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Thread: Alternative to tapped inductor in 3110a or 3115a?

  1. #46
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    EDIT: I used the wrong value. Looks like it should be .02mh. Ignore blue line and question. thx.


    With the exception of the coils, I found the exact value components of the high pass filter in my spare parts drawer (for the -10db pad). I also found a .24mh coil that I was able to wind down to .20mh (measured it in speakerworkshop).

    Then assembled the network and took freq resp measurements, without the big coil, here are the results. 2445J on 2380A.

    Green = no hf boost
    Red = 2uf bipass
    Blue = .20mh + 3uf bipass

    Something unexpected is happening with that coil I unwound. It's a Solen S18.24 that I unwound from the outside.

    I'd like to get the response above 7khz up by about 3db. Any thoughts?

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  2. #47
    Senior Member Baron030's Avatar
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    Hi GrooveControl

    Yes, the correct value is 0.02mH. By using a 0.20mH coil instead, you can clearly see how the 0.2mH coil and the 0.3uf cap is resonating at about 6,499Hz. Call it one of those D’Oh moments . We have all had them at one time or another. When you finish un-wounding that coil down to 0.02mH, you will see just how tiny that thing really is.

    Baron030

  3. #48
    Senior Member Baron030's Avatar
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    Well, as you all know 4313B has already come up with a non-tapped autotransformer version of the 3110A network. But, I thought I would do some tests and see if I could come up with some suggestions for constructing autotransformers for use in other crossover network designs for which a non-tapped autotransformer version has yet to be created. And also possibly bust a few autotransformer myths in the process. Pictured below is the high pass section of 3110A network made from some stuff that I just had lying around. And since, I did not want to destroy a perfectly good Jantzen 15 gauge Air Core inductor for these tests, I decided to nick the insulation in just 2 places and then solder some leads to create taps. There are 3 things that I wanted to determine in my tests.
    1. When using air-core coils so the voltage drives come close to the mathematical formulas or not?
    2. And which formula appears to be more accurate? Turn Ration or Percentage of Inductance?
    3. And does it make a difference where the secondary is? Should it be near the core or can it be near the outside of the coil?

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  4. #49
    Senior Member Baron030's Avatar
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    The first big surprise was measuring the tapped coil’s inductance. It would appear that when it comes to inductance the total is much greater than the sum of its parts. So, any myth of stringing together a bunch of inductors together to create an autotransformer is totally “BUSTED”. The values just don’t add up right.
    Since I wanted to determine if there were any differences in secondary winding locations, I purposely tried to created two separate tap points with as close to same turn rations as I could.
    Starting with the secondary windings closest to the core, that is to say that pin #1 is the primary input and pin #5 is the secondary output and pins #2 & #6 are the common leads. The turn ratio formula calculates the output to be -6.7db. And the percentage of inductance formula calculates the output to be approximately -8.2db.
    Alternatively, with the secondary windings being closest to the outside of the coil, that is to say that pin #2 is the primary input and pin #4 is the secondary output and pins #1 & #3 are the common leads. The turn ratio formula, it calculates the output to be -7.5db. And the percentage of inductance formula calculates the output to be approximately -6.5db.
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  5. #50
    Senior Member Baron030's Avatar
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    Now we come to some test results. I should point out that in the 3110A network, when the HF Boost is set to its minimum setting that a 3.9 ohms resistor (R4) is bypassed. And so it would appear that the output level is actually higher than when the HF Boost is set its other positions. And that I used a 0.025mH coil instead of the required 0.020mH value. So, the “MAX” HF Boost comes in at a little lower frequency then it should be.
    Inside Secondary:
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    Outside Secondary:
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  6. #51
    Senior Member Baron030's Avatar
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    Admittedly, looking at the test results, it does have me scratching my head a lot. There are a few things that I kind of expected like the measured output voltage drives being lower than their calculated values. And if you think about it, when was the last time you ever saw an air-core power transformer? Or a tube amp with an air-core output transformer? So, it’s not surprising that measured values are lower because the mathematical formulas are not taking into account the greater core losses associated with air-core coils.
    Since, the curves are fairly flat at 2511Hz, I will use that frequency for direct comparisons between measured and calculated values.

    Inside Secondary:
    Calculated: Turn Ratio Formula: -6.7db, % of Inductance Formula: -8.2db
    Measured: No Boost: -9.34dbV, Mid Boost: - 12.26dbV Max Boost: -12.29dbV

    Outside Secondary:
    Calculated: Turn Ratio Formula: -7.5db, % of Inductance Formula: -6.5db
    Measured: No Boost: -7.4dbV, Mid Boost: - 10.24dbV Max Boost: -10.46dbV

    It would appear to me that the “Percentage of Inductance” formula might be far more accurate than a “Turn Ration” formula when it comes to constructing coils. And in the case of an air-core coil, the location of the secondary does not appear to make much of a difference. And yet it is strange, how 146 turns near the outside can generate more inductance and voltage drive then 160 turns wound near the center. So, I am thinking of creating another tap point near the core with hopefully something close to the outer tap’s 0.726mH of inductance and I will run some additional tests.

    Baron030

  7. #52
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    Since, the results of my first tests just did not seem to me as being very conclusive as to which secondary location came closest to fitting the mathematical model. I decided to create another tap and rerun my tests. And then I soon realized that I did count the total windings correctly. The correct total count is 386 turns and this changes all of the “Turn Ratio” formula calculated values in my previous test.
    Pictured below are the correct values and the new Clio Results:

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    Since, the curves are fairly flat at 2511Hz, I will use that frequency again for direct comparisons between measured and calculated values.

    0.785mH - Inside Secondary: Calculated Turn Ratio: -5.7db, % of Inductance: -6.8db
    Measured: Min Boost (R4 shorted): -7.12dbV, Min Boost (with R4): -8.90dbV, Mid Boost: - 9.90dbV Max Boost: -10.12dbV

    0.726mH - Outside Secondary: Calculated Turn Ratio: -8.4db, % of Inductance: -6.5db
    Measured: Min Boost (R4 shorted): -7.37dbV, Min Boost (with R4): -9.16dbV, Mid Boost: - 10.20dbV Max Boost: -10.43dbV

    Well, I think the differences between the two formulas and the measured results appear closer together in the case where the secondary winding is wound closest to the core.

    So, my recommendations for anyone wanting to make an autotransformer would be to use an iron core from an Erse I-Bar coil and follow the percentage of Inductance formula, and wind the secondary windings (taps) closest to the core.
    Baron030

  8. #53
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    So, my recommendations for anyone wanting to make an autotransformer...
    Mine would be to try and live life a little, travel a bit, see the world, enjoy a cold beer, volunteer in your community... something, anything, besides sitting around making tapped autotransformers. Hell, worst case, fire up a game of World of Warcraft for fuck's sake.

    Of course, if someone has already done all that other shit and the only thing left to do on the bucket list is wind an autotransformer then by all means...

  9. #54
    Senior Member Baron030's Avatar
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    But, what about the people that want that real “Autotransformer” sound?

    Whatever that is?
    I am kidding of course…

    No, I only did this research just to prove or disprove a few ideas and to kill a few myths.

    Baron030

  10. #55
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    I think it's great.

    Thank you for taking the time.

  11. #56
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    Quote Originally Posted by GrooveControl View Post
    EDIT: I used the wrong value. Looks like it should be .02mh. Ignore blue line and question. thx.


    With the exception of the coils, I found the exact value components of the high pass filter in my spare parts drawer (for the -10db pad). I also found a .24mh coil that I was able to wind down to .20mh (measured it in speakerworkshop).

    Then assembled the network and took freq resp measurements, without the big coil, here are the results. 2445J on 2380A.

    Green = no hf boost
    Red = 2uf bipass
    Blue = .20mh + 3uf bipass

    Something unexpected is happening with that coil I unwound. It's a Solen S18.24 that I unwound from the outside.

    I'd like to get the response above 7khz up by about 3db. Any thoughts?

    Name:  2445 a.jpg
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    Hi GrooveControl,
    Interesting tests. There is not much on the inernet about the actual, measured impact of the two HF boost circuits (the bridging 2uF capacitor on the one hand, and then the bridging 3 uF and 0.02mH coil. So I was pleased to find this thread.
    Did you eventually manage to get it right with the 0,02mH coil ?
    If so, would you by any chance have some measurements of the end result with the correct coil value ?

    Thanks in advance.

  12. #57
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    Along come a pair of LX13s...

    Hi Folks. It's been a few years since I acquired the parts for my next speaker, but still have not started the cabinets.

    Anyway, along come a pair of LX13s. The cost of the parts to build the xo in this thread isn't much cheaper, and these come in a nice jbl box, so I thought why not? I'll just add the CD compensation to it.

    So off I go to take some measurements. I thought I would share these with you guys, and I have a few questions at the end.

    The first chart shows the response of the 2445j on a 2380a horn with three different cap values in parallel with the hi-pass section of the xo. What I found interesting is how they all behaved. Note 3rd octave smoothing applied to make it easier to see.

    The blue line is a 3.0uf cap, which provides the most boost centred around 20khz.
    The red line is a 6.2uf cap, which provides the most boost centred around 15khz.
    The green line is a 9.1uf cap, which provides the most boost centred around 10khz.

    To my aging ears, the 9.1uf cap provided the most pleasing overall sound, even though it rolls off at the top end quicker than the other caps.

    Chart A


    The next chart shows the difference between having the 9.1uf cap in and out of the circuit. Again, 3rd octave smoothing applied.

    Chart B


    Finally, here is the second chart again, but without smoothing (just gating). Except for the dips around 8khz, I think it looks pretty good.

    Chart C


    Now I have a few questions:

    1 - Is there any way to keep the response from falling when using the 9.1uf cap? Ideally I would like the boost at 10khz offered by the 9.1uf cap and the boost at 20khz of the 3.0uf cap combined.
    2 - Looks like the biggest boost in chart B is about 7db at 11khz, but the HF gain is in the min position which should be a 10db reduction. Any ideas why I'm not getting a 10db boost?

    Thanks.
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