THE ANCIENT AUDIOPHILE'S QUEST FOR THE
ULTIMATE HOME SYSTEM
Do you long for the old days --do you fondly remember the
JBL "wallbanger" sound? Knocking that hideous art-deco kitchen clock off the
wall with Mercury's Antal Dorati recording of the 1812 and evoking ooh's and
ahh's from dumbstruck friends who couldn't believe their ears on hearing
your massive 35 watts per channel and the sound of Bob Prescott's "Cartoons
in Stereo"?
In 1961, those of us who could capriciously defy our
wives or parents and spend $355.80 plus the outrageous $10-$12 cost of high
grade plywood lumber to build our own 14 cubic-foot cabinets, lived in bliss
with the reverently held belief that a pair of D130's and 075 bullets was as
good as a speaker system ever needed to be, that recorded music could never
challenge such a system, and that some day if we ever got a huge tax refund
we might think about adding a pair of 175DLH's to make the ultimate system.
We were the audio elite--the cognoscente who held court for those who
thought we were geniuses because we could plug together a Mac 60 and a
preamp and actually set the correct disc equalization for any one of the
many individual record company disc cutting EQ's used back then--to the
chagrin of non-engineer music lovers.
If your like me, a child of the fifties, chances are your
memory of those early high-efficiency systems nags at you and makes you
wonder what in the world all the fuss about "digital-ready" speaker systems
is all about. Yes, JBL was digital-ready 45 years before digital was ready!
Of course, you can still get an E130 and a 2402 (the
current model numbers of the old components) and fool yourself into thinking
that's hi-fi, but if you're still the audio tweak you were in 1961, the
results of this out of date thinking will prove uninspiring--the sound you
remember won't be good enough anymore. The sound you get won't live up to
the memories of it in view of what you've probably heard over the last 45
years.
Well wallbanger fans, since 1961, some progress has been
made in understanding the listening experience, the reasons sound systems
never sound like live performances, and how to improve that enigmatic
situation . We know how all the hardware works now, and we know a lot more
about why there are so many ways to make bad speaker systems. As Danish
philosopher would probably have said, "audio is like philosophy; at every
step it sloughs off an old skin into which creep its useless hangers-on". If
you want stay on the leading edge of your favorite technology, you have to
have an eclectic view of past design improvements. never mind the fact that
today's" recording engineers" have green hair and can't read music and that
most of what goes into the bites of pop cds emanates from a programmable
box. To be fair, there are plenty of well recorded cds available to listen
to, and every reason to expect that good program material will be made by
those who care about music and audio quality more than mass marketing.
Although JBL's sales of raw components to the home-grown
market has been continually expanding since the fifties, JBL as a company,
has done so well in professional audio (permanently installed sound systems,
touring sound systems, movie theater sound and musical instrument speakers
and components) over the past decades, and the hi fi marketers have
inundated the public with so many ready-made speaker choices, that the
proportion of JBL's total sales of raw components to the hi fi market has
been overshadowed to the point where supporting that market segment is now
unprofitable. JBL loves its loyal fans, but the time it takes to answer the
thousands of questions from them offsets the resulting sales. As a result of
this and the fact that almost all the consumer calls received by JBL
Professional are now inquiries into building ultimate systems (despite our
efforts to send you to Harman America and sell you the spectacular 250 Ti
top of the line consumer loudspeaker system) , I have decided to answer all
your questions in writing in hopes you won't call and nag me.
This then is what I personally think I would do if I had
a lot of money to spend on my home system and could not countenance the
occasional doctor with his super-expensive Krell/Rowland/Cello system,
challenging for bragging rights.
HOW GOOD CAN IT GET WITH PRO
COMPONENTS ?
A sizeable number of dissatisfied audiophiles, bass
freaks and a lot of JBL hi fi old-timers have called and written insisting
on my recommendations for larger home stereo playback systems that might
deliver all the gut-thumping reality of curling up in fetal position inside
a rock-n-roll kick drum. Although the merits of self-inflicted hearing
destruction escape me, I offer what I consider a useful alternative (for
those so inclined) to hiring a live band and a tour sound company when the
urge for auditory self-abuse arises.
The "dream system" described here won't peel the paint
from your walls or suffice as a P. A. system for rooms larger than a typical
vertical assembly building, but it should satisfy the auditory cravings of
even highly altered punkers, disco-droids, rappers and the most masochistic
metal-rock fans, while still providing adequate subtlety for delicate
baroque chamber music, your annual Hogwood Brandenburg, and those
ubiquitously popular insect sound-effects records.
THE SYSTEM CONSISTS OF THE FOLLOWING:
[4] 2245H 18" subwoofer drivers
[2] 2220H 15" midbass drivers
[2] 2123H 10" midrange drivers
[2] 2445J 4" compression drivers
[2] 2382 Flat-Front Bi-Radial horns
[2] 2405 Diffraction tweeters
[1] 6290 power amplifier
[4] 6260 power amplifiers
[2] 6230 power amplifiers
[2] 525 active crossovers
[2] 3105 passive crossovers
The total system cost for these components only, is
around $18,000 so dust off the old mortgage papers and gas up the Rolls for
the trip to the bank for a second on your house.
The amplifier complement listed will, on demand, deliver
1200 watts to the four 18" woofers, 1200 watts to the two 15" mid-bass
drivers, 1200 watts to the two 10" midrange drivers and 600 watts to the two
compression drivers and tweeter units. the total on-demand power is a clean
4200 watts. This may also place demands on you -- from your neighbors and
local police.
Now before you gasp and expectorate "yech ! horns ?" be
aware that everything you've heard is history and most of it was wrong. The
model 2382 is a two-inch throat, 120 degree waveguide type device with a
rapid flare rate and virtually non-existent "horn sound" which is due to
horn throat non-linearities associated with smaller one-inch throats and
exponential flare rates and it won't be used in this system to reproduce low
enough frequencies to be obnoxious anyway.
Remember the JBL catalog copy of the sixties: "Steep
wavefronts of explosive loudness are taken in stride by the powerful magnet
and 4" diaphragm". Keep an open mind if you expect to be rewarded with high
sound pressure levels. Its one of nature's immutable laws that you must make
at least some concessions to get certain benefits (like extremely high sound
levels). Sorry, but you can't change the laws of physics with money. Your
buddy's 7-foot tall Acoustats are going to sound like a $4 transistor radio
next to your system, so stop biting your nails and write the check.
ENCLOSURES
You will need to build or otherwise acquire (don't call
us, we can't help) cabinetry that will provide an internal volume of 20
cubic feet for each pair of 18" bass drivers, a separate enclosure of 1.5
cubic feet for each 15" midbass drivers, a sub-enclosure or separate
enclosure of .3 cubic feet for the 10" midrange drivers and mounting
surfaces for the horns and tweeters. The whole affair (one left or right
member of the pair) will probably be between 48 and 60 inches tall, about 5
feet wide, about 3 feet deep and will weigh a lot.
Build the low-bass enclosures out of something stiff like
6-inch poured concrete cast around woofer mounting rings made from 14 ply
Finland birch plywood, or just use the plywood and two-by-four bracing glued
and screwed down on- edge anywhere where you can detect any panel resonance
when pounding on the panel with your 2-pound framing hammer. The goal here
is to make the finished cabinets as rigid as concrete or at least as rigid
as possible. Keep in mind that the system will sound better if you build the
whole thing into flush-mounting soffits in the wall, so you better have a
long lease or own the house you intend to modify.
The exact interior box dimensions for the subwoofer
enclosures are 41 X 33.5 X 29 inches. The 29 X 41 side is used for mounting
the woofers. the ducted vent consists of two boards, 9.25 X 29 inches
installed in the center between the two woofers. This slotted vent tunnel
serves both to tune the enclosure and brace the side panels. The open area
of the vent and tunnel is 4.5 X 29 inches (the width of the box), with a
total depth of 10 inches.
Line the box interior on all sides with a single layer of
1-inch thick, half-pound density fiberglass for internal reflection damping.
There is no benefit, and in fact, there could be deterioration in
performance if lots of fiberglass is used. Fiberglass adds virtual volume to
an enclosure. Wear a mask and gloves when you staple the stuff around the
bracing (unless you have put the bracing on the outside of the box) or onto
the panel interiors and then take a cold shower when you finish. Maybe you
won't itch and cough for a week.
A word of warning for animal lovers: if you have a cat,
you should use a screen of chicken wire on the inside end of the ducts in
the woofer enclosures to prevent curious felines from losing any of their
nine lives when the cannon from the 1812 overture awakens them from their
cozy nap inside the box.
For the midbass driver, you have to create a very solid,
resonance-free enclosure to set on top of the bass enclosure. Once flushed
into the wall, it won't matter if your boxes don't match width and depth
dimensions. The exact interior dimensions of the midbass enclosure are 18.9
X 15.4 X 13.4 inches and you need a vent consisting of a 2-inch X 5 inch
slot, cut in the 3/4-inch material of the baffle, somewhere near the edge of
the midbass driver. As with the woofer enclosure, apply a layer of
fiberglass to the interior walls of the box.
The midrange driver is housed in a separate sealed
enclosure whose inside dimensions are 10.7 X 8.7 X 7.6 inches. This
enclosure too, should be lined with the same fiberglass padding, with an
extra layer against the back of the box. It is best to build the enclosure
onto a large flat baffle to accommodate mounting the driver since it is 6
tenths of an inch larger than the inside width dimension of it's ideal
enclosure and some relief routing will be needed to furnish a good mounting
and ensure a good air seal.
The horns can be mounted on 3/4-inch baffles made of the
same lumber, and don't require sides or boxes, just the front baffle and
some way to support it is enough. If you're a golden-eared audio wizard, you
may wish to "align" the timing of the acoustical signal arrivals at your
listening position; to do this, all you need to do is move the tweeter back
over the flat-front horn to a point where the backs of the two magnet
assemblies are lined up vertically, and move that whole assembly forward to
within 3 inches of the position of the magnet assembly of the midrange
driver. If you do this and baffles or horn walls end up shadowing the
mounting surface of the midrange driver, simply line all facing surfaces
(those that have a view of the midrange driver) with Sonex or similar sound
diffusing, irregular-surfaced foam. Do the same on top of the 2382 horn so
the tweeter won't be spraying sound down onto a reflecting surface.
CONNECTING THE SYSTEM
Once you've finished the cabinets and mounted all the
drivers and horns and done all you can to assuage your family that you don't
need outpatient psychiatric help, you can hook everything up. Start by
making speaker cables out of the heaviest wire you can find--battery cable
is not too large! The only advantage the "esoteric" cables have over
ordinary speaker cables is that they are usually a heavier gauge, beyond
that there is no measurable (or it would have been published) difference.
Cut your cables 50% longer than you think you'll need for the minimum run,
but be careful to locate the power amps close to the speakers so there is no
extra cable length. Carefully label all your cables (VLF, LF, MF, HF) for
left and right and mark polarity if necessary so you won't get confused, and
to be helpful, you should be able to feel any markers in the dark or around
in back of the amp rack if you're working in confined spaces. The amp rack
(crossovers and power amplifiers) should be wired according to logical
engineering practice, crossing signal and speaker wiring at right angles and
isolating any chassis grounds as necessary to prevent ground loops and hum.
It should be possible to assemble and wire your amp rack so there is no hum,
just some hiss (associated with high-sensitivity loudspeakers) from the
amplifiers when their gain controls are wide open.
The 525 crossovers should be set to divide the subwoofer
(VLF) and midbass (LF) drivers at 100 Hz. the 6290 power amplifier, in turn,
is connected to the two pairs of 18" drivers wired in parallel to each
channel, and the two 6260's are switched to bridged mono mode and each drive
one of the midbass drivers. The MF outputs of the 525's feed one each 6260,
set to bridged mono mode, which are connected in turn, to the midrange
drivers. The dividing frequency for the LF-MF drivers should be set to 500
Hz. The HF outputs of the 525's feed the remaining pair of (bridged) 6230
power amplifiers which in turn feed one of the 3105 passive crossovers. The
dividing frequency for the MF-HF section should be set to 1200 Hz.
The 2445J compression drivers are connected to the low-
frequency output of each 3105, and the 2405 tweeters are each connected to
the high-frequency outputs of the 3105 crossovers.
Hook up the 15" midbass drivers in reverse polarity from
the 18" drivers. Hook up the midrange drivers in reverse polarity to the
midbass drivers (the same polarity as the 18" drivers). The horns and
tweeters, through the 3105's, should be connected according to the red-black
instructions on the 3105 crossover instruction sheet and wired so the input
to the 3105 (red terminal) is reversed polarity from the midrange driver,
unless you have physically aligned the horn and tweeter forward over the
midrange driver, in which case you will have to flip the polarity of the
3105's input. (NOTE: This one item may require some fudging and adjustment
including polarity experimentation, to achieve the best group delay
characteristics).
TUNING AND TWEAKING
After you're finished putting everything together and
flushing it all professionally into your living room wall, you will need to
get 1/3 octave spectrum analyzer or an audio engineer who has one, and set
everything up properly by adjusting gain controls and the like. If you live
in a metropolitan area, you might find someone with a TEF machine who is
curious enough to measure and tweak a system the likes of which he has
almost certainly never seen. I recommend that you don't try to play any
music through the system until some measurement and adjustment can be done,
so that you will have no chance to suffer buyer's remorse when, because the
system is not properly adjusted, it doesn't sound right. If you've spent
this much money, you owe it to yourself to finish the job properly.
The best procedure for setting correct gain between all
the amplifiers is by the use of sharply defined, octave-wide bands of pink
noise. If octave band filters are not available, use the rule of the thumb
that the subwoofers are the least sensitive portion of the system, so they
have to be used as the level reference for the other components, in other
words turn them up all the way, then turn up the midbass, midrange, and
horns, in that order, until the levels sound like they match. The frequency
response measurement capabilities of the TEF measurement system are probably
the best way to ensure proper system setup and the machine's time-energy and
phase measurement capability make it easy to properly physically align the
components along the listener's Z axis, forward or back.
THEORY OF OPERATION
My philosophy on speaker system design is in accord with
JBL's. Simply stated, the acoustic power output of the speaker system in a
diffuse, reverberant field, should be as flat as possible. Individual driver
elements should be smaller than the wavelengths they are asked to propagate.
I also feel that none of the system elements should be stressed during
operation at typical listening levels. For the latter and I believe most
important reason, I have chosen midbass and midrange drivers that are the
most efficient available in order to start out with the advantage of
operating nominally below 1 percent of rated power capability. You should
find, when listening to this system, that there is an effortless,
bigger-than-life sonic quality that makes for a very detailed and revealing
reproduction of the input signal. This is due in large part to the high
sensitivity of the system components. Although there is every reason to want
a single small driver to reproduce the entire audio spectrum, we know from
direct experience that small drivers can't handle enough power to produce
sufficient acoustic output. The cone of a 4-inch speaker would have to be
able to move back and forth 4 feet to move as much air as the subwoofers in
this system are capable of moving. In addition, the wider the frequency
range one driver has to cover, the more it is subject to Doppler
distortions; non-harmonic and non-musical irritating sounds caused by the
modulation of higher frequency sounds caused by large diaphragm movements
associated with simultaneous low-frequency reproduction. The answer for
Doppler distortion and power handling capacity is to divide the audio
frequency spectrum into bands, each of which represent a small portion of
the total required power and each of which require only successively smaller
drivers to propagate the successively smaller wavelengths those frequency
bands require.
The essence of the system's performance is its ability to
track transients, which, in well recorded musical software, will have peak
levels 20 to 30 decibels higher than the average power used to play at
reasonable listening levels.
Lower efficiency speakers suffer heating of their voice
coils and subsequent output compression, from high-power inputs. My thinking
is that for a loudspeaker to faithfully reproduce incoming signals, it must
at each moment in time, act as though the signals are the first stimulus
received; that is, it is impossible for a loudspeaker to be accurate if the
signals just reproduced alter the loudspeaker's electrical or mechanical
characteristics, by for example heating the voice coils or stretching the
active materials that make up the loudspeaker's moving parts. In the case of
electrostatic speakers, losses occur as the result of finite power and
motion capability. Electrostatic speakers also suffer from extremely low
efficiency. The solution is to keep input power levels nominally low so
heating is minimized, and to do this it's necessary to use high-efficiency
drivers as system elements. the disadvantage of high efficiency drivers is
that they cover narrower frequency bands as their efficiency increases.
Conversely, wide-bandwidth drivers (the JBL LE8 is an example) always
exhibit low efficiency--a direct manifestation of physical laws.
You may wonder why it's necessary to provide a bridged
600-watt amplifier for a driver that will be operated nominally at a watt. A
20 decibel musical transient peak requires 100 times the power required by
the average signal and a 30 decibel peak requires 1000 the power required by
the average signal. The 600-watt output capability of the amplifier driving
the midbass units represents just a bit less than 28 decibels above 1 watt
of power reserve for the tracking of transients. If you are an electrostatic
or Bi-polar speaker fan, you will loathe the sound of this system until you
get used to it, after that you will loathe the electrostatic and bi-polar
types. An analogy of the perceived effect is that this type of system
(high-efficiency type) is like removing an electronic compressor from an
otherwise good speaker system.
There is bound to be "time-smearing" or "image-smearing"
from any sound source that is not a simple point in space, but by aligning
the system elements in a straight vertical line (except the subwoofer
drivers), horizontal time and image smearing is eliminated. Humans don't
perceive vertical time and image smearing unless they jump up and down in
front of the speaker system--a practice I don't recommend for critical
listening (divides your attention). Since JBL's individual component
loudspeakers are matched very closely as a matter of manufacturing practice,
the stereo imaging of the system is spectacular.
DISCLAIMER: A SERIOUS WORD OF
WARNING
The system described here is easily capable of producing
sound pressure levels far in excess of that which will cause irreversible
hearing loss--don't take this lightly. You might suffer not only permanent
hearing loss, but also constant ringing in the ears that can cause insomnia
and lead to nervous disorders or emotional problems. JBL and this writer
make no claims and take no responsibility for the design, operation or
consequences of using the system described here.
© 1988 Drew Daniels