Connect with us

Speaker overload (tweeter) protection using bulbs (repost)

Discussion in 'Electronic Repair' started by Eeyore, Nov 14, 2008.

  1. Eeyore

    Eeyore Guest

    This is a well established technique for preventing voice coils burning
    out under conditions of 'overdrive'.

    There is a stage monitor I'm having problems with that uses this method.

    The HF sounds very distorted and almost cuts in and out.

    I looked closely inside and found some damaged push-on terminals. Ah, I
    thought, probably a poor contact causing the probelm, replaced them,
    checked driver DC resistances etc, reassembled thinking I'd probably
    fixed it.

    But no, the low level HF and distortion continued.

    I'd checked the DC resistance of the protection bulb but later it
    occurred to me that it might have 'very nearly' burnt out and have a
    weak spot that wouldn't show up on a DVM but passing signal would heat
    it and cause this trouble. I'll be able to find out soon enough but I
    wondered if anyone else had ever encountered this ?

    In the meantime I brought the HF driver home to check it's not voice
    coil rub.

    Graham
     
  2. jakdedert

    jakdedert Guest

    I imagine that there are variations in lamps, given that they are
    designed to provide 'light', not speaker protection. I suspect you'll
    find an issue with the driver, though. Swap them around to check and/or
    do a sweep test.

    I just picked up a set of Bose (I know...they were cheap) 201s at a
    thrift store. The only thing wrong with them--besides being Bose--were
    the blown out protection lamps in series with the tweeters. Not bad for
    $7.99, plus the cost of two lamps...altogether less than $10. They'll
    make decent computer speakers for my 12 year old.

    jak


    jak
     
  3. Eeyore

    Eeyore Guest

    A 'swap', or rather replacement, was done earlier with no good effect. Hence I
    laid the suspicion on the damaged push-ons. I will still test the drivers
    though. I have 2 drivers that measure ok plus a third blown one and a
    replacement diaphragm.

    Nice steal. For undemanding work they aren't *that* bad.

    Graham
     
  4. Eeyore

    Eeyore Guest

    I will double check that. Thanks for the tip.

    Interesting point.

    Graham
     
  5. Eeyore

    Eeyore Guest

    24V truck lamp here.

    That's EXACTLY what I'm getting. Thanks for the effective conformation.

    Well it has been but the bulb's nicely cushioned.

    Graham
     
  6. Eeyore

    Eeyore Guest

    The amps aren't but a blast of HF feedback will do it since it will whistle
    through the HF filter. I am as it happens going to fit an 80W P-Audio driver
    in the damn second JBL JRX112M that's blown its puny HF driver for the second
    time, despite improving JBL's original ineffective bulb protection which was
    100% ineffective. Jesus that design is CRAP. 1st order crossover !

    Bear in mind these are the 15 year old monitors made of MDF and have been very
    little trouble generally over the years.

    Graham
     
  7. Eeyore

    Eeyore Guest

  8. Eeyore

    Eeyore Guest

    And it is. Another new house engineer recently for example.

    Well this is where it gets slightly complicated.

    But a simple level limiter still won't stop the amp delivering full power say
    @ 8kHz. Tweeters aren't meant to see that. It would have to be a frequency
    sensitive limiter matched to the crossover. Which has given me a product idea
    of course.

    Graham
     
  9. the easiest way to overdrive a speaker is to use a[n] amp
    Actually, the easiest way to overdrive a speaker is to play it at a level
    where it produces audible distortion. But as most listeners have no idea
    what distortion sounds like...
     
  10. Although what you say is true in terms of physics (the speaker's excursion
    limits do not have a necessary relationship with its heat-dissipating
    capability), in practice, audible distortion is a warning that you should
    turn down the volume. William Michael Watson Dayton-Wright told me horror
    stories of how he could not design a speaker with sufficient power-handling
    capability to keep "deaf" listeners from overdriving and damaging it.

    The distortion I was talking about was the sort that comes from pushing it
    into its excursion limits.
     
  11. I grant you all your points as valid, and bottoming (or farting)
    Agreed. I've never heard it called that, but it's an apt description.

    What gets you up so early on a Sunday? (I'm on the west coast, and have been
    up since 2AM.)
     
  12. Tony

    Tony Guest

    I like to protect gear from whatever might come, so I like lamps and polyswiches. I would
    be almost as happy with a frequency-selective limiter that could independently account for
    the thermal and displacement limits of the tweeter, mid and woofer (with appropriately
    different time constants - an even greater challenge), but the reality is that a lamp in
    the tweeter circuit handles 80% of abuse, so it's cheap insurance.

    But an interesting twist is that with the simple 2nd order HPFs I have been trying (with a
    very high Fc, eg 8 kHz, to flatten CD horn response and match sensitivities), a lamp or
    polyswitch in the INPUT to the crossover has the nice benefit of also seeing more MF, and
    so potentially account for both thermal and displacement limitations. Admittedly the time
    constants cannot be independent as would really be required, but it's still better than
    having the lamp in the crossover output circuit, and a LOT safer for the amp, which
    becomes UNLOADED when the protection operates, instead of shorted.

    Tony
     
  13. liquidator

    liquidator Guest

    Jeez...a little knowledge is a dangerous thing.
     
  14. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
  15. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
  16. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
  17. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
  18. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
  19. Phil Allison

    Phil Allison Guest

    "Eeysore = incorrigible pommy fuckwit TROLL "



    ** 100% WRONG.

    Your autism is showing - big time.

    CROSSPOSTING IS AN ABOMINATION !!

    Stevenson is the BIGGEST TROLL on Usenet





    ..... Phil
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-