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Thread: I Made an Air Core Autoformer for the LX5 .... and it didn't work

  1. #1
    Senior Member Eric M.'s Avatar
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    I Made an Air Core Autoformer for the LX5 .... and it didn't work

    I used 18 gauge wire to wind (I was hoping to get the DCR between taps a little closer to the original iron core version)
    I got the inductance at the taps pretty close to the original JBL LX5 autoformer (The tolerances of the autoformers was pretty sloppy back then)
    I installed it in a rebuilt crossover, everything new with air core coil for the 6 mH at the low end.

    I tested the crossover with the new air core autoformer and then swapped in the original iron core.

    They both crossed over at about the same, the slopes were similar, both flattened out at about 1K.
    The iron core only dropped about 1.25 DBV all the way to 20K.
    The air core was down 3 DBV at 7K and then dropped another 5.5 DBV on it’s way to 20K.

    DCR at all the testing points between the iron and air were almost double but the max was only 1.2 ohms from the start to the end of the air core autoformer vs. .64 ohms for the iron core. I can't see how a higher DCR would only alter the high end.

    Any ideas? Why would it roll off at the high end like that?
    I did wind a second autoformer and tested that. It had the same results

    I can post data if anyone has ideas.

    Thanks,

    Eric

  2. #2
    Administrator Robh3606's Avatar
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    Hello

    Is the roll off consistent on all taps?? It doesn't change from tap to tap?? So the whole coils is 3.85 mH and DCR 1.2 ohms.

    How tightly wound is it?? Just thinking stray capacitance but not sure it would be enough to matter at audio frequencies.


    Rob
    "I could be arguing in my spare time"

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    Hi Eric,

    I spent a few hours simulating tapped air cor high pass filter - attenuators tonight.

    Your measurements are consistent with my simulations. Unfortunately the tapped air core approach is only effective in band pass filters because the inductance is unavoidable.

    I did find a fix however by shunting an RC bypass circuit across the inductors to the load. The values shown need to be adjusted in SPICE each time a different attenuation is required. Overall 2.1mH seems to work best. The R6 resistance will need to be trimmed with different taps.

    I have provided you with two scenarios by swapping the inductors from top to bottom. The 1.5mH inductor is 1.2 ohms (just a guess of a typical air core of the value. The 600u inductor is 0.47 ohms again a typical dcr for this value.

    There is no real advantage in attempting a tapped inductor air core attenuation circuit when you can employ a T section constant impedance attention as used in the 4343 network.

    The green curve is the 500 hertz Butterworth crossover created by Linear X Leap 5 which is actually an 800 hertz voltage drive with the std 8 ohm load. The C value was 17.5 uF and the L value was 2.26mH).

    JBL compression drivers typically have an impedance of 12 ohms so that is fine.

    The RC values l have shown seem to work the best so give that a try. The impedance is a flat 8 ohms on this circuit.

    I hope this helps solve your request.

    Ian
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  4. #4
    Senior Member Eric M.'s Avatar
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    Hi Ian,

    Thanks for running the simulation, at least I know it wasn't a mistake on my part. For this project, I was trying to to rebuild the LX5 but keep it as original as possible. Same design, just newer, better performing components. As soon as I start adding components, shunts, circuits, it's no longer an LX5 but a tweaked or modified or really just a different 500 Hz crossover al together.

    As you mentioned above, there are better ways to attenuate. If I'm just looking for a better 500 Hz crossover for my JBL drivers, there are (and I know JBL purists will cringe when I say this) better crossover designs I could start with. I would be better off designing around the small thiele parameters of my drivers instead of using a universal type crossover like the LX5.

    I searched the net, from one end to the other, for anyone who had taken on such a task and couldn't find a definitive answer. Now it's posted for the next person who is contemplating purchasing a coil winder to make his own air core autoformers for their JBL crossover. If it's a 3-way and the autoformer is used on the mid band, maybe, but test it first to make sure it isn't rolling off before your high crossover point starts it's slope.

  5. #5
    Administrator Mr. Widget's Avatar
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    I applaud your dedication, but if your goal is to maximize performance rather than maintaining a museum piece I suggest you save the LX5s as original and put them on the shelf.

    If you can take the appropriate measurements you should be able to design and build an improved crossover. And even if you don't have all the needed measurement gear you can certainly wing it and play around with it until you achieve the desired results.


    Widget

  6. #6
    Senior Member Eric M.'s Avatar
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    The goal is to maintain a museum piece. The key word being "Maintain". I want to use the crossover and have it perform as it did when it left the factory.
    Think of it like a vintage 64 Corvette. Almost 50 years old, the original engine probably has 250K miles on it and is missing on 4 cylinders. The suspension is shot, the brakes leaking.
    You have a few options:

    Park in in your garage and show it off to anyone who comes by. Maybe winch in on a trailer and roll it off when you go to car shows. (Kind of boring)
    OR
    Restore it to it's original specs. Original motor, factory spec. compression, cam shaft etc. New bushings on the suspension and new brakes. (You get to drive it around which is always fun)
    OR
    Put that 640HP, supercharged motor in it, 8 speed manual transmission, high performance coil over suspension at all 4 corners and 6 piston Wilwood disk brakes all around. (It's the coolest ride in the neighborhood, maybe city. All the Corvette purists hate you for destroying an original 64 Vette when you could have done the same thing with a worthless old Vega!)


    I'm in the middle. I want an LX5 that I can actually listen to. When it comes time for the best of best crossovers for my set up, I'll do it on a Vega, I mean PC board!

  7. #7
    Administrator Robh3606's Avatar
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    Inductors have no moving parts so just use the originals they are for all intent and purposes indestructible. You will have more issues with the switch

    Rob
    "I could be arguing in my spare time"

  8. #8
    Senior Member Eric M.'s Avatar
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    Quote Originally Posted by Robh3606 View Post
    Inductors have no moving parts so just use the originals they are for all intent and purposes indestructible.

    Rob
    Good point. I was hoping I could improve on the sound with an air core for the autoformer and eliminate saturation / distortion.
    Really, I think I got a little obsessed with the idea of an air core autoformer. It was a fun venture, it would have been nice if it netted a bit of success

  9. #9
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Eric M. View Post
    I'm in the middle. I want an LX5 that I can actually listen to. When it comes time for the best of best crossovers for my set up, I'll do it on a Vega, I mean PC board!
    What system is this part of?

    FWIW: A friend who restores cars had a '60 Vette in his shop and suggested I sit in the driver's seat for a laugh. Jeez... I have no idea what GM was thinking. The steering wheel and pedal positions are just terrible. Just driving it to the grocery store would be unpleasant. Sometimes museum pieces are best left in their display cases.


    Widget

  10. #10
    Senior Member Eric M.'s Avatar
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    [QUOTE=

    FWIW: A friend who restores cars had a '60 Vette in his shop and suggested I sit in the driver's seat for a laugh. Jeez... I have no idea what GM was thinking. The steering wheel and pedal positions are just terrible. Just driving it to the grocery store would be unpleasant. Sometimes museum pieces are best left in their display cases.


    Widget[/QUOTE]

    LOL. A friend of mine has a 70 Hemicuda. At 425 HP we thought that was the fastest, wildest ride back in '83. He now also has a 2016 Viper with 640 HP. It's twice the ride, comfortable, has A/C and can run on pump gas. I will say the Hemicuda is still a heck of a ride, I don't think I'd ever make that a "Look at" only car. But, I don't think I'd ever take it on a road trip where as the Viper you could.

    Back to the LX5. This wasn't for any particular system, mostly an experiment.

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    Hi Eric,

    Thank you for elaborating on your plan.

    If the LX5 is still intact you could conceivably replace specific parts with like values using more modern and improved parts. For example the 16.5uF capacitor could be replaced with an Audience Auricap capacitor. This would be a noticeable improvement. A number of restorations have been done with Auricaps. Then replace the compression driver diaphragm with a Radian equivalent diaphragm which are almost replica of the JBL and Altec originals but with an improved alloy and suspension. Again you will hear a noticeable improvement.

    It will look the same but it will put a smile on your face.

    Of course todays compression drivers, horns and woofers are superior in many respects but that is beside the point. The benefits of the driver efficiency are the most appreciated aspect of these hey day components. But if you want to listen to the original design in all its glory then the above suggestions would most certainly win applause if demonstrated at an Audio Club Meeting.

  12. #12
    Senior Member Eric M.'s Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    Hi Eric,

    Thank you for elaborating on your plan.

    If the LX5 is still intact you could conceivably replace specific parts with like values using more modern and improved parts. For example the 16.5uF capacitor could be replaced with an Audience Auricap capacitor. This would be a noticeable improvement. A number of restorations have been done with Auricaps.

    That was my plan. I have replaced all the caps with film caps, resistors with non-inductive type, replaced the 6 mH iron core coil with an air core version and replaced the rotary switch. The only component I was reusing was the autoformer. I was so close to all new, improved components using the original design!
    Like you said, unless you plug that autoformer into the wall, there isn't much you can do to damage it.

  13. #13
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    In more detail most of the JBL consumer woofers had a very low Qts due to the large magnets and large Vas. So JBL could get away with relatively high dcr values of 0.5 ohms in the woofer chokes. The upshot of this was that iron core chokes were a much lower cost alternative at the time.

    It might not be worthwhile but you might consider an ERSE Super Q laminated core 6.00 mh choke which would have lower saturation than the original core. ERSE might even be interested in winding you a new tapped laminate core choke with improved magnetic properties for the horn filter. My suggestion is to maintain the original electrical properties by measuring the output signal voltage drive of the crossover with an 8 ohm resistive load on graph paper for each tap using REW or an osciliscope. Then provide that to ERSE if they are interested in winding a custom tapped transformer. They would have test equipment, winding machines and cad to design a transformer. Jensen transformer may also be able to help.


    There was a guy running a cottage business in the USA winding tapped audiophile audio transformers for line level preamplifier attenuators. You could see if he is able to help. https://intactaudio.com/atten.html. But that looks awfully expensive.

    That way you are staying with the original design but with modern replacement parts. You could even build a replica crossover with a aluminium container and foil cal. Some people have done that.

    So there are still possibilities.

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    Quote Originally Posted by Eric M. View Post
    The goal is to maintain a museum piece. The key word being "Maintain". I want to use the crossover and have it perform as it did when it left the factory.
    Think of it like a vintage 64 Corvette. Almost 50 years old, the original engine probably has 250K miles on it and is missing on 4 cylinders. The suspension is shot, the brakes leaking.
    You have a few options:

    Park in in your garage and show it off to anyone who comes by. Maybe winch in on a trailer and roll it off when you go to car shows. (Kind of boring)
    OR
    Restore it to it's original specs. Original motor, factory spec. compression, cam shaft etc. New bushings on the suspension and new brakes. (You get to drive it around which is always fun)
    OR
    Put that 640HP, supercharged motor in it, 8 speed manual transmission, high performance coil over suspension at all 4 corners and 6 piston Wilwood disk brakes all around. (It's the coolest ride in the neighborhood, maybe city. All the Corvette purists hate you for destroying an original 64 Vette when you could have done the same thing with a worthless old Vega!)


    I'm in the middle. I want an LX5 that I can actually listen to. When it comes time for the best of best crossovers for my set up, I'll do it on a Vega, I mean PC board!
    ...OR...

    Engine was already a replacement block with non-matching numbers so build the car the way I want. Bring the engine up to ~375-400 hp, with a 3.70 :1 rear end, put in a 5-speed with 0.64 overdrive for freeway cruising, upgrade the suspension with Dick Guldstrand parts, put on front disk brakes with a dual master cylinder and line lock on front brakes (for burn-outs and hill holding), redo the paint and interior adding personal, custom touches, and let the purests gawk.

    My '64:

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    But why stop there? Use the car to start a whole new "business venture" for parts that were never made but should have been.

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    http://headerbracket.com/index.html

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