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Thread: JBL L Pad Question

  1. #1
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    JBL L Pad Question

    I notice that some of the JBL crossover schematics, like the 3133, have a strange looking L pad drawn in for the 2420 level control. My assumption is that it is actually a stereo pad with the wipers connected together.

    I guess that this was done to present a somewhat constant DCR to the preceeding components.

    I haven't actually seen one of these crossovers, could someone confirm that this is indeed the case?

    Charles.

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    Re: JBL L Pad Question

    The 3133 uses a standard L-Pad on the 2420. It's 16 ohm but doesn't present a true 16 ohm load to the components in front of it since it doesn't have a 16 ohm load behind it ("16 ohm" 2420 paralleled with a 20 ohm resistor doesn't present the required 16 ohms for a 16 ohm L-Pad to present a constant 16 ohm load).

    The "trick" part would be the 1.55 mH tapped autotransformer which in this particular case is center-tapped (-6 dB). Once you know the required DCR of this component it is easy to recreate.

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    Thanks Giskard, for the info. I thought that I may be able to get the center tapped inductor/autotransfomer fom JBL. I wonder if they still support parts for the 4133/L300? Probably not.

    I do have a something similar center tapped from a pair of Altec 3000 Hz crossovers (from 601D's) although I can't measure the inductance, I have looked at the frequncy response and they extend well below 1 Khz into a purly resistive load.

    Worst case, I guess I could lower myself to using something like the 3122 resistance voltage dividers.
    Charles.

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    Yeah, try ordering the tapped autotransformers. I think JBL Pro recently "found" a bunch of those 1.55mH for the L300/4333. If you can't get any from JBL Pro let me know and I'll dig mine out and measure them for you.

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    Or, maybe I can try something like two 0.775 mH in series!
    Charles.

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    An actual stock 1.55mH measures as follows:

    input to ground = 1.51 mH @ 0.79 ohms

    input to -3 dB tap = 0.15 mH @ 0.42 ohms
    input to -4.5 dB tap = 0.26 mH @ 0.46 ohms
    input to -6 dB tap = 0.39 mH @ 0.52 ohms

    -3 dB tap to ground = 0.76 mH @ 0.60 ohms
    -4.5 dB tap to ground = 0.52 mH @ 0.54 ohms
    -6 dB tap to ground = 0.35 mH @ 0.47 ohms

  7. #7
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    Giskard, I am confused. The crossover schematic I have for the 3133 and 3133A shows a simple inductor that is (center)tapped. Where are the taps you are describing?
    Charles.

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    I have the actual core that that particular inductor is made from so it has various taps on it. If you take apart a stock L300/4333 you may notice that the superfluous taps have been snipped off. When one ordered the particular N333/3133 part number in the past, JBL would often send it out with the -3 and -4.5 taps already snipped off. I asked that this particular one be sent with all the taps intact.
    Attached Images Attached Images  

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    Oh, I should probably point out that this tapped autotransformer was also used in the LX80 and LX80A.

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    Thumbs up

    Nice Pic

    And I remember you lamenting about your photo skills .

    What changed ??

    <> EarlK

    Say, what's the @ physical size of that inductor ?

  11. #11
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    I used sunlight and I sacrificed a chicken to the God of Kodachrome.

    The size as you are looking at it in the picture is 1-7/8" wide x 2-5/16" high x 1-3/4" deep.

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    Hi Chas

    Re; Your 3133 crossover mockup;

    You might consider substituting a 1.5 or 1.6 mh regular-style inductor (coil) in place of that center tapped 1.55mh inductor . The center tapped coil that JBL uses, does offer a 6 db padding for the circuit - which BTW, you will still need the padding from somewhere in your circuit. But you might as well do what JBL did in the 3133A Circuit - which in "Biamp" mode - forgoes the 6 db of attentuation from the center tapped inductor and instead employs a regular fixed pad a little further upstream. In the 3133a , JBL makes a fixed Lpad from a 4ohm series resistor and a 5ohm shunt resistor. Dependant on the rotational setting of the 16 ohm Lpad - I figure this fixed Lpad offers about 7.5 db of attentuation - if it is "seeing" a 7.2 ohm load ( which it will - in certain areas of that 16 ohm Lpads "rotation".

    Do as JBL did and omit the 7.5 ohm resistor that occurs in front of the 1.55 mh inductor - if you go with a fixed Lpad instead of the centertapped inductor. The 3133A does this when it switches the crossover into biamp mode - (study the schematic and the effect of the switch positions ) .

    Solen "On Line Parts" sell a 1.5 mh inductor , 18 gauge wire, air core, for about $ 7.25 cdn. This coil has a dcr of .6 ohms - meaning - to satisfy the 3133 circuit requirement for DCR as posted above by Giskard - you should put a .2 ohm resistor in series with the coil.

    To get wirewound resistors , when in the Toronto area, I'll go "Supremetronics Inc." if I'm in downtown . They are located at the 300 block Queen St West area. They have a decent selection of 10 & 20 watt values - mostly 10% tolerance - 40¢ for the 10 watters - I believe .

    regards <> Earl K

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    Earl, you provide a very simple, elegant solution. I had completely discounted the 3133A because I had intended to go pasive for now.

    I should have looked a little closer at it!

    Okay, now I am ready to order some parts and get moving on this thing!
    Cheers,
    Charles.

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    I don't believe it is that simple.

    Do a search on this forum for my non-tapped autotransformer solution for the N333/3133A.

  15. #15
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    Search is a no go!

    I just did a quick look around - & couldn't find a rework of that circuit .

    Might have sunk, along with the last forum to the bottom of the Dot-Com shoals.

    & Speaking of "forcing a submarine to the surface" - how about answering your PMs .

    <. Earl K

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