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Thread: Modded and restored JBL L212 speaker system

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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 uh... uh... it's down there somewhere, let me take another look...

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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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    Senior Member rdgrimes's Avatar
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    I see these are on eBay now. Wish you luck, good work should be rewarded.

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    Junior Member DHL's Avatar
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    Quote Originally Posted by rdgrimes View Post
    I see these are on eBay now. Wish you luck, good work should be rewarded.
    Thanks.

    I just hooked them up to my reference system (Cambridge Audio CD/Bryston BDA-1, Parasound JC2 line preamp, JC1 monoblock power) and disconnected my B&W 803ds.

    WOW is all I can say. Those crossovers have made an incredible difference. Gorgeous 3d sonic image, very detailed and spacious. Rivals my B&Ws which cost over 10X! They are a bit leaner on the low bass end compared to the B&Ws with the HSU sub, but that is to be expected. The upper bass lower midrange "honkiness" that I remember seems to have gone. They are so efficient that the high bias setting on the JC1s runs Class A for most program material. These are an incredible bargain for $1500, esp compared to what you can buy for that kind of money today.

    I'm re-considering the sale, but will probably have to go through with it now

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    Senior Member jblsound's Avatar
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    I just saw this thread, most of two years late. Beautiful job. Did you ever sell them?
    I did an upgrade to my L212s making them mirror imaged with biased networks.
    I had thought of sloping back the boxes to time align, but I decided not to do so.
    I figured that was more to gain with the mirror imaging, than time aligned.
    Never thought of adding pieces to the front. Guess I could add them.
    Living in the Land of the Sun

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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
    Currently: JBL 4343 (4353?) with 2235H/2123H/2441/2405 and 4312MKII (for sale)
    The Dutch JBL fanatic

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