L26 Decade has been in the attic for some time
and a mail from the US about a possible second round on
the L26 got me started on the final chapter on this
vintage JBL speaker.
As discussed before, restoring old speakers always leaves you with a lot of dilemmas. What should be left "as-is" and what should be modified to possibly enhance performance. You easily get into modifying everything, leaving little of the original design and end up with a modern speaker made from highly tweaked old components with little resemblance of the real thing. The L26 may not be one of the most precious gems from the JBL company and I have tried taking the middle road and added bracing to the cabinet, done only little to the drivers and most importantly, kept the LE25 for treble. This is really not a bad tweeter, but it needs some equalisation to deliver the best. I could very easily have taken the SEAS MCA12 for mid and the 27TFFC tweeter for treble - but then it definitely wouldn't be a JBL speaker any more.
Having a pair of JBL LE5-12 middrivers, these might be a possible option for a suitable midrange relieving the bass of handling the troublesome upper mid/lower treble range, but these LE5 drivers can be major trouble and I almost gave up in the process. The LE5 drivers are found in a number of variants and I have a pair of LE5-12 and a pair of LE5-2 (the latter the alnico version from the L100 and other JBL speakers). The LE5-2 is worse than the LE5-12 but I'll get back to this later.
Seen above (left) is the response
of the LE5-2 (blue) and LE5-12 (red) on a 20 cm wide
baffle. Overall sensitivity appear to be almost the same.
The ceramic magnet on the LE5-12 is doing very well and
from this presentation the LE5-12 seems like a much
better driver compared to the LE5-2, having a smooth
response in the upper mid and lower treble.
Right: The response of the LE5-12
with alu dome (red) and with rubber dome (blue). This
really doesn't look bad. The driver now has a steadily
rising response up to 6 kHz, a minor peak at 8 kHz and
it's gotten rid of all the rubbish above 10 kHz. The
sensitivity seems a little reduced, but there's plenty
for an L26 3-way.
Left: Above the response
of the two LE5-12-RD on the L26 baffle after the rubber
dome glue has settled. Not bad at all.
Right: Above the
impedance of my two LE5-12-RD drivers in free air. These
LE5 mids really behave more like giant tweeters than
middrivers. They would fit in well with the old
Wharfedale open baffle speaker with its up-firing 3"
Another thing about the LE5 mids - and this is the last one - is consistency. As can be seen on the graph above, the impedance of these two drivers are not exactly the same. 1 ohm difference in basic impedance and again - this is getting boring - the LE5-2s are even worse. I have no idea why JBL wasn't able to make voice coils with the same DC resistance. It doesn't make sense. I mean, you wind so and so many turns one way and so and so many turns back again to make a two layer voice coil. How tough can this be? Nevertheless, I've had three pairs of LE5 drivers in my time and none of these have neither measured the same, nor sounded the same. I guess it's difficult to make a very rigid suspension the same all the time, in particular when the stiffness of the suspension is partly determined by a manually applied layer of coating. A fabric spider and a rubber or foam outer suspension is likely to produce the same compliance all the time.
Restoring these old cabs takes at least the same time as making a new pair. Quite some work!
Front panel driver layout and port
Left: As always: The
placement of the drivers have a major influence on the
response of the drivers
Right: The JBL L26 vent
made from cardboard tube with an internal diameter of 68
More pics from cabinet rebuild:
There are several reason for these two
simulations. We have to go back in time - some 40 years
at least - and think of the tradition of loudspeaker
making of those days. Basically the main drivers were
able to handle both the bass and significant part of the
midrange, if not all of it. Obviously the upper mid would
start suffering from beaming due to the large cones and a
smaller middriver might be added to enhance dispersion
and add some sparkle ( = treble) to the overall sound.
3" tweeters were often seen and today on eBay you
can find "green cones", fullrange German cinema
speakers with a 3-4" small speaker for treble and
I've recently seen the BassZilla/Dich
Olsher system fitted with green cones, an
8" running fullrange for mid and a 3-4" for
treble, the latter run by a single small capacitor. So
the practice of having a true midrange driver covering
the frequency range from 300-3000 Hz came in later, hence
the high Fs of the LE5 drivers. This is an old time
driver in the tradition of the Fifties and Sixties. What
appeared from the simulations was the possibility of
running the LE5-12 down to around 450-500 Hz, close to
the Fs and the second simulation is in the tradition of
running a small mid from around 800 Hz. As disussed
before I've seen the JBL L100 Century (123A + LE5-2 +
LE25) having only two capacitors for crossover, 8 uF for
the mid and 3 uF for the treble. I've tried modelling the
L100 from this set-up and it looks horrible. The LE5-2
and the LE25 run from only two caps are peaking away like
crazy. I can't believe we thought this was "it"
back then. Anyway, many of the JBL drivers are very
capable of handling most of the midrange - in particular
the 123A - from its very flat cone geometry and
well-behaved frequency response in general. The 125A is a
little more trouble due to the major peak at 2 kHz and a
little more sophisticated crossover is needed to bring
out the best of this driver.
The cabs were finished during the summer
holidays and the sanded veneer was given a fresh oil rub.
Front and rear panels had a dark grey colour and to put a
long story short, I'm very pleased with the result. And
it's very much time to finalise the crossover after a
couple of months' break. Gut feeling tells me the
800-3500 crossover option is the most likely to succeed.
Gut and gut.....a friend came by with a pair of JBL L100
Century speakers a month ago and I spend quite some hours
recording what they were doing. This will be reported
later, but what was apparent was the point of crossover
between bass and mid. The LE5 drivers really aren't meant
to go very low due to high Fs and much of the JBL sound
is derived from the bass handling lower and middle
midrange. The LE5s are more like mid-treble hybrids and
should be treated accordingly.
Left: In all of the midrange and up to 10 kHz a smooth response is achieved and minimum phase tells it all seems to work fine.
Right: SPL including
merged nearfield reading of bass response. Disregards the
200-300 Hz region - all things can be achieved when doing
nearfield measurements at 1 mm distance to the cone.
Left: Mid, tweeter and summed response. Green = reverse polarity of tweeter displaying nice phase tracking between the two drivers.
Right: Inpact on frequency response of RC circuit bypassing bass series coil. It's impotant that the huge 2-3 kHz peak from the bass drivers is reduced.
That JBL sound!
Connecting the first speaker from the test crossover set-up, my first reaction was: Well, I'll be damned - or something similar in Danish. My surprise was due to the treble. My expectations from the LE25 tweeter had been so and so. This tweeter is much better than measured performance would suggest. Sibilance is handled gracefully and there's an overall ease in presentation. Hmm... a large, rigid paper cone... It takes a minor equalisation to get the balance right and there's not much we can do about the 12 kHz notch - except for lowering the response in the 10-15 kHz range a little. You can increase tweeter level by 1-2 dB and yes, there's too much treble, but it doesn't become bothersome in the way most dome tweeters do. Strange. Does this tweeter really have such low distortion? Whatever it does, it points to the fact that we should take great care with what a tweeter does at 3-10 kHz and what's beyond may be less interesting. Oh yes, the 10-20 kHz provides the airiness to the overall sound, but whether it has to be ruler flat is a good question. Supertweeters? Honestly, forget about supertweeters and let's talk about the things that matter.
My most troublesome,
sibilant female vocal recordings? Probably the best
speakers I've had with regard to this particular problem.
This speaker can play loud,
really loud. Almost as loud as the old SEAS 503 kit - with it's
13" bass driver. There's nothing like a big bass
driver! Due to the acoustic vent, the bass is firm, solid
and up in level all the way down to 50 Hz, from where it
starts rolling off.
Detailed descriptions of sound is boring and I'll stop here. If you love that JBL sound and want to hear what a modern filter can do to the JBL sound, you may try what's been described here. It's some journey and some of the effort going into such a project has to come from the satisfaction of restoring vintage equipment. I'm certain some of the JBL drivers hold qualities worth the effort and the end product can easily compete with modern equipment if not sometimes leave it way behind.