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    Junior Member DHL's Avatar
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    Modded and restored JBL L212 speaker system

    After many years of collecting dust, I have finally completed the restoration and modification of my JBL L212 system. It was originally purchased by me in the late '70s. After many hours of sonic experimentation, I added time alignment plates, fabricated from solid walnut planking to match the wood finish on the originals. This was done soon after I purchased them, and significantly improved the depth of the soundstage and imaging.

    One of the weak points of this system, in my mind, was the cross over network. I had always felt the quality level of the components was never quite up to the quality of the drivers. Originally, I had tossed the stock crossovers for a tri-amped active crossover system, but never got around to building and purchasing all the electronics required. So the system sat in limbo and failure of the surrounds on the 121A sub bass driver and the two 112A bass drivers stopped things for quite a while. When I decided I wanted to get these back in action, I had new surrounds put on the drivers. Luckily, I was able to keep the original cones and spiders.

    I constructed new crossovers from two stacked 2mm thick epoxy glass turret boards. The inductors I used where Jantzen and Erse air core 18 ga. To simulate the low Q versions JBL originally used, I added non-inductive resistors of the appropriate value in series with the inductors. The capacitors were upgraded from the mylar film types JBL originally used to polypropylene film types. I used the OEM schematic for the crossover network, including the original Zobel (driver impedance correction) networks. I wired all the drivers with 14 ga (which is the largest wire size that will fit the spring terminals on the drivers).

    I relocated the LPads from the front to the rear and placed them on a panel mounted internally. The time alignment plates had blocked access to the mid-range adjustment L-Pad on the original positioning.

    Here are some photos of the finished project:
    Attached Images Attached Images      

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    Obviously... not a golfer grumpy's Avatar
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    The large beveled surfaces aligning the drivers looks very reminiscent of the period.
    Nice job with that.

    Q: (perhaps for other L212 owners) Wasn't there a later crossover with different slopes
    than the original? (a revoicing perhaps, vs. any changes needed to go from alnico to ferrite)
    The same responders might be able to provide opinions on whether this was an audible
    improvement. I'm too lazy to look it up right now
    It's nograveconcern to me, but it appears a fan on AVS "borrowed" my avatar...

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    Senior Member rdgrimes's Avatar
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    I've always wanted to do the alignment thing with the PT800. Do you hire out?

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    Junior Member DHL's Avatar
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    Quote Originally Posted by rdgrimes View Post
    I've always wanted to do the alignment thing with the PT800. Do you hire out?
    The simplest method to use, without a lot of experimentation, is to align the rear of each cone (or the spider position) to the same plane. That should get you pretty close. I had a pair of L100s (or L110s can't remember exactly) that I pulled the tweeter amd midrange out of and mounted in a separate box. I placed this on top of the main cabinet with the woofer still in it, and by moving the tweet/mid box back and forth I could change the imaging and sound field. I could also rotate this box toward the listener and really focus the central image. That was before I got the L212s and was the inspiration for building the front plate. B&W used the same method in their Nautilus line of speakers many years later.

    There are of course other methods like tilting (as is done with some monitor speakers placed near floor level) or even rasing the entire cabinet. Each of these alternative methods has its issues though.

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    Junior Member DHL's Avatar
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    Quote Originally Posted by grumpy View Post
    The large beveled surfaces aligning the drivers looks very reminiscent of the period.
    Nice job with that.

    Q: (perhaps for other L212 owners) Wasn't there a later crossover with different slopes
    than the original? (a revoicing perhaps, vs. any changes needed to go from alnico to ferrite)
    The same responders might be able to provide opinions on whether this was an audible
    improvement. I'm too lazy to look it up right now
    I think we went round and round on crossovers for the L212 sometime back. All I was able to determine was that the main components were pretty much the same for all versions, except for some changes in the Zobel networks for some of the drivers. I built the crossovers from the specs in the Service and Repair manual published by JBL.

    I considered altering the values of the bandpass and bass lowpass to alter in the response hump between 200 and 500 Hz in the lower midrange (you can see this in an audio magazine test report of this system circa the late 70's), but later decided to leave the components as spec'd by JBL. Anything that can be done to reduce this emphasized lower midrange response is a sonic improvement, as it adds too much "nasality" to the sound. Back in the day, I used active EQ to reduce this.

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    I like the component mount pins on your crossover boards, where were they sourced from?

    Thanks.

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    Junior Member DHL's Avatar
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    Quote Originally Posted by wsilva View Post
    I like the component mount pins on your crossover boards, where were they sourced from?

    Thanks.
    You can get both the boards and "turrets" from

    http://www.tubesandmore.com/

    Look under "Hardware and Terminal Items", then "turret boards". Both the boards and turrets should be listed. Note that their current turrets will not stake properly with the 3mm thick boards, but I had no problem using them anyway. After mounting everything I used a little epoxy around the base of the turrets to hold them in securely. It also helps if you stake them in before adding components, so you will need the staking tool. I used the 3mm boards due to their stiffness as they had to support those very heavy air core inductors.

    These boards are also used to prototype tube amps so if you have a DIY tube amp project, these may come in handy as well.

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    Thank you Sir

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    time aligned L212

    Thanks for the inspiration DHL - I have done similar to mine now ! I've gone the other way, abandoning my B&W 804s - which are great speakers - especially with a few mods - but...the JBLs are more fun to listen too.
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    Senior Member martin2395's Avatar
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    Looks great but you are going to cook that Sony receiver on the KSA-80
    JBL 4343 and L100S user...

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