You can get them at Radio Shack or any Hardware Store...Check out the info below:
By Mark Mattson
In lieu of buying comparatively expensive electronic limiters, you can use
standard AGC fuses to protect your loudspeakers. AGC fuses are those small
glass cylinders with metal end caps. Electronic limiters are more
sophisticated and can "read" the content of an audio signal. They can apply
protection as needed depending on signal content and the limiter settings.
By comparison, fuses are dumb devices. They respond only to current flow and
to a fixed time constant (i.e. fast-blow or slow-blow).
WHAT YOU NEED:
The hardware required for fusing is simple and available from Radio Shack or
other electronic supply sources. You need AGC fuse holders, fuses, and a
little labor. A good AGC fuse holder to use is the block type. It has two
metal "U" clips to hold the fuse mounted on a plastic block with solder tabs
and holes for mounting it. Another type of AGC fuse holder, though not as
good, is an "in-line" holder with pig-tail wires. This type is used on
after-market electronics for automobiles and may be available at automotive
supply houses. A third good one is the panel mount type with a removeable
caps found on the back of much electronic equipment. They can be mounted in
most jack plates, and keep all connections internal. Current capacity is
fully adeqate. Any shop that services guitar amps will have these on hand.
The AGC fuses themselves are usually sold packaged in multiples, so you will
automatically be "forced" to buy your spares!
INSTALLING THE FUSE HOLDER
Mount the fuse holder in a convenient place. You might mount the holder on
the back of the loudspeaker enclosure in the input panel recess, behind the
front grille, internally in the enclosure, or at the amplifier output.
Remember, if a fuse blows, you have to be able to replace it, and sometimes
you may need to do this in a hurry during a performance! So choose your
mounting location accordingly. Solder the fuse holder in series with either
of the wires going to the loudspeaker or driver. It does not matter if it is
the plus or minus wire. Also, the fuse and its holder have no polarity. If
soldering is simply not an option, you can use the in-line type holder with
wire nuts, although this is not the preferred method.
WHAT TO FUSE
The best approach to fusing a loudspeaker with a passive crossover is to put
the fuses post-crossover, with the fuse values calculated for each driver.
This will require opening up the loudspeaker to access the wire leads going
to each driver. If you choose to mount the fuse holders on the outside of
the enclosure, you may need to extend one of the wires to each driver to
reach. Bring the wires out through a small hole in the enclosure or input
panel to where you are mounting the fuse holders. You MUST seal this hole
with caulking to avoid any air leaks around the wires. It is better to use
the panel mount type in this case as it will preclude the issue of sealing
the cabinet back up and provides a much more finished appearance. These
generally come with a fiber washer, which will do the job of sealing the
hole. You can certainly use a single fuse for a multi-way cabinet and avoid
the added work required for fusing individual drivers. However, the
protection will not be as exacting. For multi-amped systems, you must fuse
the individual drivers.
The only other question is what type and ampere rating of fuse to use. If
you are fusing the overall system, use fast-blow fuses. If you are fusing
individual drivers use fast-blow fuses for horn (compression) drivers, and
slow-blow for cone drivers. This is because cone drivers are much more
forgiving of momentary excessive power than horn drivers. You must decide
which power rating to use for your calculations. RMS will give the most
protection, but may also blow too easily. If the audio content will not be
substantially bass heavy, you can probably safely use the program power
rating, which is generally twice the RMS rating. Calculate the fuse rating
using this formula:
Amps = square root (watts / Ohms)
Example: A loudspeaker (or driver) is rated at 300W (RMS or program power,
whichever you choose), 8 Ohm nominal impedance.
Amps = square root (300/8)
Amps = square root (37.5)
Amps = 6.1
Thus, the fuse rating to get is 6 amps. Once you get beyond 3 amp ratings,
increments smaller than 1/4 amp may be hard to find so use the nearest fuse
value you can find: higher rating to be a little less conservative, lower to
be more conservative. If you are fusing the overall system, start using the
RMS rating of the loudspeaker. Work you way up in 1/2 amp increments if you
find the fuse blows too easily and you are sure you are not overdriving the
system. If you have two drivers in parallel, such as two woofers, and use
one fuse for both, you must double the fuse rating calculated for one
driver. If the drivers are in series, use the rating calculated for one
Fusing will usually prevent damage to the drivers from excessive power, but
it is not absolute, so you still need to exercise caution when driving any
system hard. Does fusing work? You bet. The store I used to work at used to
rent out gear. It was a Monday morning ritual to replace horn diaphragms. I
finally got fed up and put fuses in-line, but inside the cabinet where the
user wouldn't know about them. From then on, plenty of inexpensive fuses
were replaced, but no more expensive diaphragms.
[ December 06, 2003, 01:16 AM: Message edited by: david mancuso ]