,000 Hz., and
the impedance varies greatly with frequency: very low frequencies cause the
impedance to drop to one ohm or less; high frequencies cause the impedance to
rise to perhaps double and above the nominal specifications. This phenomenon
of frequency-varying impedance has much to do with the overall interface of the
loudspeaker to the amplifier's drive-power and their combined ability to deliver
the volume required to a given room's acoustics and size. Planar (quasi-ribbon
and electrostatic) speakers do not always yield the most informative information
at I metre- I watt (because of their large diaphragm area) so sometimes different
LOUDSPEAKERS 15
volumes and distance cletails. are statecl. Electrostatic loudspeakers also exhibit
tlifferent impedance/fre quency measurements. . . often clet rt'asing in irnpedance as
frequency ri.sc.s.
Loudspeakers
Loudspeakers, their types of design, inherent character and elficiency warrant
some explanation. Any loudspeaker centers around a 'motor' of electro-magnetlc
or electrostatic principles. The motor causes the transducing diaphragm or menl-
brane to move the air, thereby creating sound-waves, at a certain distance for a
given amount of excitation voltage (the music via the amplifìer).
Now, if this motor operation were to be of the regular electric rotating
variety, we'd be talking of the motor's eflìciency, predicated by its design and
build, in terms ol actual horse-power produced fiom the amount of electrical
energy required. One horsepower is related to 750 watts of electricity, so if the
motor under test only producecl a half-horsepower from 7-50 watts, it would be
deemecl lobe 50o/c cffìcient: not the best, but more effìcient than any loudspeaker.
We don't use percentage to relate to loudspeakcr effìciency (except discountl)
preferring inslead to quantily the sound level produced by onc watt at one meter.
Hence the statement ". ..such and such loudspeaker's effìciency (or sensitivity)
was measured to be 84 dB/l W/i m. .." Let us examine some of the various
technologies employed in the basic 'motors' or drive-units themselvcs.
Gone-type Moving-Coil or'Dynamic' Drivers
These are by far the longest in service (and therefore in development). This is the
most commonly used system for reasons ol effìciency against thc manufacturing
cost. While seeming to be a very simple magnetic device, the cone driver is a
complex design which involves electrical, mechanical and acoustical disciplines.
In essence, a coil (the 'voice-coil') is wound around a cylinder or tube attached
to the inner diameter of the cone. The outer diameter of the cone is attached by
a flexible suspension to a mounting-frame or'basket', as is the throat assembly
with its attached voice-coil, So as to enable it to move forward and backward
in a piston-like motion. Inside the voice-coil is a magnetic pole-piece which is
attached to the heart of the motor, the main magnet that surrounds the assembly'
The connection of electrical energy to the coil causes the cone to move back and
forth in the magnetic field, thereby moving air and 'transducing' the electrical
energy to audible acoustic excitation. Here is where the efflciency factor depends
upon:
16 THE VTL BOOK
a) The strength, size and quality of the magnet (expressed in Gauss-units of
flux-density, weight and the composition of the magnet). When the magner is
very large in strength and size, as is not uncommonly found in drivers capable
of handling high power, there is an interaction of 'magnetic-braking' known as
'hysteresis effect': this dampens the cone movement, and thereby retards "speed"
and curtails "highs'l
b) The gap-size between pole-piece and voice-coil.
c) The 'cold' resistance of the coil-winding, dependent on length, its thickness
and type of material (copper, silver, aluminium) plus its shape (round, square,
rectangular etc.)
d) The size and mass of the cone.
e) The cone's composition and flexure characteri stics. Cones have been made
from paper, acrylics and mylars, treated cloth, aluminium and recently such 'won-
der' materials like "Kevlar" a woven and chemically-treated fabric.
-
f) The cone's mechanical resistance to being pushed in and out together with. ..
g) Its recovery time.
h) Its 'air-load' both against its own cabinet and the driven acoustic.
i) Its own resonance or'Q' factor, coupled with that of the enclosure. ('Q"
of a resonance is broadly defined by its magnitude; in order to produce a distinct
note, the value of 'Q' must be greater than 1 a wineglass may have a 'Q' of up
to 100 and a loudspeaker enclosure should be - complerely dead with a negligible
'Q'factor. This is why they are made from very dense composition-board rather
than raw timber planking which has a higher 'Q' or, say, sheet-metal with an
unusably higher 'Q' yet.
j) The total combination of all these variables culminate in the overall 'damp-
ing-factor' a number which is inversely proportional to 'Q'.
k) And then the unit's intended frequency coverage, which will in tum dictate
its impedance and how it varies. The design of the essential enclosure obviously
plays an important part; were the bare driver to be connected to an amplifier,
there would be virtually no low-end response nor any power-handling capability;
this is primarily caused by the front-to-back cancellation effect of the bass notes.
Merely fitting the drive unit to the centre of a large flat board about twenty rimes
the drivers' size, called an "lnfinite Baffle", starts to correct cancellation, but
does not do much for balancing resonances nor give the driver optimum loading
to produce good bass.
The first problern of a cone driver's fiequency response occurs with its size and
type: big, heavy and well-damped designs yield good bass with high power-
handling but poor mids and treble. Light and fast attributcs offer the micl ancl top
but cannot produce any bass with power. Hence the commonly adopted dual or
LOUDSPEAKERS 17
triple driver system, with each driver being dedicatecl to a portion of the fiequency
spectrum.
Enter new cans ol worms: the crossover network and the arrival times of
the different f'requencies. The crossover divides and allocates the spectrum to
the various drivers and should also seek to equalise their inherently disparate
efficiencies, exacerbated by 'speed' or transient response. Phase shifi may occur
in the crossover, which will require correction in the vertical placement-line of
the drivers to realign or optimise time-arrival (possible delay) and these factors
are combined with the electro-acoustical properties of the enclosure itself. The
enclosure can be dcsigned according to many possible concepts and can assume
various proportions and sizes, all affecting overall perfbrmance. A smoothly
integrated three-way crossover system is a rare beast indecd, requiring hefiy
calculations and much ad justment by ear. Richard Vanderstecn builds some of
the best.
The Dome Tweeter
This is the most commonly employed unit designed to handle the treble frequen-
cies. It is similar to the cone driver in conccpt, the difference being that a little
dome or half-sphcre ol about three-quarters to one inch diameter is attached 1o
the mouth of the voicc-coit cylinder. The dome-shapc is chosen for its smooth
dispersal of high frequency inlormation and the material ol the dome varies from
compressed paper to doped cloth fabric to fine metals such as aluminium, beryl-
lium and titanium. The difÌèrent materials impart their own sonic characters and
'speed' or rise-time. Dome-tweeters do not need the size of magnet commonly
found on larger drivers, since they do not handle such low frequencies or as much
power.
A small-cone or larger-dome variety of driver (which is a vcry small bass
unit or an oversized tweeter, depending on how you look at it) is often used as
the mid-frequency driver. This was lovingly (and dead accurately) dubbed the
"squawker" by older American audiophiles, who also coined the name "woofer"
for the bass driver. Just one of the problems encountered in the design of a three-
way unit to include a mid-range squawker is that the human ear is not, as already
mentioned, all that linear; it is around the frequencies between 1000 and 4000 Hz
that the ear peaks in sensitivity by up to seven or eight dB, thereby highlighting
the squawker's potential to squawk. Purists (and we certainly choose to be among
them) preferring instead the two-way system with a 'more seamless' transition
from the bass unit to the tweeter. Of course, a single full-frequency driver would
overcome all crossover vicissitudes and would seem to be the ideal solution. Yet
like many ideal concepts, the problem of sheer physics stands defiantly in the way.
18 THE VTL BOOK
An older design comes to mind from the Lowther factory in Britain. Be-
cause it is light and fast enough for the upper frequencies, its power handling
is limited. Being of only six inches in diameteron its own, it requires a pretty
complex labyrinth enclosure to produce good bass; but smooth and homogenous?
Very. Efficient? And then some: about 96 dBs worth, so that a very convincing
sound indeed can be achieved with a sanely-operated 25 watt amplifier of high
quality. We enjoy very pleasin-e results with three Lowthers handling almost the
whole frequency range, augmenting them at 12 kHz with a super-tweeter on top
and under-pinning the bass with the superb Hartley 24-inch woofer. We routinely
supply these to mastering and mix-down rooms for hyper-accurate semi-nearfield
monitoring. [See Manley brochure.]
While on matters British and some of their clever and eflìcient designs, it is
perhaps worth mentioning the cone-driver designs of that acoustic titan, Edward
Jordan. Jordan probably knows more about cone drivers than any other living
person, having devoted a lifetime to their design and manufacture. He believes
in aluminium concs, both treble and bass in a two way system, for their lightness
(speed) and properties of flexurc, among other factors. Jordan's loudspeakers have
to be heard rather than described: we would go as far as to say they are, with
some German Kevlar drivers we have heard, the leacling edge of cone-technology
available today.
Horn and Pure Ribbon Tweeters
These are two of the other options available to loudspeaker designers using two or
more drivers. Horn tweeters (and mid units) are esscnlially tiny diaphragm units
not unlike that of a dynamic microphone, and is enclosed in a magnetic field of
considerable proportions under unvented or pressure conditions. This diaphragm
is positioneci in the throat of a trumpet-hom (to give it even more efficiency)
and is sometimes utilised with a re-entrant or labyrinth design to enable it to go
lower and louder. Homs are not popular in today's audio world as the result their
tendency to 'shout' at the listener; it is not fbr nothing that hom systems are
used at baseball diamonds and railway stations! For all that, when a horn tweeter
is well designed, constructed and smoothly crossed-over with an excellent bass
system, some very pleasing results can be achieved.
Some of the older American twelve- and 1ìftcen-inch cone drivers employed
centre-mounted pressure horns (Lansing, JBL, Altec) and were efficient to the
tune of 100 dB. They still find much favour in thc Japanese marketplace, ofien
being driven by Class A amplifiers of 4 to [ì watts of power. We fìnd this
very interesting, and conclude that the hom's inherent clarity plus the fact that
traditional Japanese music does not have vast low-frequency content combines in
a way they like. Not to mention the famous American acoustician, Paul Klipsch
LOUDSPEAKERS 19
(and his well-loved Klipschorn) would be both rude and unfair. You hardly ever
see Klipschorns written about in the press, and we think that this is a pity. The
units are in abundant production in three or four sizes up to the 'killer' unit with
its absolutely excellent labyrinth enclosure for the bass; it produces a quality of
bass that is hardly equalled, let alone beaten. Too, it should be stressed that
the bass-labyrinth enclosure couples very favourabty with the roorn acoustics,
whereas the fairly common "sealed-box" enclosure battles, to a large extent, the
natural acoustics.
We need to make a distinction between 'pure' ribbons and 'quasi-ribbon'
units, as we shall explain. Pure ribbons only exist as tweeters in the sense of a
crenellated foil ribbon hanging in space, suspended at top and bottom, within the
cheeks of a powerful magnet. In essence they are a near-replica in shape and size
of the classic ribbon microphone (such as the RCA 44B. Stanley Kelly designed
some tweeters of this type for Decca London, and in more recent times the
justly-famed Magnepan Corporation has offered a winner of a long-dimensioned
(40 inches or so) pure ribbon tweeter which goes far lower than Mr. Kelly's
designs and are, because of their length, easy to drive. Kelly's ribbon itself was
only two and a half-inches long, easily four inches or so if you stretched and
destroyed the accordion-like crenellated fotds. This meant that thc DC resistance
of the foil was akin to a paper-clip (1.e., about one-fìfih or near-zcro ohms)
and could not be driven at all without a transformer. Subject transformer had a
secondary of negligible impedance to face the ribbon being wound from about
ten inches of rectangular or flattened copper section of about 6 or 8 gauge. The
primary was wound to eight or ten ohms nominal to face the crossover. A
daunting design and execution, but nonetheless one capable of very seductive
high frequencies.
Quasi-Ribbon Speakers
These units are comprised in the main of a section of foil attached with adhesive
to a diaphragm or planar membrane with a bar-like magnet-structure being placed
in front or in rear (or both) close up to thc diaphragm. These units are usually
full-range. Because of (again) the resistance or impedance of the foil being on
the low side (one, two or three ohms) along with the magnets being inherently
less-powerful, they are often what is termed'hard to drive'and classed as being
somewhat less than efficient: from around 70 to 80 dB. The leading brand offering
this style of technology is Apogee, and they certainly have their fbllowing. We
hasten to point out that when Apogees appeared on the market place we took
cognizance of their