Part I: 24 liter
floorstander, 9700 option
Danny from Norway wrote me and asked if he
should "eBay" his SS 8542 drivers, which I
certainly think would be a very bad idea, so here we go.
My 8542 drivers have been on the shelf much too long.
This is one of my favourite SS drivers, long fibre paper
pulp, foam surround, symmetric drive, etc. A driver you
can listen to for hours and hours - that is if the
tweeter is properly integrated.
Only bad thing about this driver is the
dip at 6 kHz. I'm not sure what causes this problem. I
had a suspicion on an un-damped polepiece, but by closer
inspection it turned out the polepiece actually is
damped. However, with a 2nd order filter this dip in
response is way down and probably doesn't impact sonic
performance.
The 8542 has been seen in studio-monitors and I'll later
publish a construction in a 14 liter cab with the SEAS
27TFFC tweeter, as this is the classic set-up. But Danny
asked for the 9700 tweeter option and this is where it
will start, although not in the small cabs. I have
pre-veneered, pre-cut panels for the mini, but later...
The front panel used here is 190 mm wide and the 14 liter
monitor version will have a 205 mm wide baffle and
slightly different driver placement, so there may be
changes to the crossover.
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The Crossover
Crossover for 18W/8542 + 9700
Suggested crossover components from Jantzen Audio
Contact Jantzen Audio at: contact@jantzen-audio.com
The Cabinet
Basically this is the
same as the TJL2W, only 240 mm deep. Construction
details and damping materials can be found in TJL2W and SP38 files.
Cabinet volume, front panel width and driver
placement are the most important things to follow
here.
Problem! Cabinet depth = 200 mm, vent length =
200 mm, won't work. (My test cabs a deep enough
for a 20 cm vent). Use a 60 mm vent with a length
of 12 cm, or make the vent towards the floor.
Leave at least 40 room (spikes, etc.) for the
vent to breathe.
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8542 drivers, TS data, box
simulation.
My measurements compared to the SS data, no major
surprises.
Added mass method used.
LspCAD box simulation. From measured data: QB3 = 12.9
liter, SBB4 = 14.8 liter and SC4 = 13.8.
This looks fine for the 14 liter monitor version, but we
can get deeper bass from this driver.
With a vent tuning of 45 Hz we have flat response down to
50 Hz - and it sounds good too.
Measurements
 
Left: SPL from 9700 and
9500 tweeter, red and green = 9500. As seen
before the 9500 has higher sensitivity and the
9700, due to the symmetric drive feature, has a
rising response towards higher frequences, thus
needs equalising not to sound pointy.
Right: SPL from 8542 drivers and 9500 tweeters
again. The dip at 5 kHz is sad, as this driver
might have worked with 1st order filters had it
not been for the design problems.
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Left:
impedance of all drivers, also displaying the
spread in performance. This doesn't get much
better.
Right: SPL from driver with crossover and summed
response. Point of crossover is 2 kHz. |
 
Left:
Summed response at 1 m/tweeter height from same
polarity (red) and reverse polarity (blue)
Right: Left and right speaker, one 8542 driver
had a minor notch at 2 kHz making its way through
to the final performance. However, hardly
audible. |
 
Left:
Impedance of final system +/- mid impedance
flattening circuit: 1 mH + 10 uF + 15 ohms across
input terminals.
Right: Step response showing positive polarity of
both drivers and a smooth decline from the 8542
driver. It also shows this speaker is not
time-coherent. |
Part II: 24 liter
floorstander, 9500 option
Suggested crossover components from Jantzen Audio
As always: Swapping
tweeters can be major trouble and this one was no
exception. The 9500 and 9700 are very different
tweeters and the whole thing had to be worked
over again. What made things worse was a minor
difference in 8542 performance, which leaves you
with task of making a crossover that will work
for both drivers and hopefully for all 8542
drivers. I can only guess what the variation is.
Worst case is a minor bump at 2 kHz, but I think
one of my drivers is an exception.
Red = true resistors, "empty" resistors
= resistance of coil. For the L1041, the total
resistance of coil and resistor should be around
0.8 ohms.
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Measurements
 
Left: This is what
happens when we swap tweeters without fine-tuning
the crossover for the new tweeter. Blue = 9500
option with 9700 crossover. Right: Here we can
see the 9500 response (green) from the 9700
crossover. No further comments.
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Left: Summed response
(red) and response with inverted tweeter
polarity. Measurements taken a tweeter height, 1
meter distance.
Right: Summed response and minimum phase.
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Left: Impedance from
system in 24 liter cabs. Red is with stuffed
vent.
Right: Step response displaying same polarity of
tweeter and bass and a smooth decline from the
8542 is seen.
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