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Author Topic: Pinging PJ re JBL D208's  (Read 6204 times)

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Offline Aural Robert

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Pinging PJ re JBL D208's
« on: December 14, 2009, 07:27:06 AM »
I've been doing some searching regarding the JBL D208 drivers, and Paul Joppa's name keeps coming up.

I used to own a set of these in a previous life and am thinking about picking up a set to mess around with. Paul, did you ever get yours boxed up?

Everything I read tells me I should use a 1.5 cu ft or so bass reflex enclosure, with a tweeter for the highs. I figured I'd start with the 8 incher and add some kind of tweeter later. Not sure how big to make the port either. Time for some research.

Aural Robert.
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Offline Paul Joppa

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Re: Pinging PJ re JBL D208's
« Reply #1 on: December 14, 2009, 01:01:13 PM »
Here's a link to the specs for the 2110, which is the pro division number for the D208:

http://www.lansingheritage.org/html/jbl/specs/pro-comp/2110.htm

This is from 1977, according to the Lansing Heritage site where I found it. The D208 goes way back though, and has undergone a few changes. I bought mine around 1967(?) and used them for several years in a 3-way box of nearly 2 cubic feet. Crossover to the midrange was 700Hz IIRC. I sold those boxes to a friend, and then after a couple decades bought them back when he got new speakers.

Box design is hard to come by. My original boxes were designed from Thiel's papers which had just been (re-)published around that time. Today I ran BoxPlot which suggests 0.5 cubic feet tuned 77Hz for -3dB at 87Hz - yuck. Then I raised the electrical Q by 50% to approximate using an SET amp, and got 1.64 cubic feet tuned 55Hz for -3dB at 51Hz - that's pretty workable. A 2.4" diameter hole in 3/4" cabinet material would do that.

Three things:

1) response goes to heck above 3kHz, so cross no higher than that. I'd go as low as 1200 if I have an appropriate (horn) tweeter, but there's a tradeoff - as the crossover drops below 3kHz it happens more and more in the critical voice range (bad) even while you reduce rough response above 3kHz (a good thing). Figure on experimenting! JBL crossed them at 2.5-3kHz to their "baby butt" tweeter; you might find that crossover on the net somewhere.

2) Note that even in the highly smoothed "plot" this driver has a rising response, picking up 6dB between 100Hz and 2kHz. A BSC would probably help, or just live with a midrange emphasis - they are more punchy than rough as I remember.

3) On the high-efficiency asylum board there have been some recent discussions of the Altec Bolero and replacements for its fragile original tweeters; the same tweeters would be appropriate for these drivers.
Paul Joppa