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 outragous $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 thats hi-fi, but if you're
still the audio tweek you were in 1961, the results of this out of date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 ecclectic 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 programable 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 marketeers have innundated 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 comsumer 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, challlenging 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 recommemdations 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 arround $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 40 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 accomodate
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 surfaceof
the midrange driver, simply line all facing surfaces (those that have a view of
the midrange driver) with Sonex or similar sound difusing, 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 advanage the "esoteric" cables have
over ordinary speaker cables is that they are usually a heavier gauge, beyond
that there is no measureable (or it would have been published) difference. Cut
your cables 50% longer than you think you'll need for the minmum 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 compresson drivers are connected to the low- frequency output of each
3105, and the 2404 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 TWEEKING
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 tweek 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 measurment capabilities of the
TEF measurement system are probably the best way to ensure proper system setup
and the machine's time-energy and phase measurment 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 belive 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 wavelenths 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
closly 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.