JBL2425 + JBL2342

JBL2342 Bi-Radial Horn

One of the iconic JBLs, the JBL4425 made it's appearance sometime in the mid-80s.

The 4425 is a fairly compact 2-way Studio Monitor that was popular as main speakers in small recording studios of that era. It has a 12" woofer and a 1" compression driver loaded with a distinctive Bi-Radial Horn.

This horn has always intrigued me. I've never had the opportunity to own one until lately.

The units I acquired have the JBL2425HS screwed on.

This 1" throat compression driver sports a titanium diaphragm and weighs in at 10 lb.

On paper, it's frequency range spans from 800Hz to 20kHz.

Power handling is 70W continuous program at 800Hz with a 12dB/octave slope. For 100W continuous program, the recommended frequency is 1.2kHz or higher at 18dB/octave.

Let's take a closer look at the JBL2425HS when it's mated with a JBL2342 Bi-Radial horn.



Impedance Plot

On first impression, one can easily mistake this impedance sweep for a vented box.

The two sharp peaks with a valley in between is so charateristic of a bass reflex until on closer examination, the tuning frequency is found to be at 1kHz.

A second unit was swept to ascertain that the driver is not faulty. The subsequent plot is indeed a matching twin of the first.

I can only speculate that there is either some sort of tuning within the JBL2425 at 1kHz or it's due to horn loading.



Impedance Sweep (200Hz - 20KHz)

Gated SPL Plot

The initial SPL sweep (Green plot) of the JBL2425 with the JBL2342 Bi-Radial horn was disappointing to say the least.

I was totally unprepared for the huge hump at 2K. In comparison to my JBL2345 horn, also with a JBL2425H, the cast iron 2345 horn is not only devoid of this peak but even has a much gentler and linear roll-off.

It is obvious that something must be done with the 2kHz peak if the Bi-Radial horn is to be crossed at 1.2kHz like in the JBL4425.

Having given it much though, I decided to cross at 2kHz for a start.

In doing so, I hope to avoid the 2kHz peak.

Fortunately, it's easy to obtain a flat contour with a simple 3uF+0.33mH in series.

With a working range from 2kHz-6kHz, the Bi-Radial horn can now be put through it's paces.

Later, a suitable Super Tweeter can always be inserted for frequencies above 6kHz.

It should be quite straight forward as the



Gated Frequency Response Sweep (1KHz - 20KHz)

0.33mH, in combination with the natural roll-off of the horn, results in a 12dB/octave acoustic slope at 6kHz.

What is important is to quickly establish how this horn sounds like.

This is, after all, not a budget component, so it's only natural that expectations are higher.

Considering it's cost, is it worth it?


Listening Test

With the Synergy crossover set for 2kHz (18dB/oct) and paired with the Oriole woofer, the JBL2342 is now ready for music.

From the opening tracks, I'm relieved that the typical "honk", so common in horns, is absent with the contour network in place.

What is immediately obvious is also the ease with which the JBL combo delivers the highs. I can only describe it as velvety smooth yet exhibiting an uncanny perception of depth. It's actually possible to hear what is in the background. Maybe it has to do with less harmonic distortion or the way the horn is designed.

Whatever it is, the cheaper competitors are nowhere close to this level of quality. The only driver I know that exceeds this JBL combo is the JBL2440.

Afterthoughts

The JBL4425, which the JBL2342 is a vital part of, is quite advanced in concept.

For one, the woofer and compression driver acoustic centers are physically aligned in the vertical plane. In conjunction with the even-order, allpass crossover network, the JBL4425 exhibits only 400 micro seconds of group delay from 300Hz to 1.6kHz. Amazingly, group delay from 2.5kHz to 20kHz is virtually zero.

Dispersion is outstanding. The JBL2342 Bi-Radial horn maintains a smooth coverage in the horizontal and vertical axis (100x100 degrees) from 1kHz to 12.5kHz.

In on-axis and normal off-axis listening postions, the JBL4425 is free of lobing in the horizontal plane. In the vertical plane, lobing is minimized over a +/- 10 degree window.

Quite an accomplishment, even by today's standards.

It will be most interesting to marry the JBL2432 with a newer woofer to emulate the JBL4425.

Something definitely worthwhile to ponder over. Meanwhile, as a stand alone horn, the JBL2432 Bi-Radial horn with a JBL2425 is truly in a class of it's own.

 

Mike



60 Downes Street | Calais | ME 04619 | USA