Results 1 to 10 of 10

Thread: L 101 crossover point and polarity

  1. #1
    Senior Member davidpou's Avatar
    Join Date
    Jul 2010
    Location
    Paris
    Posts
    121

    L 101 crossover point and polarity

    Hi everyone,
    I have been torturing my L 101 since I have them (around 6 years) and recently my new torture instrument is a mini DSP 2x4 HD. So the passive crossover has been removed (a condo and a pad is protecting the compressions) and the speakers are run in bi amp with two crown D75 in mono for the woofers and a MC275 mkII for the tweeters.
    I am playing with the crossover part. I know JBL crossed the drivers at 1500 Hz but there is plenty of space to move this point as both drivers once equalized, overlap from 400 Hz up to 4000 Hz ! (Amazing drivers to me…)
    My question bears on polarity: I was convinced that both drivers were of negative polarity. (I checked my self the woofer with a 9V battery). But if I cross them in LR 24dB and put both drivers in phase I get a dip while if I inverse the tweeter polarity I get the nice flat result you see on the picture….
    I precise that I respected JBL principle and double checked the wiring: everything is black on black and red on red and the polarity is reversed (for both drivers) at the speaker terminal….
    By the ear I have difficulty to tell the difference honestly but since I inverted the tweeters it seemed more harmonious and homogeneous…..
    How come ?
    What crossover point would you choose ?
    Name:  SPEAKER WITH WITH AND WITHOUT PHASE INVERSION ON TWEETER.jpg
Views: 415
Size:  76.9 KBName:  SEPARATE DRIVERS LEFT SPEAKER.jpg
Views: 344
Size:  75.4 KB

  2. #2
    Senior Member grumpy's Avatar
    Join Date
    Oct 2003
    Location
    SoCal
    Posts
    5,341
    Both amp types preserve or invert phase similarly?

  3. #3
    Senior Member davidpou's Avatar
    Join Date
    Jul 2010
    Location
    Paris
    Posts
    121
    Well I would think an amp cannot invert phase... can it ?
    They are on different plugs though and that might be the cause....if the plugs are phase inverted ... I will check this...

  4. #4
    Senior Member
    Join Date
    Sep 2014
    Location
    Las Cruces, NM
    Posts
    114
    A L.R. filter must have the drivers phase aligned in space to have a flat response on axis. a member here, 4313b used an alignment method that seems to work well. He set the two drivers electrically out of phase. Varies the time delay to the woofer for a max null at the crossover. Set the phase back to in phase. The response should be flat on axis at the crossover. This is a common problem with horn systems. The horn driver is not in the same plane as the bass driver.

    Regards,

    John

  5. #5
    Senior Member grumpy's Avatar
    Join Date
    Oct 2003
    Location
    SoCal
    Posts
    5,341
    Well I would think an amp cannot invert phase... can it ?
    Certainly it can. Some do, some do not. Between using different manufacturer amps
    and bridging, it's definitely worth checking.

  6. #6
    Senior Member hsosdrum's Avatar
    Join Date
    Oct 2009
    Location
    Burbank, CA
    Posts
    295
    Quote Originally Posted by grumpy View Post
    Certainly it can. Some do, some do not. Between using different manufacturer amps
    and bridging, it's definitely worth checking.
    He may be quibbling the difference between *phase* and *polarity*.

  7. #7
    Senior Member grumpy's Avatar
    Join Date
    Oct 2003
    Location
    SoCal
    Posts
    5,341
    Good point...perhaps I've sine waves on the brain.

    Signal inversion (positive voltage in, negative voltage out) with no delay
    added is the same as polarity swapping at the driver terminals... hence my question
    about the amps. Sounds like it was sorted out sufficiently/empirically

    What's being done in the DSP re crossover slopes (24LR filters should sum flat in magnitude at crossover)
    or driver phase-center + filter delay compensations could affect things as well, but I'm assuming those
    are not the issues being questioned.

  8. #8
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,695
    You know, the 1/2 wave length of ( 1000hz ) is right around 6.75". ( ie; 1124/1000x12/2 )

    It's not hard to imagine that the all important "acoustic center" of the horn driver (combo) is behind that of the woofer ( by about, just that distance ).

    That half-wave differential ( in acoustic centers ) will cause a null around the crossover point (with an LR24 slope ) .



    David, your understanding of how crossovers really work ( & how they interact with acoustic devices ), is under-developed ( & only partial ).

    You need to overlay ( account for ) the acoustic realities onto the (predicable) electrical slopes, to arrive at what will really occur ( at crossover ).


  9. #9
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,695
    Quote Originally Posted by johnlcnm View Post
    A L.R. filter must have the drivers phase aligned in space to have a flat response on axis. a member here, 4313b used an alignment method that seems to work well. He set the two drivers electrically out of phase. Varies the time delay to the woofer for a max null at the crossover. Set the phase back to in phase. The response should be flat on axis at the crossover. This is a common problem with horn systems. The horn driver is not in the same plane as the bass driver.

    Regards,

    John
    David, delay the woofer just like John has suggested .

    Then your L.R.24 slopes should combine properly ( with everything being held in positive polarity ).

    FWIW, I haven't used a pro amp that inverted the output signal ( in decades ) .

    That's one good reason to use pro amps I guess .


  10. #10
    Senior Señor boputnam's Avatar
    Join Date
    Apr 2003
    Location
    northern california
    Posts
    7,007
    First of all, David - some nice measurements.

    Quote Originally Posted by johnlcnm View Post
    A L.R. filter must have the drivers phase aligned in space to have a flat response on axis. a member here, 4313b used an alignment method that seems to work well. He set the two drivers electrically out of phase. Varies the time delay to the woofer for a max null at the crossover. Set the phase back to in phase. The response should be flat on axis at the crossover. This is a common problem with horn systems. The horn driver is not in the same plane as the bass driver.
    John
    This is a pretty important post - what it does, without saying, is - it discriminates between electric and acoustic phase. The LR filter needs things in-phase electronically to produce a flat response at crossover. Fine. But what you are measuring is the acoustic phase - this has to do with the effects of (delays caused by) the LR crossover filters, and the acoustic centers (separation) of the drivers.

    There's one slight caveat to John's post: There is always acoustic time delay from the LF - the magnitude of which varies by the knee of the LPF (i.e., the amount of delay is frequency dependent). 4313B didn't "vary" the time delay "to" the LF (that couldn't be done this way), he just inverted the LF (put it 180° electrically out of phase) which in that instance smoothed the acoustic response.

    Quote Originally Posted by davidpou View Post
    My question bears on polarity: I was convinced that both drivers were of negative polarity. (I checked my self the woofer with a 9V battery). But if I cross them in LR 24dB and put both drivers in phase I get a dip while if I inverse the tweeter polarity I get the nice flat result you see on the picture….
    Unless a purist on what JBL had done, I wouldn't get too consumed by mimicking their settings. As you note, the frequency response of those drivers gives great latitude in setting a crossover point. Bear in mind the vocal range, and strive to keep the point out of the most audible portion of that range. You may find you like an acoustic dip at crossover - that is a cancellation which may clean-up the acoustic response in your setting for your taste. Or, maybe you don't.

    FWIW, I toggle to inverted subs at every show I mix - during system check, sound check and live during the show. Physics give us a good starting point but the venue behavior is dynamically affected by temperature, humidity and body count, and if I can clean-up the interaction between the subs and mains, I'm all over it...
    bo

    "Indeed, not!!"

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. 4722-HF (2432H) to 2226H crossover point/slope (active crossover)
    By Jonas_h in forum Lansing Product DIY Forum
    Replies: 6
    Last Post: 04-25-2012, 01:17 AM
  2. 2405 crossover point
    By Sootshe in forum Lansing Product Technical Help
    Replies: 9
    Last Post: 11-17-2010, 02:53 PM
  3. 808-8B/511b+515GHP/816A crossover point
    By leifchristensen in forum Lansing Product DIY Forum
    Replies: 0
    Last Post: 12-14-2009, 05:53 AM
  4. Crossover Point for JBL System
    By JBLBob in forum Lansing Product DIY Forum
    Replies: 1
    Last Post: 10-17-2008, 02:41 PM
  5. BX63A Crossover point made lower?????
    By mrbluster in forum Lansing Product Technical Help
    Replies: 0
    Last Post: 09-21-2005, 03:51 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •