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Thread: I Need Some Electronic Genius Re: Speakers...

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    I Need Some Electronic Genius Re: Speakers...

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    OK.. I have a pair of JBL G-734's that I use for live stuff. The problem is that I have the tendency to get a little ripped... forget what I'm doing... and ultimately do something dumb to get a feedback spike that fries my compression drivers. So it's back to ordering two more diagphrams at ~$150 a clip (times two...).. and putting them in. Well, this is getting old. I'm wondering if anyone out there can tell me if there's any way to put a fuse or breaker in line from the crossover to the horn in these things that'll blow just before the driver does? Any thoughts (other than to quit drinking, or buy a pair of 2247s)? If anyone could tell me if there's a way to do this, and actually look into the speaker specs so they could walk me through what wattage breaker/fuse I could put in to max this out, but blow before the speaker does (if it's possible at all) would be greatly appreciated! things just don't sound the same without horns...lol... Thanx, Tom!

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    well, the easiest thing to do is put a limiter inline with the amp, so that it keeps this from happening. Behringer makes a couple of cheap compressor limitors that I would think should be fine for this sort of thing. I'm sure someone will come along and tell me I'm wrong though, as I havent tried one. But they run about 80 bucks or so.

    My EV speaks have a circuit inline to protect against this sort of thing consisting of a zener diode and a really big power resistor, IIRC. I havent had it apart in a while, and I'm really tired, so I may be remembering it wrong.

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    Thanx bdgr! I have a pair of JBL Control Series studio monitors that have the same diode thingy in them... they're cool to watch in the dark through the bass port... but even with that in the mix.. ...I've blown those up in the past a few times as well. HELP... SOMEBODY STOP ME!!!!!!!!!!!!!!!!!!

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    HELP... SOMEBODY STOP ME!!!!!!!!!!!!!!!!!!

    Ok, why then,. do you not "get ripped" after the show?
    .........\\|//..............
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    -oOO--(_)---OOo--
    -------PEACE--------
    ><>eYEslIkEfIRE<><

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    Originally posted by Fmmahoganyrush
    Thanx bdgr! I have a pair of JBL Control Series studio monitors that have the same diode thingy in them... they're cool to watch in the dark through the bass port... but even with that in the mix.. ...I've blown those up in the past a few times as well. HELP... SOMEBODY STOP ME!!!!!!!!!!!!!!!!!!
    Differant critter. A zener diode doesnt light up, it operates in breakdown mode. Basically, it blocks the flow of current, until the voltage exceeds a set level, then it shorts out, but unlike a regular diode, which will operated in break down for a short time before it burns up, the zener just gets hot. The power resistor keeps it from blowing your amp when it does short. It works really well.

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    Thumbs up

    Yo' Tom, Behringer does have a capable tool specifically designed to eliminate your problem. Beh's FeedBack Destroyer provides ample protection limiting peak surges and signal overloads commonly associated with speaker damage. Whether you're under the extreme influence of alcoholic spirits or trying to break
    the WHO'S record for loudest live perf, the Destroyer, once set,
    allows for hi-decibal signals to be emitted thru speaker but clips
    damaging peak levels thru it's capacitor-type crossover/limiter w/o any added coloration.

    I've witness its uses at several live perf's INCLUDING our own
    LI SLIM, who perf'd at one particular live gig where he was really feelin' it and when his voc's and/or git playing reached a crescendo, the Destroyer kicked in w/o affecting his music!

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    From a fidelity point of view I really don't like this idea of using a diode. All the diode is doing is acting as a peak limiter, which is exactly the mechanism that is killing your compression drivers. When you turn up the volume too loud the top of the waveform clips creating lots of high frequency harmonics whose RMS power fries your tweeters.

    In order to limit the potential RMS power the amplifier output can deliver to your tweeters enough to be effective with this method you also need to drastically limit the available dynamic headroom. In other words, you will likely be chopping off the top of every single drum kick and bass thump causing your sound to be very harsh.

    A much better solution is to simply limit the RMS output of the amplifier. How? Well, the absolute simplest way of doing this is restrict how high you can turn the power amp up by putting an appropriate value resistor in series with the volume pot (two for a stereo amp).
    Code:
    O------------[resistor]-----+
                                |     +----------O
                               <      |
                     volume pot > <---+
                               <
                                >
                                |
                                |
                              -----
                               --- 
                                - 
    
       new max power = amp power * [     pot        ]^2
                                   [--------------- ]
                                   [(pot + resistor)]
    So, for example, if you put a 10k resistor in series with a 10k volume pot you will then only be able to turn the amp up to 1/4 of its power. Choose the ratio such that the amp cannot put out more power than the speaker rating. You limit the RMS, but you keep all of your headroom!

    If you require a lot of attenuation and the resistor value is more than say 50% of the voulume pot value, it might be best to also replace the pot with a smaller value in order to maintain about the same total resistance (resistor + new pot = original pot) to avoid any issues with the amplifier circuit.

    barefoot

    http://barefootsound.com
    Thomas Barefoot
    Barefoot Sound

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    hey barefoot, Q, bdgr, and eyeslikefire.. Thanx for the info. I'm gonna keep that schematic handy, and look into a feedback harness.. but in the mean time.. I picked up the new drivers, and while I was there I asked the folks there (not a typical place... the real deal pro audio... ATC Audio, Springfield Mass) if I could put an in-line fuse up to the diagphram. The guy looked up some specs on my speakers, and on the diagphram, and figured out how much power these are supposed to be able to handle before they fry using some calculation (which.. dumb shit me forgot to copy down), which turned out to be ~1.25 amps. So I put a fused link in between the crossover on the speaker and the horn. Hell, I'd rather be spending $2.50 replacing fuses than the $150 a pop for the diagphams(and the price seems to be going up every day on these for some reason). I've stuck a 1 amp fast acting fuse in them for the time being so I can see if it works. I'm probably gonna get some 1/4, 1/2, 3/4 amp fuses to see how much the thing can take prior to popping those, so I'll have an idea of what I can get out of them at full power. Any of you guys/gals heard of this type of thing before?? Thanx again for the input!!!!! ....but if I blow these things up again.. I'll probably have a pair of JBL SR4725X's for the next JamFest ... TOM!!!!!!!!!

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    Quote Originally Posted by barefoot View Post
    A much better solution is to simply limit the RMS output of the amplifier. How? Well, the absolute simplest way of doing this is restrict how high you can turn the power amp up by putting an appropriate value resistor in series with the volume pot (two for a stereo amp)...
    ...
    barefoot

    Barefoot Sound
    Sorry for dredging this old thread up, but I could not let this error stand uncorrected.

    The 'rms' limiting you describe is actually peak limiting that will still result in a clipped signal (at the output of the preamp) under feedback conditions.

    What you have drawn is similar to a power soak in the preamp stage.

    The whole notion of harmonic content destroying tweeters depends on a clipped waveform and a passive crossover to route the harmonic content to the tweeter. Your solution addresses neither of these factors. All it does is reduce the magnitude of the clipped signal similar to that zener.

    The one thing that you are correct about is that the dynamic headroom will decrease. The same could be accomplished by using a smaller amplifier.

    Speakers must be able to reproduce peaks cleanly and that means no clipping on their signal is allowed at any point in the signal path. Even trying to limit just the tweeter voltage with a zener will put an unacceptable strain on the crossover while trashing the sound.

    The proper way to prevent feedback from blowing drivers, or any other signal from blowing drivers either, without causing objectionable distortion in the processing, is to use compression. Compression distorts the signal also, but in a more benign and tuneable way that generates fewer harmonics that sound bad and blow tweeters.

    Modern powered speakers tend to use soft limiting protection that is basically a form of pre-tuned compression, and/or to use discrete amplifiers for each frequency band so that the power in each band is inherently limited by the amplifier capability.

    Such limiting can also be used with passive crossovers effectively. Typically the configured compression has an attack time that lets dynamic transients through while preventing sustained feedback from frying the tweeters. Compression is the only practical way to limit the average power without also limiting the peak power and destroying the dynamic range.

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    Yes, a fuse in the tweeter (only*) feed is good and simple idea. The nity comes when comes in deciding the rating and type.
    Two basic types: Slow blow or 'T' for 'trage' and F for fast. For speaker protection I would go for fast. Rating is trickier and really has to be worked up.

    Ball Park, a 50 watt rated unit will pull 2.5 amps for 8 Ohms at full power but I suspect for a tweeter that rating would be too high and I would start at 1 amp but keep some others by you at gigs. 1.6, 2A, 2.5A are the common ratings. Run the speakers and if the 1A blows PDQ, go up a notch.

    *I would not fuse the whole speaker system. Having a load go open circuit at max welly would of course be fatal to a valve amp but my experience has told me that transistors are NOT keen either!

    BTW, fuses are 'non-linear devices' as well and cause distortion, especially close to their fusing point but this is probably less than 0.1% even at rated current and we IS talking rock and roll?

    Dave.

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