From: mberg@dk.oracle.com (Martin Berg)
Newsgroups: sci.electronics
Subject: Re: Speakers loose tweeters and filter elements
Keywords: Audio, PA, Speaker
Date: 23 Dec 92 13:44:29 GMT
Reply-To: mberg@dk.oracle.com (PUT YOUR NAME HERE)
Organization: Oracle Denmark

In article <1517@chorus.chorus.fr> ngk@opera.chorus.fr (Gia-Khanh Nguyen) writes:
>
>
>	Hi netters,
>
>	I have a pair of JAMO-300 speakers. Their specification is:
>
>	- 8 Ohm impedance.
>	- 300 W / each.
>	- 4 ways SP system with passive filters.
>
>	I use them with a Samick SM-6000 Stereo Power Amplifier (250W/channel)
>for dancing parties.
>The problem is that I often have the tweeters and/or some capacitors of the
>filters broken, but never the other elements. I think that the filters for
>the broken tweeters are not well designed or it misses some sort of protection.
>So I'm wondering if any one can tell me how to avoid such a situation for the
>next time I use them (i.e. an active/passive protection circuit or a
>modification in the filters).
>
>	I give you here the diagram of one speaker system :
>
>
>        +-------+------------+----------------+------------------------+
>	|       |            |                |                        |
>        |       |        C1 -+-           C2 -+-                   +---+---+
>        |       |           -+-              -+-                   |       |
>        |      |-|           |                |                  + |       | +
>        |      | |          |-|               +----+             |\+-+   +-+/|
>        |   L1 | |       L2 | |               |    |          T1 | | | T2| | |
>  +     |      | |          | |          C3  -+-  |-|            |/+-+   +-+\|
>  o-----+      |-|          | |              -+-  | |            + |       | +
>                |           |-|               | L3| |              +---+---+
>To PA           |            |              + |   | |                  |
>  o--+          |            |              |\+-+ |-|                 |-|
>  -  |     +----+       +----+----+      T3 | | |  |               R1 | |
>     |     |    |       |    |    |         |/+-+  |                  |-|
>     |     |    |       |   |-|   |         + |    |                   |
>     |   + |    |     + | L4| |   |           +----+                   |
>     |   |\+-+ -+-C4  |\+-+ | |  -+-C5        |    |                   |
>     | B1| | | -+-    | | | | |  -+-         |-|  |-|              C6 -+-
>     |   |/+-+  |   M1|/+-+ |-|   |        R2| |  | |R3               -+-
>     |   + |    |     + |    |    |          |-|  |-|                  |
>     |     |    |       |    |    |           |    |                   |
>     |     |    |       |    |    |           |    |                   |
>     +-----+----+-------+----+----+-----------+----+-------------------+
             ^^^^^         ^^^^^^^           ^^^^^^^                ^^^^^
	     12dB/oct      12dB/oct          18dB/oct              6db/oct

>
>

<stuff deleted>

I'm sorry, but I think you are in bad luck.

The reasons for this:

	Modification of the filters will only reduce dissipated effect
	in the smaller speakers if the bandwith of the filter sections
	are made steeper - this will only be of limited value - furthermore
	the mix of 18 dB/octave and 6 dB/octave filtering seems somewhat 
	strange. A change in filter design would therefore also most likely
	result in unwanted changes in the audio performance.

	Protection circuits might work, but will result in audible
	distortion (suddenly a part of the signal is missing).

	Proper protection circuits are not easy to design - I've seen
	so-called 'protected' speakers destroyed.

	You are obvious playing as laud as possible (I deduce this from
	the repeated speaker failures :-).

	You probably (likely) therefore often are driving the amplifier
	into saturation which has always been known to be one of the most 
	effective methods to burn tweeters and the like.

	Not all speakers are really up to full load hour after hour
	(night after night) - comming from the country where JAMO is made
	I risk the possible flames this can give and state that JAMO 
	speakers (those I've seen) are not in this category.

The only thing I'm almost sure would work is to ensure that that the amplifier
AT NO TIME is brought into saturation - this would most likely mean that 
you will have to turn the volume down (supprise: turning the volume down 
will protect the speakers from overload ;-).

Of course the 'real' solution (if such a thing exists :-) would be to use
real PA-speakers - but I kind of guess that this was not your intention - 
especially because real PA-speakers costs a lot more that the JAMO's.


>	Thank you very much for any advice.
>	
>	Gia-Khanh Nguyen (ngk@chorus.fr)

For what is was worth, you are welcome.


Martin Berg
Oracle Denmark A/S

No, Oracle Denmark does not have anything to do with these opinions, so
they are mine.