wjm@lcuxc.UUCP (B. Mitchell) (01/15/85)
<gulp>
Speaker fuses are a two-edge sword. Yes, if used properly, they can protect
your speakers, but they also can contribute to distorted sound (especially
in a high-end audio system).
I agree with Phil Rastonicy and Dick Grantges in their comments about fusing.
Personally, I do not have separate fuses on my JSE-1's (in fact JSE recommends
NOT fusing, since they feel the speakers, especially the woofers, can handle
any reasonable amplifier output).
Woofers can generally handle large amounts of power (however, one should
check the manufacturer's literature to be safe) so fuse protection is probably
not required for them. Tweeters cannot, in many cases, but the cross-over
network will keep low frequencies (where most of the power is in classical and
non-hard-rock music) away from them. A fuse that is small enough to protect
the tweeter would probably blow on musical passages that are acceptable to
the speakers (since it contains mostly low frequency energy). On the other
hand, a fuse that protects the woofer is too large to provide any tweeter
protection.
Some amps, like my Hafler DH-220, put fuses inside the feedback loop, which
solves the problem of fuse-induced distortion at the cost of adding more
components to the amplifier. However, this does not solve the problem
of separate woofer/tweeter fuses (unless you biamp).
Recommendations:
Speaker fuses will not add significant distortion to most budget and mid-priced
system, and can provide additional protection for many of the "bookshelf"
speakers used in these systems. For best results two fuses per speaker should
be used, one in series with the power input to protect the entire speaker system
and the other in series with a tweeter lead to provide additional protection
to the tweeter. Of course, this requires opening up your speakers (which may
void your warranty) unless the speaker manufacturer has provided separate
tweeter input terminals (which is generally not the case).
Given that most speaker systems used in high-end systems have high power
handling capabilities, it is probably better NOT to fuse them, since the
additional distortion is likely to be greater than the risk of speaker damage.
How to fuse:
First find the maximum rated power P of the speaker you want to fuse.
Then, the maximum "let-through" current I is given by
I = sqrt( P / Z ) where Z is the impedance of the speaker.
Now "quick blow" fuses blow instantaneously at double their rating , so divide
the "let through" current by 2 to get the appropriate fuse value. Be sure
to use "quick blow" fuses like the 3AG type that are rated for at least 125 volt
and round the calculated current DOWN to the next standard value.
Regards,
Bill Mitchell ({ihnp4!}lcuxc!wjm)