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

  1. #31
    Senior Member 4343's Avatar
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    Quote Originally Posted by GrooveControl View Post
    Wow! Just what I was looking for! Thanks!

    One Q. How much impact will the ab switch have on the top end? Can you add to the chart?
    There is graph here:

    http://www.audioheritage.org/vbullet...hread.php?5376
    Mike Scott in SJ, CA
    Drive 'em to the Xmax!

  2. #32
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    Quote Originally Posted by 4343 View Post
    I believe that graph represents the different positions of the autoformer affecting the gain of the HP section of the crossover. I think GrooveControl was wondering about the two positions of the CD-compensation switch. Perhaps I'm wrong, though it would be my fist time...it the last two minutes.

  3. #33
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    Here's one showing the effects of the HF compensation switch. High, Low, None.
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  4. #34
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    Quote Originally Posted by 4313B View Post
    Nope. You cannot treat a tapped autotransformer like an inductor.
    That's easy if you just build one with a standard (non-tapped) 3.3mH inductor. I just measured mine set to 0dB, i.e. all the taps floating open and the signal drawn from the top of the 3.3 mH inductor ((the junction of R2&R3, C5 and L3 connected to the junction of S1 pin 3 and R4). Using a 16 ohm L-Pad connected to the output pins I adjusted it to read -10dB (compared to the output level when it was not there). I then measured it's resistance out of circuit. From ground to the wiper, I measured 4.5 ohms, and from the wiper to the plus output I measured 40 ohms.
    I want to address this because I think it is important to show why.

    First schematic shows substituting in a 3.3 mH inductor for the 3.3 mH tapped autotransformer as well as adding in the -10 dB point values of a 16 ohm L-Pad as mentioned above.

    Second schematic shows the same but with the other stock resistors removed as well.

    I also want to make sure everyone knows why there is a 20 ohm shunt resistor between the stock 8 uF capacitor and the stock 3.3 mH tapped autotransfomer. It is there to mitigate the inductive reactance of the tapped autotransformer so that the 8 uF capacitor behaves as it is intended to. This shunt resistance can be found in many JBL networks using a tapped autotransformer. It is usually high power, for example 40W in the 4331/4333/L300, or made up of two or three lower power resistors to handle the resulting heat build up. In this instance it is made up of two 39 ohm 10W. It is superfluous with a standard inductor.
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  5. #35
    Senior Member Baron030's Avatar
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    Thanks, 4313B for taking the extra time in showing us why.
    Wow, do those curves look really messed up.
    I guess that really proves that you can’t treat a tapped autotransformer like an inductor.
    So, in true MythBusters fashion, you know how to call it…
    Name:  Busted.JPG
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  6. #36
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    Quote Originally Posted by 4313B View Post
    Here's one showing the effects of the HF compensation switch. High, Low, None.
    Thanks 4313B, that's what I meant.

    I think I'm going to use the N4671 lo pass filter as I'm using the 2035HPL which doesn't seem to need the zobel. I measured the imp of mine a while back and they are 8ohms up to 5khz.

    http://techtalk.parts-express.com/sh...P-Measurements

  7. #37
    Senior Member 4343's Avatar
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    Quote Originally Posted by robfive View Post
    I believe that graph represents the different positions of the autoformer affecting the gain of the HP section of the crossover. I think GrooveControl was wondering about the two positions of the CD-compensation switch. Perhaps I'm wrong, though it would be my fist time...it the last two minutes.
    The map portion of that graph is labeled.
    (RED) -6dB tap, MAX HF
    (GRN) -8dB tap, MAX HF
    (BLU) -6dB tap, MED HF

    Interesting that the different attenuation taps don't affect the 20KHz level on MAX HF, so the curve appears different on the two.
    Mike Scott in SJ, CA
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  8. #38
    Senior Member 4343's Avatar
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    Quote Originally Posted by 4313B View Post
    I want to address this because I think it is important to show why.

    First schematic shows substituting in a 3.3 mH inductor for the 3.3 mH tapped autotranformer as well as adding in the -10 dB point values of a 16 ohm L-Pad as mentioned above.

    Second schematic shows the same but with the other stock resistors removed as well.

    I also want to make sure everyone knows why there is a 20 ohm shunt resistor between the stock 8 uF capacitor and the stock 3.3 mH tapped autotransfomer. It is there to mitigate the inductive reactance of the tapped autotransformer so that the 8 uF capacitor behaves as it is intended to. This shunt resistor can be found in nearly every network JBL made using a tapped autotransformer. It is superfluous with a standard inductor.
    Thanks for clearing this up!

    My next faux pas involves discovering a huge inductor in my bass cabs. I've been collecting 2226J's to populate them, and I finally have 4 of them. Today I took out the pair of K-140's and what looked to be 2225's. (On closer inspection one was a 2205A and the other one had a sticker on it saying it was a 2205A, but it covered the real model 2220B...)
    The inductor is 18mH 16ga feeding just the bottom woofer. It seems I should have been looking for 2226H. Or I can look at changing the LP filter to work at 16 ohms.
    Mike Scott in SJ, CA
    Drive 'em to the Xmax!

  9. #39
    Senior Member Baron030's Avatar
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    Interesting that the different attenuation taps don't affect the 20KHz level on MAX HF, so the curve appears different on the two.
    Hi 4343
    There is a simple explanation why the different attenuation taps don’t affect the response curve at 20KHz on the MAX HF setting.
    The 3.0uf cap and the 0.02mH coil form a RLC series resonant circuit and it’s resonating at 20,551Hz, which means the impedance drops to nearly zero.
    So, you get no voltage or signal drop.

    Here is a link to a RLC series resonant circuit calculator and some other handy calculators as well:

    http://www.lautsprechershop.de/index...ngkreis_en.htm

    Baron030

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    Quote Originally Posted by 4313B View Post
    Here is the standard value equivalent. I forgot to force it in the original.
    Not sure if I'm reading this correctly. Does R6 represent the driver? Or is that a real resistor?

    Also, are 10 watt resistors suitable for application? Or should I double the value and run two in parallel for each?

    Thanks again.

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    Yes, R6 is the 16 ohm driver load. It's easier than putting in a driver model along with a 16 ohm objective curve.
    An actual driver impedance curve is rather pointless since this is a "generic" JBL network.

    I would use double, it keeps them more linear at any rate.

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    How about Cap ratings, Solen offers 250V, 400v and 630v. I usually use the 250v, but I'm thinking 400v for this application.

    Is 400v enough for the lo pass section given the woofer can take 600w?

  13. #43
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    JBL uses lots of 100V components. I use 250V. I've never had any issues. However, I have come to the realization over the years that perhaps I don't flog my systems like some other forum members do.

  14. #44
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    Quote Originally Posted by GrooveControl View Post
    How about Cap ratings, Solen offers 250V, 400v and 630v. I usually use the 250v, but I'm thinking 400v for this application.

    Is 400v enough for the lo pass section given the woofer can take 600w?
    400V?

    100V is rated at 625W at 16 ohms... 1250W at 8 ohms. 250V at 16 ohms gives you almost 4KW.


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  15. #45
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    Some guys swear the higher voltage capacitors sound better, especially on the top end. That's why they use 400V capacitors on their tweeters.

    The higher the voltage rating the more expensive and physically larger the capacitor for any given capacitance. JBL uses 100V because they are less expensive, physically small so the whole network isn't overly large, and perfectly adequate for the intended application.

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