[net.audio] Speaker fusing

greg@olivej.UUCP (Greg Paley) (01/10/85)

A friend of mine, using an NAD CD player and Threshold amp managed
to blow all of the fuses on his Magneplanar MG-IIIB's.  Since he
had a pair of rear speakers running with a separate amp and was
doing other things while the player was on, he wasn't aware of
exactly when it happened or even which CD was being played.

In his case, this is no big deal since for $2.50 he can replace all
of the fuses.  I'm concerned, because the instructions for my
Vandersteen IIC's specifically recommend against fusing, with the
argument that it shouldn't be necessary and will inevitably cause
a deterioration in the sound.

I'm not about to blame the CD player outright.  I suspect that,
although my friend doesn't own any of the "notorious" speaker-killer
disks, such as the Telarc 1812 Overture or Wellington's Victory,
he probably turned it up loud enough to hear something like the
soft beginning of the Mahler 1st Symphony in another room, only to
be blasted by the forte sections.

The situation brings up several diverse points that I think merit
some discussion:

(1) How much dynamic range is really desirable for a home situation?
    In other words, do you really want to spend your time jumping up
    and down, turning up the soft passages to make them audible and
    turning down the loud passages so as to avoid being blasted out
    of the room?  It seems a practical limit of sorts has already
    been reached and, in fact, may have already been reached with analogue
    recordings.

(2) Is there an alternative, short of "on the spot" monitoring and
    speaker fusing to protect equipment in this situation?

(3) Perhaps the first one that should really be asked: is this a
    relatively exceptional occurence, indicating a defect in either
    the player, preamp, or amp, or is it something likely to
    occur fairly frequently when CD players are linked to high-end,
    high-powered (150+ Watts/Channel) systems in the hands of people
    who like their music fairly loud?

My personal concern is still somewhat theoretical since I don't yet
own a CD player.  However, another friend gave me the surprise of
my life by demonstrating his recent-model Sanyo player which,
though poorly isolated from shock and requiring a real struggle
to insert and remove disks from its drawer, produced sound better
than I've ever heard from a CD player in its price class.  I'm
still not about to rush out and buy one but, to say the least,
it's given me second thoughts.

	- Greg Paley

karn@petrus.UUCP (01/11/85)

The heart of the problem is reducing the level of ambient noise in
your listening area. While I am blessed (?) with a private residence
in a quiet neighborhood, there are plenty of things that start sounding
very loud when you want to listen to CDs. Air conditioners, refrigerators,
and most especially, personal computers with fans all contribute.

I think that a major effort to reduce background noise would not only
help you keep your speakers intact, it would also protect your hearing.
It is very tempting to just turn the volume up to cover up the noise.

Phil

sjc@angband.UUCP (Steve Correll) (01/11/85)

Speaker fuses need not degrade an audio system if one places them
inside the power-amplifier feedback loop, immediately after the output
transistors; I believe Heathkit used to do this.  (The negative
feedback used in audio amplifiers reduces not just the nonlinearity but
also the output impedance. The impedance of a fuse is much less than
the value of the emitter resistor used in many transistor output stages
to stabilize the bias, and a fuse is probably much more nearly linear
than the output transistors themselves.)

I'm a little foggy with regard to tube amplifiers, but I believe the same
trick might be made to work.

Apropos the comment about a Sanyo CD player not being very shock-resistant,
is it actually less immune than the usual turntable? 
-- 
                                                           --Steve Correll
sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc

greg@olivej.UUCP (Greg Paley) (01/14/85)

>> Apropos the comment about a Sanyo CD player not being very shock-resistant,
>> is it actually less immune than the usual turntable?

No, but that was not the relevant issue.  It was considerably less
immune than other CD players in its price class, which makes for a
bit of a dilemma since, at least to my ears, it offered a superior
sound quality to what I've heard from other comparable players.
Is it greedy to want both sound quality and good isolation?  This
might not be a particular problem for many people who can easily
avoid subjecting the unit to mechanical shock, but those who, like
me, have small children in the house know that it's impossible to
predict when they'll come storming through which, in the case of
this particular player in my friend's installation, caused ear-splitting
sputters to result.

	- Greg Paley

mikey@trsvax.UUCP (01/14/85)

I think fuses are worthless.  First off, how many people blow their woofers
because of DC offset?  Most amps capable of blowing the speakers have
DC sensors that will pull the speaker relay off.  You're not going
to protect the midrange and tweeters with a single fuse, you'd have to
put individual fuses at each element, and I doubt that you could get a
fuse that would act fast enough on transients (there won't be a DC
offset here, unless you bi-amp or tri-amp) and not blow on loud music.

The only purpose I can justify for a fuse is to prevent the output
stage from doing a melt-down, not to protect the speakers.

mikey at trsvax

smb@ulysses.UUCP (Steven Bellovin) (01/16/85)

I suppose I should throw in a story about how speaker fuses can help...  I
had a Heath AR-1500A receiver, which, through a glitch (caused by pressing
on the volume control knob to seat it better) sent a large spike of pure
DC down the line.  Yup, the fuses went....  Moral:  pure DC shouldn't happen,
but then again, neither should power line shorts involving the AC line fuse.

rs55611@ihuxk.UUCP (Robert E. Schleicher) (01/18/85)

Maybe someone can comment on this, but it was my impression that with most
modern tweeter designs,  the common cause of failure is not a transient,
but rather thermal damage from sustained high-frequency content.  This is
in contrast to typical woofers, in which the chief cause of damage is excess
cone excursion during transients.  Thus, a single fuse may well protect
both the woofer and tweeter.  When I bought my Advents (6 years ago), they
enclosed a specific fuse recommendation that would prevent thermal damage
to the tweeter (which was the weak point of Advents in their original design;
mine had a ferro-fluid cooled tweeter that was more robust), and presumably
protected against most woofer damage causes.

Thermal damage in tweeters makes a kind of intuitive sense, as with dome
tweeters, or ribbon designs, or EMIT (Infinity), there isn't really a cone
to "pop out", and you're really worried about power levels, not peaks

Also, many "experts" advise that the most potentially damaging music
(for speaker damage) is synthesizer music, which typically has much
more sustained high frequency content.

Bob Schleicher
ihuxk!rs55611