Custom Bodied JBL 4430 Studio Monitors
After about a decade, I finally finished my speakers. These custom units
are based on the JBL 4430 studio monitors of the mid eighties. Technically,
they have a 15" woofer, and a 1" compression driver into a "bi-radial" horn.
Sonically, they have deep, flat bass and a smooth high-end with great dispersion
for a well-defined image. And, accrding to my wife, they merge sound generation
with symbols of fertility. They make for an interesting 3-d Rorshach test.
When I started the project, I had the compression drivers and a pair
of JBL E-140s. The JBL horns were out of my budget, so I made a pair out
of alder. To get the dimensions, I got a copy of the original JBL patents. I also had
access to a real pair of 4430s at my workplace, and I spent hours measuring
those. To my surprise, the contour of the actual JBL horns doesn't exactly
match the mathematical equation given in the patent. Being an engineer, I copied practice, rather
I cut my original pile of 3/4" alder into shape with my skill saw and
a scroll saw. This was based on a stack of templates I made, based on mathematics
and dead reckoning. I made each segment flat and square with my trusty Stanley
block plane. I then carved the slices closer to the real contours with a
3/4" Stanley wood chissel. Alder is reasonably soft and has a straight, closed
end grain, which helped ease the task. I couldn't imagine doing this in hard
oak. After the rough carving was completed, I glued the pieces together with
carpenters glue. I used straps and wedges to press things together. These days,
now that I have more money and tools, I would used a planer, a jointer and
One thing I learned was that it's easier to sand the edges than the center,
and that the hard, concave angles are really hard to smooth. The result
is that my horns flare a bit faster than the real JBL bi-radial horns. That
probably reduces the dispersion at lower frequencies, but they sound great
to my ears.
After I first assembled the horns, I had lots more carving and sanding
to do. Once I got them to their basic shape (but still pretty rough) I made
throat adapters, hooked them up, and set them on my woofer boxes. They were
already a great improvement over my old elliptical horn and louvers (JBL
2307 and 2308). I think I had them set up like that for at least a year.
Next came the cabinets. I did them in 3/4" MDF. The design requirements
I used 2x2s for edge bracing and dowels for cross bracing. The angles
were a challenge, but I was able to cut things close enough with my skill
saw, and then I used my block plane to get the fit to be tight. After assembly,
I painted the cabinets black, temporarily mounted the horns and drilled a
hole to snake the speaker wires through. I used the cabinets in this state
- 5.0 cu. ft., taking components and bracing into account.
- 25 sq. in. port with a depth of 8.5". This tunes to about
- Tower design - the HF driver is high for sofa sitting, low for
standing, but just right on an upright chair.
- Narrow front. This was governed by the 15" woofer.
- Non-parallel sides. This reduces resonances. Only the top and
bottom are parallel, and the top gets pretty small.
- Get the woofer as close to the horn as is comfortable to minimize phase errors.
One year ago at the end of Spring I got the bug to complete the project.
The goal was to "skin" the cabinets in alder. I wanted smooth contours,
so I didn't go with veneer. I cut the alder into strips, jointed them to
make panels, and planed the panels to 9/16" with a surface planer. I then
cut them to fit and glued them to the MDF box with Gorilla Glue. I'm hoping
that the flexibility of the alder and the glue will keep the alder from
splitting over time. Oh well. The alder wouldn't be "dead" enough, the MDF
was ugly, and veneer wouldn't allow for complex curves. I skinned the left
cabinet one Summer, and the other one the next. Each box has about 110 individual
strips of alder. I'd hate to know how many hours of labor I've invested.
Keep in mind that I had E-140 woofers rather than the 2235Hs that I wanted.
The E-140s were fine back when I was using the drivers for sound re-inforcement,
but they didn't have the bass I wanted for my home studio / general listening
application. I had been watching eBay, but the $500+ prices per pair were
putting me off. I finally found a pair for $300 from Cherokee Studios, and
bought those - only it turns out that they were really 2231H drivers reconed
to 2235Hs. Checking the Lansing Heritage
site, I found that this upgrade really makes a 2235H, so I bought them.
But the bass was lacking. It turns our that Cherokee left off the mass control
rings by design. That made these 2234Hs, which are more efficient, but don't
drive down as low. I already had an EQ for the E-140s, so I redesigned the
circuit to boost the 2234Hs, which worked pretty well.
Checking again with Lansing Heritage, I learned that the mass control
rings were available individually, and that they could be added without
a full recone. I got the parts through Robert G. (Thanks for both the parts
and the info Robert!) With a steady hand I cut out the old dust caps with
an Xacto knife, used mastic to glue in the MCRs and glued on new dust caps.
I painted the mastic with black model glue and, voila!, I had two 2235Hs.
The story of the woofers wasn't quite complete. The 2231H baskets were
battleship gray and had mastic stains from previous rework. I wet sanded
the edge, and masked, primered and painted the visible parts of the speakers.
From the front they now look great.
In the middle of all of this I made more accurate throat adapters for
the horns, and installed a nice terminal housing from some stock aluminum,
and some plastic wall connector plates from Radio Shack. Each plate houses
four gold seven-way connectors. The plastic just acts as an insulator, and
keeps the interior hex lugs from spinning.
The final touches included a week of hand oiling and waxing the cabinets
with Watco natural oil and wax. I also needed to re-paint the exposed MDF.
The Gorilla Glue had made a mess of the previous black paint job. The biggest
hassle was masking the alder.
On September 23rd, 2002 - the Autumnal Equinox - I did the final assmbly,
and carted the beasts from the workshop into the studio. For the first time,
I got to hear real 2235Hs playing with the 2426J drivers through finely
finished throats and horns. I set my Behringer Super X-Pro crossover, Haffler
P3000 amp and Samson Servo 120 amp by dead reckoning, hooked the CD player
into the Mackie 1402-VLZ Pro Mixer and powered it all up. The result? It
sounds awesome. I didn't have to touch a knob - except to crank the levels
up! The bass is deep and controlled. It can be cranking away and stop on
a dime. Very nice. The highs are smooth and clean. The bi-radials don't have
the resonances and peaks that I sense from other horn designs. Frankly, the
2307s sound like conch shells to me.
So, what's next? I need to redo the grilles. I bought some JBL blue grille
cloth, but I don't like the result with the alder. It makes the wood look
too yellow. I need to pony up for some JBL black cloth. I'd also like to
upgrade my amps. A Bryston 4B for the lows and a 3B for the highs sounds about
right. My DVD player supports DVD-Audio, which is 24-bits per sample at 192kHz
in stereo. I need to pick up a disk and try it out.