THE ANCIENT AUDIOPHILE'S QUEST FOR THE ULTIMATE JBL 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 60
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 return 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 you're 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 35 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but if you're still the audio tweek you were in 1961, the results of this out
of date thinking will prove uninspiring–the sound you remember won't be good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 any more. The sound you get won't live up to the memories of it in
view of what you've probably heard over the last 25 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 Kierkegaard
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 to 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 grooves of pop records emanates from a programmable box. To be fair,
there are plenty of well recorded compact discs 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 decade, 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 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 own home system and could not countenance the occasional doctor
with his super-expensive integrated MacIntosh 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 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-inch subwoofer drivers.
[2] 2220H 15-inch midbass drivers.
[2] 2123H 10-inch midrange drivers.
[2] 2445J 4-inch 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 $12,000 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-inch woofers, 1200 watts to the two 15-inch midbass drivers,
1200 watts to the two 10-inch 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. It's 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-inch bass drivers, a separate enclosure of 1.5 cubic feet for each
15-inch midbass driver, a sub-enclosure or separate enclosure of 0.3 cubic
foot for the 10-inch 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'd 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. The
13 inch diameter tube thus replaced, wouldn't fit on the baffle anyway, and
slots work the same as tubes–there is no difference at all in performance.
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 the 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 and you
will 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 its 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 jumper 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 measureable (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 audible 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 subwoofers (VLF) and
midbass (LF) drivers at 100 Hz. The 6290 power amplifier, in turn, is
connected to the two pairs of 18-inch 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 each 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 outputs
of each 3105, and the 2404 tweeters are each connected to the high-frequency
outputs of the 3105 crossovers.
Hook up the 15-inch midbass drivers in reverse polarity from the 18-inch
drivers. Hook up the midrange drivers in reverse polarity to the midbass
drivers (the same polarity as 18-inch 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 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 a 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 even 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 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 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 be should 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 frequency 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 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 times the power required by the average
signal. The 600-watt power 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 should be 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.