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In the previous article we went through the history of the famous JBL 431x series monitor speakers. This series of speakers share a lot of debates over the years. The supporters think they are a precise sounding piece of equipment while the objectors think they are noisy and harsh sounding. In this article I tried to use a modern speaker design point of view to investigate the pros and cons of the design of the 431x series. When these monitors speakers were first introduced in the 60s they are considered the state-of-art. There is a reason that these speakers stayed in many recording studios for so many years. But are they still capable of delivering the elements required for a recording studio 50 years? I don't have an answer for it.

 

JBL 4312e, source: The Internet

Let's start by looking at the evolution of the JBL 4312X speakers. The change from 4310 to 4311 is a simplification process on the enclosure. The designer of 4310 mount the tweeter and the mid-range driver on a same plane protruding above the bass driver by around 1cm. General speaking, this kind of design is to achieve a time propagation alignment above all three drivers but usually the bass driver protrude mode due to the depth of the diaphragm and the accompanied time delay. Some famous examples include B&W Matrix 801 and Wilson Watt/Puppy. 4310 did the alignment in a reversed way. The reason is unknown to me. But by the fact JBL decided to cancel this kind of design it probably doesn't help much in the sound.

The change from 4311 to 4312A extended the high frequency response to above 20 KHz to support the wider frequency spectrum provided by modern sound sources such as CD. Around year 2000 JBL moved the 4312 series outside the PRO line and introduced a new LSR (Linear Spatial Reference) line. The first generation of the LSR line contained 4 models: LSR-12P,LSR-25P、LSR-28P與LSR-32. The first three models with a P suffix are powered monitors while the last one, the LSR-32, shares a lot of similarity with a 4312. They both have the same number of drivers with a similar topology. The enclosure are pretty much the same size. It's a fair guess that the intention of JBL is to replace 4312 with LSR-32. The latest LSR series was introduced by JBL in year 2010. LSR-32 was replaced by LSR-6332. The major change was the high frequency driver that extends to 40kHz in the new model. Compared to the 4312, besides a re-designed enclosure, the major changes for LSR-32 and LSR-6332 series include a detachable square panel  and a much complicated 4th-order Linkwitz–Riley crossover design. The former is to ensure the high frequency driver is always mounted above the mid-range driver. The later is following the modern speaker design trend ensure a flat frequency response across the spectrum. It is a completely new design philosophy compared to its predecessor.
LSR6332, JBL current production line. Source: JBL official Web



The crossover circuit of a LSR6332. Much complicated to the crossover design of a 4312. Source: JBL official Web

Now let's take a look of the crossover design of a 4310. The model number is 3111 in JBL's document:

Crossover of a JBL 4310, source: JBL official Web

Picture of a JBL 4310 crossover, Source: Internet

If we look at this circuit closely we can probably figure out the thinking of the designer. There is no any filter connected to the bass driver. In other words, the low frequency driver works as a full-range driver. From the measurement, the bass driver, model name 123A, has a flat response all the way up to about 1kHz then start to fall off to around 5kHz and rise again and eventually fall-off rapidly after 6KHz. The connection to the mid-range driver and the tweeter was done by merely two capacitors valued at 3uF and 13.5uF respectively forming simple first-order high-pass filters. Like the connection to the bass driver, there is no low-pass-filters connected to the mid-range-driver to limit it's response. According to the measurement data from JBL, the response of this mid-range-driver, model name LE5-2, although nicely crafted, is very bumpy. The frequency response deviation can be as high as tens of dBs around 6kHz. The LE- series of drivers seem to share a similar response curve, be it the LE5-2, LE5-6 or the later LE5-10. Following the bass driver and the mid-range driver, the tweeter join the chorus starting from around 6kHz. So above 6kHz all three drivers are singing loud in medley. This is probably the major contribution to the characteristic of the sound of the 431x speakers.


During the time L100 was introduced, it looks like JBL was trying to do something in the crossover design. While keeping the low-frequency driver drive in full-range, the filters for the mid-range and the high-frequency drivers are changed to a second-order design hence two inductors were introduced. There were also two resistors added in series with the mid-range driver and the high-frequency driver. The rest of the design is the same as the crossover of a 4311.

An early L100 crossover circuit, source: JBL official Web

Photo of an early L100 crossover, two inductors can be observed. Source: Internet

According to JBL's document, the crossover design of the 431x series was based on countless listening and tuning and the result was perfect. Guess I am not smart enough to tell the differences. On the internet there are two schools when it comes to modifying the crossovers for the 431x speakers. One school insists the original design is the best while the other school proposes a stronger filtering for the individual drivers.


Reference:
http://www.jblpro.com