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Conference napalm::guitar

Title:GUITARnotes - Where Every Note has Emotion
Notice:Discussion of the finer stringed instruments
Moderator:KDX200::COOPER
Created:Thu Aug 14 1986
Last Modified:Fri Jun 06 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:3280
Total number of notes:61432

993.0. "speaker protection" by CGFSV1::EDMENG () Wed Nov 23 1988 17:05

    	
    	I have just blown the horns in my monitors for the third time
    in as many years. (Mike too close to monitor, feedback, horn goes
    pop!!). Does anyone know of any methods to protect monitor horns
    from excessive power input and/or transients?
    
    	I can think of four methods so far, but I don't know if they
    are practical.
    
    1) Use a limiter (too expensive).
    
    2) Use a fuse to protect the horns. How do you calculate the fuse
       rating given the horn impedance, power rating, and crossover freq.?
       Are fuses fast enough to protect against transients?
    
    3) Put a varistor across the horn. (those things used to suppress
       line voltage transients). Can you get them at the voltage ratings
       required to protect a horn? (and what might that voltage rating
       be?) Are they fast enough?
    
    4) Back to back zener diodes across the horn. This should clip the
       signal at a specified input voltage. How do you calculate the zener
       specs.? Is the clipped waveform harmful to the horn?
    
    	Any information on this topic would be greatly appreciated.
    
    	- Bruce -
T.RTitleUserPersonal
Name		DateLines
993.1Buy The LimiterAQUA::ROSTHum-dum-dinger from DingersvilleMon Nov 28 1988 08:5819
    
    Some things to think about:
    
    1)  A limiter is not too expensive.  You can get them brand new
    for about $150 (of course better ones cost more).  Compare that
    with replacing your horns a few times.

    2)  For fusing, use Ohm's law.  I.e. for 50 watts at 8 ohms, 
    
    P= I*2 x R  --->    50 = I*2 x 8   -----> I = 2.5 amps
    
    You may want to overfuse slightly so that the fuses don't blow every
    10 minutes!!!!
    
    3,4) Not a very good idea, if you clip using varistors or diodes,
    you will generate high frequency harmonics that can be more damaging
    to the horn than the original transient!!!!
993.2Limit it - don't fuse it!RCKRLL::STANLEYTim StanleyMon Nov 28 1988 10:2817
I agree with .1.    

It seems that a limiter would be the 'right' solution.  The other
solutions are simply ways to work around the problem instead of
really solving it.

Also, a normal fuse cannot protect against transients too well,
especially high frequency transients that you are having trouble
with.  A fuse blows because excessive current melts to wire in
the fuse.  High frequency stuff just isn't going to have the kind
of sustained high current needed to blow in time to protect your
horns because there is not much power involved at the high frequency.
The Ohms law equations in the last note are fine for
DC (power supply) applications, but not for AC/audio because they
do not consider the frequency related components of impedance.
You don't see a frequency variable in those equations.
993.3Compression will work too.ELESYS::JASNIEWSKIAh, the road within withoutMon Nov 28 1988 11:1112
    
    	You could also solve the problem with a little compression between
    the board and the amps. Not only would this reduce the peak wattage
    levels needed to produce the same apparent loudness, it would also
    serve as a feedback guard of sorts, by automatically reducing the
    volume level for large signals from the board.
    
    	I happen to have one for sale, a DBX unit, that I want $50 for.
    Let me know at this E-Net address if interested -
    
    	Joe Jas