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  1. #1

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    Default Need Help Diagnosing Passive Crossover Trouble

    I've got a pair of crossovers that I need help diagnosing. I've attached diagrams of the crossovers, which are just high-pass networks in a biampable speaker.

    When I first got the speakers, used, the tweeters were muffled. I pulled the xover, and found that the 1 uf cap was blown out. So I ran an electronic xover to the tweeter, and it sounded great.

    I finally got the replacement caps, and soldered them in place. I hooked up the speakers, and bleh, still muffled, but perhaps not quite as much as before.

    The two resistors measure fine. However, as you can see in the pic, one lead on the 30 ohm resistor was cut. I don't know if this was done at the factory or by a user. It seems that the two resistors are in parallel, so I think that hooking it back up would lower resistance to the tweeter and increase output.

    My understanding of circuits is very weak, so I would really appreciate any help.
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    Hello,
    Thanks for posting on the Forum. I believe that second resistor is the problem, with it in place the combined resisitance will be around 3.5 Ohms. This will raise the level of the tweeter slightly.
    Regards, Ken

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    Quote Originally Posted by Kenneth Swauger View Post
    Hello,
    Thanks for posting on the Forum. I believe that second resistor is the problem, with it in place the combined resisitance will be around 3.5 Ohms. This will raise the level of the tweeter slightly.
    Regards, Ken
    Thanks for the response, Ken. That was my conclusion, as well. I hooked up the 30 ohm resistor, and I believe I noticed an increase in tweeter output. After hooking it up, I got 3.5 ohms across one, and 3.6 across the other.

    The increase in output may not be adequate. I'll need to hook up both channels and have a listen, but it's still a little muffled.

    Here's more: I put the xovers back in place, and measured the resistance across the binding posts, or I tried to. I got 3.9 ohms on the low on one speaker, and 3.8 on the other (sig figures may mean they are 3.84 and 3.86, so could be ok). BUT I have NO CONTINUITY ACROSS THE HIGH SECTION. Now this is really blowing my mind. I don't get continuity across either of the caps on the high board, but the caps on the low board give me continuity. Could this be because the caps on the low board are directional, and those on the low board are not? I don't get continuity across the extra replacement caps I ordered, either.

    Then I measured across the tabs that connect to the speaker wires (the crossover output tabs) above and below. The highs measure .6 ohms, and the lows measure 8.2.

    The tweeter is nom 6 ohms, and the woofers are nom 8.

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    Hello,
    I believe the capacitors are preventing a DC resisitance measurement since they are not conductors. Do you happen to have a CD that has some specific test tones on it? If so, you could measure the AC output of the two sections, of the crossover with a 4 Ohm resistor as a load. Then repeat this with the other speaker's crossover. You would use several frequencies to get a very basic frequency response of the crossover.
    Regards, Ken

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    Quote Originally Posted by Kenneth Swauger View Post
    Hello,
    Thanks for posting on the Forum. I believe that second resistor is the problem, with it in place the combined resisitance will be around 3.5 Ohms. This will raise the level of the tweeter slightly.
    Regards, Ken
    Could you educate me a little on what the formula for the resistance is?

    Is it the square root of each, added together, and divided by the number of resistors? That's my guess..

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    Quote Originally Posted by DaveMuell View Post
    Could you educate me a little on what the formula for the resistance is?

    Is it the square root of each, added together, and divided by the number of resistors? That's my guess..
    1/(1/x +1/x) for parallel (the inverse of the sum of the inverses)

    series just adds

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    Quote Originally Posted by Kenneth Swauger View Post
    Hello,
    I believe the capacitors are preventing a DC resisitance measurement since they are not conductors. Do you happen to have a CD that has some specific test tones on it? If so, you could measure the AC output of the two sections, of the crossover with a 4 Ohm resistor as a load. Then repeat this with the other speaker's crossover. You would use several frequencies to get a very basic frequency response of the crossover.
    Regards, Ken
    I don't have one, but I need to get one, for sure. I hooked them up in stereo, and they sound a little better, but still a little muffled. Hard to describe, but I could say they sound overly warm, not bright enough. I can't believe that this was PSB's actual design, especially since it sounded SO much better with the active xover. With the active, I had to turn down the gain on the high freqs significantly to balance the response.

    I do have a Rane spectrum analyser/eq with a pink noise generator that I could hook up to see what they are doing. They are just in my living room, but if they are that far out of balance, I figure the response should be obviously skewed.

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    Hello,
    Another method to determine the resistance in parallel is the product of the two resistors divided by their sum. The inverse method is convenient when dealing with more than two resistors in parallel.
    Having a spectrum analyzer would work especially if it has a line level input as opposed to a microphone. Amplify the pink noise give it to the crossover, put a 4 Ohm load on the crossover and look at it as a line level signal. Keep the levels low to not overload the analyzer.
    Regards, Ken

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    Thanks, Ken. I'll try that later.

    I've got them running in stereo, and just gave it a fresh listen. Sounds pretty good. I'm sure re-connecting the 30 ohm resistor helped. I've gone from 4.2 ohms to 3.6, and the tweeter is louder now. I just wonder why it was cut. Seems like one of those factory "running mods" or whatever they're called, but I guess some uninformed person may have tried it later.

    Now to begin the real A-B-C testing:
    PSB Stratus Bronze
    Paradigm Studio 20 V1
    Polk LS-50

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    No problem, glad to be of help!
    Take care, Ken

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    Quote Originally Posted by soiset View Post
    Seems like one of those factory "running mods" or whatever they're called, but I guess some uninformed person may have tried it later.
    Im only guessing, but in the QC stage PSB may be adding varying values of parallel resistance to compensate for slight sensitivity differences between tweeters.

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    Quote Originally Posted by soiset View Post
    BUT I have NO CONTINUITY ACROSS THE HIGH SECTION. Now this is really blowing my mind. I don't get continuity across either of the caps on the high board, but the caps on the low board give me continuity. Could this be because the caps on the low board are directional, and those on the low board are not? I don't get continuity across the extra replacement caps I ordered, either.

    Then I measured across the tabs that connect to the speaker wires (the crossover output tabs) above and below. The highs measure .6 ohms, and the lows measure 8.2.

    The tweeter is nom 6 ohms, and the woofers are nom 8.
    Caps don't pass DC. If you short a cap out, then read the DC resistance, you notice it starts low and just increases to nearly infinite. That's a good cap.

    You should never be able to get continuity across a cap. If the woofer has slide on terminals, slide one off and try it again. Make sure those caps are good. Measuring DC in a circuit like this has very limited uses.

    You're getting a short from the tweeter terminals because there is one, the inductor. It shorts very low frequencies and ignores higher ones.

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    response from PSB:

    The 30ohm being cut sounds like a production line adjust to better match
    the speakers to the golden reference (its changing the level of the
    tweeter by ~1/4-1/2dB).


    It definitely sounds brighter with the resistor connected, but perhaps not brighter than the Paradigms. Dunno what "the golden reference" is.

  14. #14
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    So I guessed correctly.

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