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Bruce, MikeyD, Joris ...Anyone? I blew tham again!!!

Discussion in 'Amps and Cabs [BG]' started by mikemulcahy, Dec 17, 2001.


  1. mikemulcahy

    mikemulcahy

    Jun 13, 2000
    The Abyss
    I just got sponsorship from a local Music store for a regional minitour. They gave me some 350w Black Widoes to try in my 18 cabs. After two prctice sessions, the voice coils fried...again. I have been using a Mackie 1400i they asked me to try just to run the bottom. I have it on th sub woofer out put and crossed over @ 128Hz. There was no distortion that I could tell and the amps were nowhere close to clipping. Any ideas?


    Mike
     
  2. MikeyD

    MikeyD

    Sep 9, 2000
    Sorry to hear about that! Last time (as I recall) I had thought it might be a nearby lightning strike. Since this happened again, and you were probably paying even closer attention, it makes me wonder about that power amp. Spacegoat has described situations in which power amps sometimes fail in a way that they can blow DC directly into their speaker outputs. If the supply rail voltages got connected directly, that might be enough to burn the coils. I would very seriously suspect your power amp at this point. You might want to have it checked out by a really good technician. Maybe you can do a search on the web for problem discussions on that model. You could also try putting up a query in the Usenet professional sound forums. I recall that many of the regulars weren't too fond of Mackie equipment, though.

    Another thought - if something in your signal chain is generating very low frequency subharmonics, it might give a similar outcome. If so, you might want to insert a subharmonic filter (say 30 Hz.) and/or have a technician do a spectrum analysis on your signal chain to see if something is getting generated that will fry coils.

    - Mike
     
  3. mikemulcahy

    mikemulcahy

    Jun 13, 2000
    The Abyss
    Thanks guys. I suspect from what you have said its in the signal chain because it happened with two different amps, The Crown and now the Mackie.


    Mike
     
  4. Another thought: you'd be surprised how inaudible distortion is in the bass range. Especially below 128 Hz. While playing, I wouldn't be able to tell 30% distortion. Been there, done that....
     
  5. mikemulcahy

    mikemulcahy

    Jun 13, 2000
    The Abyss
    I was surfing around and couldn't find a subharmonic filter. Where can I find one and what exactly does it do(aside from the obvious).


    Mike
     
  6. drade

    drade

    Sep 24, 2001
    search for "subsonic filter".
     
  7. I have a variable low pass filter in my Rane signal chain. It is sweepable through a wide range, from 10 Hz and up. It blocks all signals below the set point. This keeps turntable rumble, string motion, etc, from influencing your drivers.

    Moving your hands over the strings at volume, can cause the drivers to move in and out. This very low frequency is deadly for drivers at any kind of power.

    [ edit ]

    Yep. Wrong word.. cranial-rectal inversion on my part. I have a sweeapable high-pass filter that blocks subsonic lows.
     
  8. MikeyD

    MikeyD

    Sep 9, 2000
    Blocking frequencies below a setpoint is done with a high-pass filter, and Drade is correct: "subsonic" filter is more accurate. Sorry for any confusion from my previous post.

    Joris is right about any distortion happening where the higher frequencies are filtered out. LF distortion is harder to hear without the HF artifacts that would normally be present in a full-range system. Anyway, the more I think about this and the kind of power I recall you were pushing, you would do well to block subsonic frequencies - preferably just before the power amp inputs.

    In a pinch you could play with inserting blocking capacitors at the power amp inputs, but I'd think eventually you'd want a decent quality filter. Note that some power amps (e.g., some QSC's) have built-in, switchable subsonic filters.

    - Mike
     
  9. geshel

    geshel

    Oct 2, 2001
    Seattle
    OK everyone, listen up:

    subsonic: moves slower than the speed of sound (as opposed to supersonic)

    INFRASONIC: below our hearing range (as opposed to ultrasonic)

    You are talking about infrasonic filters. Also known as rumble filters, or simply a high-pass filter at around 20Hz. Many PA amps have them built-in, switchable on the back.

    Glad to be of service. :)
     
  10. geshel

    geshel

    Oct 2, 2001
    Seattle
    BTW, that would be a HIGH-pass filter. :D As in it allows the highs to pass through.
     
  11. I already fixed it, and edited my post. Guess you have to take a ticket to beat me up...

    :D
     
  12. geshel

    geshel

    Oct 2, 2001
    Seattle
    Oh duh, I even read the [edit] part and it didn't sink in the first time, then I read the post again and saw "low-pass" - off goes the reply button! Too much caffeine today, sorry. :D
     
  13. drade

    drade

    Sep 24, 2001
    very true but many companies do call them "subsonic filters" and using this as a search term does bring results.
     
  14. geshel

    geshel

    Oct 2, 2001
    Seattle
    Ugh. I guess that's like when luthiers say "extended-B headstocks" and "strings-through-body" increase the string tension, you just have to wince a little and remember that in most respects, they know what they are doing. :)
     
  15. MikeyD

    MikeyD

    Sep 9, 2000
    Damn! Geshel, you done caused MikeyD to be embarrassed for the first time in TB, I think! I should know better [**tucks tail between legs and waddles away with bright red face**]. You are 100% correct. As Drade noted, "subsonic" is used in error all too commonly in the audio field. Still, no excuse (especially for one who majored in acoustics as an undergrad).
    - Mike
     
  16. It's amazing how much stuff (pro level no less) that have things called "subsonic filters." I never really thought of it as being incorrect before, but it's true. Of course I just call 'em HPF's.

    Anyway, that's a pretty bizarre problem. Are the voice coils actually getting burnt or are they getting blown out of the gaps? I'm stumped, I'd have to see them and hear your rig to make any judgements. It's not an amp going DC, because that only happens if an output device shorts ( i.e. P-N junction breakdown in a transistor that lets the power supply voltage go straight to the outputs). This usually results in total amp failure, a loud "CRACK" and smoke coming out of the cabinet's ports :D. Actually most amps have pretty good DC protection circuits on their outputs.
    Maybe the HPF thing is part of the problem, did you notice a whole lot of cone movement?

    I'm ashamed of you Mike! :p Just kidding, subsonic is used so universally to mean infrasonic that it's entirely excuseable. :D
     
  17. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Doc, you're doing something wrong. It's YOU, man. I mean, c'mon. How could this happen? Coincidence? I think not. Karma, perhaps. You've got to get your MIND RIGHT! Wise up!
     
  18. MikeyD

    MikeyD

    Sep 9, 2000
    What I'm wondering is if there's something in the signal chain generating, say, very LF AC (say 0.5 Hz. or less), then it's nearly as bad as DC - if it can get through the power amp. In that case, it could both kick the coil to its extremes AND burn it up. I agree - I'd want to do a post-mortem on those drivers and find the telltale. Doctor, prepare for surgery.
    Nope. Inexcusable, especially for someone who studied the terminology in college and prided himself on knowing the difference. [**Red face of embarrassment yields to red face of slapping self.**]
    - Mike
     
  19. I'm glad to find I'm not the only one who knows what means, but is incapable of expressing it correctly.

    Maybe I should run for public orifice...

    As for the problem at hand, I've seen this tail-chasing adventure while servicing mainframe computers. A blow B, B blow A, and around and around you go. Expensive way to bug shoot though.

    It would be interesting to see the post mortem on the coils..