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Thread: L-100 Century questions and observations

  1. #1
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    L-100 Century questions and observations

    Hi all,

    I'm new to this forum having just joined this morning. It looks like a great place to gather information on JBL loudspeakers. I'm looking forward to participating here.

    I've owned a pair of JBL L-100 Centuries since the late seventies that I purchased used. I'm assuming my version was the earliest as the centers of all three drivers are in line with one another.

    The high frequency driver is the one with the round metal surround. I'm not sure what model this is but would it benefit from a foam surround ring?

    A lot of craziness (diffraction?) seems to be going on behind the speaker grills as the speakers sound better with the grills off. With them off, the depth of the sound stage is increased as well as definition. The original foam was removed from the grills long ago and recently I purchased a set of grill inserts on eBay to finish off the refinishing of the cabinets. I removed the original screen that supported the foam to try and improve the sound though they still sound better with the wooden frames and speaker cloth removed.

    BTW, does anyone know why my model doesn't have a tube extending into the cabinet from the port? Seems like a crude way to do a port tuning.

    And if anyone is interested, here is a list of equipment connected to the L-100's:

    Pioneer A-77X integrated amplifier
    Sony 620 ESII CD player
    Pioneer PL-71 turntable with Stanton 681 EEE
    Behringer 8024 Ultra Curve Pro for room eq (modified with new Pioneer FC power supply capacitors and Pioneer bi-polar capacitors elsewhere)

    Richard

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    Cabinet tuning

    Re your tuning port question - whether or not a tube is required just depends on the tuning frequency desired. With a little luck, sometimes you can hit the optimum just by adjusting the area of the port. Adding the tube lowers the tuning frequency, the longer, the lower. You could experiment by adding a tube - there are even adjustable tubes for the purpose! Good source is Parts Express just south of Dayton, OH. A lot us here have found we like those older book shelf jobs - L100, 4311, 4312, et al with no port at all - closed box. Smooths the response, less boomy, tighter bass. More Hi Fi, less "rock and roll." So, depends what kind of music you listen to, personal taste. I have it blocked on my 4312.

    Re the grills - I've never heard a speaker system that doesn't sound better (to me) with the grilles off. Always a loss in definition, etc. I'm not savvy re the tweeters, perhaps someone else can jump in here. JBL produced quite a few - what's on the baffle was what they found optimum at the time. R&D process.

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    Quote Originally Posted by Mannermusic View Post
    Re your tuning port question - whether or not a tube is required just depends on the tuning frequency desired. With a little luck, sometimes you can hit the optimum just by adjusting the area of the port. Adding the tube lowers the tuning frequency, the longer, the lower. You could experiment by adding a tube - there are even adjustable tubes for the purpose! Good source is Parts Express just south of Dayton, OH. A lot us here have found we like those older book shelf jobs - L100, 4311, 4312, et al with no port at all - closed box. Smooths the response, less boomy, tighter bass. More Hi Fi, less "rock and roll."
    Thanks for all the information. It's much appreciated.

    I sent some sine waves through the system this morning and discovered the peak tuning frequency is around 80Hz. The boost drops off sharply below 80Hz though rolls off a bit more gradually beyond 80Hz to around 200Hz. In any case, I tried blocking the ports off and so far, I prefer them this way. As you indicated the boominess is reduced and the base is tighter and more defined.

    My listening room is equalized using a 1/3 octave equalizer, mainly in the area of 100Hz and below due to the L-100's being located near corners. I couldn't help this due to the room dimensions. After plugging the ports I increased 80Hz by 2db in each channel as I felt the base was a little thin using the same eq settings used with the ports open. I neglected to mention I'm also using a powered subwoofer with the crossover set at 40Hz to achieve a flatter response down to 20Hz.

    Thanks again for your suggestions and explanations.

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    Administrator Mr. Widget's Avatar
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    Have you tried using the search feature? You might want to do a few L100 searches looking at some of the older posts... I think you would discover quite a lot about your speakers.


    Widget

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    Quote Originally Posted by Mr. Widget View Post
    Have you tried using the search feature? You might want to do a few L100 searches looking at some of the older posts... I think you would discover quite a lot about your speakers.
    Thanks for the suggestion. On a quick search I just now learned the tuning frequency for my L-100's is 41Hz and my tweeter is an LE20. I guess I'm a bit confused. I thought the tuning frequency was the frequency given the biggest boost. With a tuning frequency of 41Hz, I'm getting the biggest boost at 80Hz of about 4db.

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    Quote Originally Posted by rlovison View Post
    I guess I'm a bit confused. I thought the tuning frequency was the frequency given the biggest boost. With a tuning frequency of 41Hz, I'm getting the biggest boost at 80Hz of about 4db.
    It is far more complicated than that and you probably don't have measurement equipment with the resolution necessary to accurately determine the cabinet tuning frequency. There are two methods I use. I use a measurement system called CLIO which requires a calibrated mic, specialized hardware and software. With this system or one like it you can determine the tuning frequency by looking at the impedance curve of the system... there will be a low impedance point between two peaks at the low end of the curve... a more precise method is to place the mic 1/4 inch from the cone and run a frequency response measurement of the woofer in the enclosure, there will be a small sharp dip in response at the tuning frequency.

    The 80Hz peak you are seeing is likely a room/placement issue and not the speaker. That said, a 4dB peak is negligible... I'm sure with proper measurement gear you would find much bigger peaks and dips throughout the low frequency spectrum.


    Widget

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    Quote Originally Posted by Mr. Widget View Post
    The 80Hz peak you are seeing is likely a room/placement issue and not the speaker. That said, a 4dB peak is negligible... I'm sure with proper measurement gear you would find much bigger peaks and dips throughout the low frequency spectrum.
    OK, here's what I did. I placed an inexpensive calibrated microphone (Behringer ECM8000) about two feet from the speaker with the port open, sent a 80Hz sine wave through the speaker and came up with a measurement. I then closed the port and the level of the 80Hz tone dropped about 4db.

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    Quote Originally Posted by rlovison View Post
    OK, here's what I did. I placed an inexpensive calibrated microphone (Behringer ECM8000) about two feet from the speaker with the port open, sent a 80Hz sine wave through the speaker and came up with a measurement. I then closed the port and the level of the 80Hz tone dropped about 4db.
    Not to be flippant or short, but that doesn't really tell you much.

    If you are interested in exploring this topic I suggest you read this inexpensive book: Testing Loudspeakers


    Widget

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    Obviously... not a golfer grumpy's Avatar
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    To lay bare my na´vetÚ on the subject... Didn't the L100/4311 type cabs have a short elbow type port (that can fall into the cabinet? Or am I improperly lumping the two together? An Original L100 impedance sweep would be handy (if published), to have an idea where to look for appropriate cab tuning (similar to near field mic placement ... at or almost in the port).
    It's uh... uh... it's down there somewhere, let me take another look...

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    Quote Originally Posted by Mr. Widget View Post
    Not to be flippant or short, but that doesn't really tell you much.

    If you are interested in exploring this topic I suggest you read this inexpensive book: Testing Loudspeakers
    LOL, it looks as if my ignorance is out there center stage with bright lights focused on it. Thanks for the link to the book.

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    Senior Member Don C's Avatar
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    Quote Originally Posted by rlovison View Post
    OK, here's what I did. I placed an inexpensive calibrated microphone (Behringer ECM8000) about two feet from the speaker with the port open, sent a 80Hz sine wave through the speaker and came up with a measurement. I then closed the port and the level of the 80Hz tone dropped about 4db.
    When you did this, you converted your l100s from bass reflex to acoustic suspension. It makes them less efficient, but the roll off at low frequencies is more gradual. Some people do like their L100 like this, and plumbers plugs can be used to make it more permanent.

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    Junior Member DHL's Avatar
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    I agree with other posts that removing the grills will sound best. Esp if the grill material was not provided by JBL, but an EBay seller. Who knows what the acoustic characteristics are?

    I am using ported B&W 803ds (I know they are not JBLs) with an HSU sub and they definitely sound better with the ports on the B&Ws plugged (I used foam Nerf balls). I suspect it has something to do with phasing of the port emitted sound vs the sub generated sound, but it is just speculation. Lower bass was a bit muddy and less transparent with ports open.

    BTW, sine wave testing is just about useless, esp in a room the speakers are used in. Best to use your ears with some good program material.

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