New crossover for JBL L100 Century

New crossover for JBL L100 Century
1
New crossover for JBL L100 Century
by
JANTZEN AUDIO DENMARK
integrating well with midrange, actually better than many
softdomes in my opinion. I've recently bought some LE
tweeters on eBay: LE20, LE25 and LE26 and will make a
special report on these some day.
Designed by Troels Gravesen, March 2007
While I'm enjoying the sound from newly renovated JBL
L26 Decades, I might as well start writing about the one and
only, JBL L100 Century, one of the most common studio
monitors of the Seventies plus. A month ago one of my
friends threw in these speakers, having bought them from an
old fellow for 300 DKK (50 US $) - a total give-away. All
drivers in mint condition and the cabs with minor scratches.
Tweeter foam had gone but can easily be replaced from
eBay sellers.
I won't go into historical details on the L100 as so much can
be found at:
http://audioheritage.org/html/projectmay/pmintro.htm, and
in particular about the L100 here:
http://audioheritage.org/html/profiles/jbl/l100.htm
A rare opportunity to measure what the L100 did back then
where I could only dream of owing a pair of these speakers
from "over there". I don't recall the retail price, but they
were very expensive. Having recently experienced the SEAS
503 kit, I now realise the 503 was actually a better speaker.
Quite similar in size and power handling, the 503 had a
better midrange and a better crossover and the 33FWK bass
drivers may beat the 123A drivers. But the SEAS drivers
didn't have the gorgeous looks of the JBL drivers, the whitecoated bass cone and the exquisite finish of the LE5-2
driver. I've always had a week spot for the LE5-2 due to how
well it is built. Take a look at how the dust cap is glued to
the cone and the fine touch of paint around the edges. It
takes great skills to make such fine work. Only sad the LE52 is such a troublesome driver as we shall see later.
The tweeter foam surround usually deteriorates over time.
Above my own stock drivers for recreating the JBL L100. I
have to say it's much cheaper buying an old pair of L100s
compared to buying the drivers separately on eBay. On
eBay, sellers will completely strip an L100 speaker and sell
it bit by bit down to the pins holding the front grille.
Above my friend's JBL L100, some scratches to the cabs,
but drivers in mint condition.
This first part of the my L100 story will not engage in a new
crossover, merely try to find out what the L100 was in terms
of driver performance and crossover construction. And we
might as well take a look at the original crossover right
away, because it's simplicity itself. 8 uF to the mid and 3 uF
to the tweeter. Both drivers connected via an L-Pad for
attenuation. Mid connected with inverted polarity. Having a
12" bass driver running full range is a bold choice and a lot
of work must have gone into designing the 123A driver to
produce a smooth roll off. The LE25 is a better tweeter than
we might think, maybe not from measuring performance, but
well equalised, it can deliver some excellent treble
Cabinet dimensions and driver placement.
Cabinet modelling from TS-data reveals the below seen
response profile. Basically the 123A driver has a too high Qt
to be used in a vented cabinet and the result from a 44 liter
2
volume is a 3-4 dB increase in response at 70 Hz, which is
confirmed by a nearfield reading of the response.
Place the classic L100 Century on the floor, close to a wall,
and we will have an overall 7 dB response increase from 50
to 100 Hz. Talk about a boom box! So, two options
available. Add some acoustic foam to the vent, lift the
speaker some 50 cm from the floor, which will raise the bass
driver to ~70 cm from the floor, move it some 100 cm
(distance to front panel) out from the rear wall, place in on
the long side of the room with 1.5-2 meters to side walls,
and we will reduce the bass peak to 3 dB above average
level. Much nicer. Placing the L100 in corners is a no-no.
Response from L100 as-is.
The LE25 is much better than I had anticipated. It doesn't
matter much whether the foam is in place or not. It may
change the sound, but not measuring performance. File not
shown.
Individual response of drivers from original crossover with
mid and tweeter attenuation set to zero. Some serious traffic
jam around 3-10 kHz - three drivers all trying to make the
most of it here. No wonder the original set-up fails utterly on
vocal recordings.
The 123A bass drivers really shouldn't try to play upper
treble. The mid really shouldn't peak at 6.5 kHz and some
proper low-end roll-off might improve performance. And the
tweeter might improve performance from smoothing overall
impedance.
Response in 44 liter closed box (Fb set to 1 = closed box, for
the sake of ease)
Driver performance
Individual response of drivers without crossover. Smooth
bass roll-off? Well, maybe not from this cabinet, but take a
look below where I compare the 123A response from the
L100 cab and a 60 cm wide, curved baffle.
The LE5-2 is extremely sensitive. If it hadn't been for the
dip around 1 kHz, we would have had 96-98 dB/2.8V
sensitivity. What is not so nice is the 10 dB peak at 6.5 kHz.
The SPL response of both speakers with "0" attenuation for
the tweeter and -3 dB attenuation for the mid to get the
terrible peak at 6.5 kHz down to reasonable level. I found
this attenuation setting to produce the best overall balanced
sound. Nearfield bass response is merged at 350 Hz.
Disregard difference in bass response. Probably not correct.
Now, listening to some music from this setting may be close
to what we heard back then. Quite an experience to hear that
JBL L100 sound once again. Good? Hmm... Very
impressive, lots of bass, speedy midrange. The sound of
acoustic guitars is very good, but vocals? Something is very
much wrong in the treble area, the 5-10 kHz range I guess although I'm strongly biased due to the 6.5 kHz peak. The
danger of knowing what is going on. As soon as you know
how it performs on paper, you can't help instantly having an
opinion about what the sound is like - and what is wrong
about it. But the 6.5 kHz peak is so severe that it's most
likely the cause of some of the major problems of this
3
simplistic crossover set-up. The peaky nature of the 5-10
kHz range is very much audible and vocals really aren't
good. Sibilance is highly emphasized and you can't attenuate
the mid to get rid of the trouble without losing tonal balance.
Response of midrange alone with crossover and attenuation
set for -3 dB. Severe dip at 1300 Hz and that 6.5 kHz
peak..... This looks like a poorly managed tweeter response.
A response up to 15 kHz! I mean, a single coil to the mid
might at least have produced a more flat response, but then
the upper treble may run into trouble from this approach.
Step response showing reverse mid polarity. The strange
thing about these drivers is that the red terminal on the 123A
bass is correct. The cone moves out when applied positive
voltage. The LE5-2 cone in moving inwards when positive
voltage is applied to the red terminal. Why would JBL do
this? To ensure correct polarity of drivers when plus wires
from crossover were connected to drivers? But the LE5 was
used in other constructions... Doesn't make sense.
Modelling the original crossover performance
Mid attenuation fixed at -3 dB and tweeter response shown
from +3 dB, 0 dB and -3 dB settings.
Modelled response of drivers where the drivers are
attenuated with two resistors in order to recreate the
measured performance. Not that easy, but this is close. Phase
tracking between bass and mid is better that we might
expect, but the tweeter is very much living its own life in the
upper midrange.
When I reverse the polarity of mid and tweeter in the
LspCAD, things at least get worse. It's not that bad.
Mid response from attenuation at -3 dB, 0 dB and + 3 dB.
Red = summed response from mid attenuation at -3 dB. If
you run the L100 from "mid +3 dB" it'll rip off your ears!
4
A new crossover designed for all variants of LE5-2 drivers
is a challenge and fortunately it can be done due to the very
high sensitivity of the middriver. Placing a resistor before
the driver will increase impedance and a standard crossover
will have a deviation in midrange level of +/- 1 dB. This is
manageable and for those who want to do some fine-tuning,
the mid level may be increased or decreased by the input
resistor.
New crossover design criteria
January-March 2007
The 123A bass in L100 cab, blue, and from a wide curved
baffle (red). Now, find a 12" driver today that will perform
flat up to 6 kHz and have a smooth roll-off. Remarkable!
The (troublesome) LE5-2 middrivers:
A lot of work has gone into designing a new crossover and
numerous simulations and actual crossover constructions
have been tried in order to try taming the LE5-2 middriver
and provide an overall balanced sound from the L100.
What I wanted was to maintain the basic virtues of the L100,
i.e. the 123A bass driver working almost full range, thus a
1st order filter was soon in place. To maintain 1st order
filtering as high up in frequency as possible, the LE5-2 had
an LCR circuit to flatten the rise in impedance at Fs,
allowing the high-pass section to be one single capacitor. To
cope with amplitude and phase between mid and tweeter, the
mid needs a 2nd order low-pass section and the LE25 needs
a 3rd order filter at 5 kHz to get amplitude and phase in
place. See graphs below.
Three significant things have happened here: First of all, the
LE5-2 middriver no longer peaks at 6.5 kHz. Secondly the
mid-section was constructed in such a way that the common
variation in driver impedance (LE5-2) only produce minor
deviation from target response. Last but not least, both the
tweeter and mid produce a smooth roll-off below points of
crossover. Should you want to increase mid and tweeter
levels, you may bypass R2011 (mid) and increase R1061
from 3R3 to 5R6 or 8R2 (tweeter).
I have two LE5-2 drivers asigned my future L100
reconstruction. So my friend's L100 gave me a rare
opportunity to compare four LE5-2 drivers - and my
goodness, is there some difference!
In the upper midrange we have some 4 dB difference in
sensitivity. Basically this is due to differences in voice coil
impedance. See below.
Impedance of four LE5-2 drivers. Some difference! From 5
to 7 ohms. Unfortunately both my friend and I have one of
each and the cones are so different in colour that we can't
swap drivers. If you ever buy LE5-2 drivers on eBay, be
sure to have a DCR reading of both drivers before bidding.
New crossover topology. Pay notice to terminal lay-out of
JBL drivers.
5
Installing the new crossover kit
Drawing of crossover board. Full size drawing is included.
Remove old terminals including the thin masonite panel.
Scrape off residual glue and sand the rear cabinet panel to
make a smooth surface.
Prototype of new crossover kit.
The kit will include:
1. 180 x 280 x 12 mm MDF board.
2. Terminals.
3. Two resistors for tweeter attenuation will supplied, both
can be fixed to the crossover board and a short wire to
ground provides the preferred attenuation.
4. Soldering tag strips.
5. All crossover components: Baked non-resonant coils, all
polypropylene capacitors, MOX resistors and cables for
connecting drivers.
Not supplied is silicone glue, screws and optional damping
material. As can be seen below, it's highly recommended to
replace old damping material. I've used soft 40 mm
polyurethane foam and a sheet of MDM3. It changed the
overall sound a lot - and for the better.
Kit components for one speaker.
New damping material.
6
Summed frequency response, red, and response of individual
drivers from new crossover. All drivers now display smooth
roll-off characteristics.
Six page instruction manual included in kit. Contains basic
crossover design with all component values and wiring
instruction.
Performance of L100 with new crossover.
The litmus test: Reverse mid polarity. Reading taken a 1 m
distance at middriver height. Due to placement of drivers,
this reading may somewhat depend on placement of
microphone. In my own recreation of the L100 I'll have a
standard vertical placement of drivers with mid and tweeter
slightly off-set.
L100 frequency response from new crossover. The dip at 1.3
kHz is caused by the small front panel. Some of the LE25
tweeter display a peak at 12 kHz. Below is shown one of my
LE26 tweeters, where JBL apparently succeeded in taming
this peak. However, I must say the treble from the renovated
L100 is remarkably smooth. If your L25 tweeters are intact,
do not discard. They're excellent tweeters - only need a
proper crossover.
Final remarks to the L100:
I dare say it's quite a different speaker compared to the
previous "two-caps" set-up. Classical music is a delight,
vocals are handled very well and should you throw a
Saturday evening party, give it a 150 wpc amp and let it
rock! It runs well from my 20 wpc PSE valve amps, though
not at very high levels.
One last thing: The L100s need stands, at least 45 cm high to
bring the 123A drivers off the floor. Remember, the 123A is
handling most of the midrange and a 5 deg. tilt is doing even
better.
Enjoy!
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