"Graham Holloway" wrote
Jim, for the purposes of seeing how a fuse might affect linearity,
some sort of frequency response of resistance change vs frequency
would be needed. If a fuse has a very low thermal inertia, it might
change its resistance significantly during a cycle, particularly at
low frequencies. This would cause distortion if it was in series with
a speaker. At higher frequencies which it could not track, it would
simply cause amplitude compression - I've used a light bulb for this
in the past.

d

Pearce Consulting
http://www.pearce.uk.com


When I worked for ILP Electronics many years ago, they suggested the use of
a fuse in series with the loudspeaker as protection against a DC fault. I
found that distortion was significant due to the modulation of the fuse
resistance by low frequencies. I have no records of the tests I did.
However, I did suggest an alternative, and that was to put a fuse in each (+
and -) supply rails. It was possible to use fuses of a lower rating than
that in the output line because of the lower (half cycles) duty.

One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

Fleetie wrote:

"Graham Holloway" wrote
Jim, for the purposes of seeing how a fuse might affect linearity,
some sort of frequency response of resistance change vs frequency
would be needed. If a fuse has a very low thermal inertia, it might
change its resistance significantly during a cycle, particularly at
low frequencies. This would cause distortion if it was in series with
a speaker. At higher frequencies which it could not track, it would
simply cause amplitude compression - I've used a light bulb for this
in the past.

d

Pearce Consulting
http://www.pearce.uk.com


When I worked for ILP Electronics many years ago, they suggested the use of
a fuse in series with the loudspeaker as protection against a DC fault. I
found that distortion was significant due to the modulation of the fuse
resistance by low frequencies. I have no records of the tests I did.
However, I did suggest an alternative, and that was to put a fuse in each (+
and -) supply rails. It was possible to use fuses of a lower rating than
that in the output line because of the lower (half cycles) duty.

One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to guard against
excessive DC. Whatever else - the speaker won't see prolonged DC. Supply rail
fuses will blow.


Graham

"Pooh Bear" wrote in message
...
Fleetie wrote:

"Graham Holloway" wrote
Jim, for the purposes of seeing how a fuse might affect linearity,
some sort of frequency response of resistance change vs frequency
would be needed. If a fuse has a very low thermal inertia, it might
change its resistance significantly during a cycle, particularly at
low frequencies. This would cause distortion if it was in series with
a speaker. At higher frequencies which it could not track, it would
simply cause amplitude compression - I've used a light bulb for this
in the past.

d

Pearce Consulting
http://www.pearce.uk.com


When I worked for ILP Electronics many years ago, they suggested the
use of
a fuse in series with the loudspeaker as protection against a DC
fault. I
found that distortion was significant due to the modulation of the
fuse
resistance by low frequencies. I have no records of the tests I did.
However, I did suggest an alternative, and that was to put a fuse in
each (+
and -) supply rails. It was possible to use fuses of a lower rating
than
that in the output line because of the lower (half cycles) duty.

One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to guard
against
excessive DC. Whatever else - the speaker won't see prolonged DC. Supply
rail
fuses will blow.


Graham


If either fuse blew, the output would float to zero.

We used it because it was cheap. In fact, the 100W amplifier modules cost
under one UKP (circa 1982) in parts, including the heatsink.

Graham Holloway.

In article , Graham Holloway
wrote:


"Pooh Bear" wrote in message
...


One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to guard
against
excessive DC. Whatever else - the speaker won't see prolonged DC.
Supply
rail
fuses will blow.


Graham


If either fuse blew, the output would float to zero.

My experience was similar. I tried various kinds of 'problems and faults'
on the designs I played with and they tended to either:

1) Blow one fuse and the output floaded down to zero with no real ability
to o/p current.

2) Blow both fuses almost at the same moment.

I assume this depends a lot on the design details, but I concluded that I
could omit any d.c. crowbar, etc and just depend on the fuses. My concern
was more for the amp than the speakers, though... 8-]

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Jim Lesurf" wrote in message
...
In article , Graham Holloway
wrote:


"Pooh Bear" wrote in message
...


One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to
guard
against
excessive DC. Whatever else - the speaker won't see prolonged DC.
Supply
rail
fuses will blow.


Graham


If either fuse blew, the output would float to zero.

My experience was similar. I tried various kinds of 'problems and faults'
on the designs I played with and they tended to either:

1) Blow one fuse and the output floaded down to zero with no real ability
to o/p current.

2) Blow both fuses almost at the same moment.

I assume this depends a lot on the design details, but I concluded that I
could omit any d.c. crowbar, etc and just depend on the fuses. My concern
was more for the amp than the speakers, though... 8-]

Slainte,

Jim

--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Jim

My boss was more concerned about £10 amplifiers failing, followed by a claim
for £100 speaker damage.

Graham

In article , Pooh Bear
wrote:
Fleetie wrote:



What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to guard
against excessive DC. Whatever else - the speaker won't see prolonged
DC. Supply rail fuses will blow.

I avoided using 'protection' circuits as I always had the feeling that:

1) They would fire when not needed, thus either blowing fuses or
interfering with the music when it wasn't warranted.

2) They could also go wrong, and took design effort and cost away from the
actual amp.

However I agree that by avoiding them, the designer takes a risk. Swings
and roundabouts. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Fleetie" wrote in message
news
"Graham Holloway" wrote

When I worked for ILP Electronics many years ago, they suggested the
use of a fuse in series with the loudspeaker as protection against a
DC fault. I found that distortion was significant due to the
modulation of the fuse resistance by low frequencies. I have no
records of the tests I did. However, I did suggest an alternative,
and that was to put a fuse in each (+ and -) supply rails. It was
possible to use fuses of a lower rating than that in the output line
because of the lower (half cycles) duty.

One of the fuses will blow before the other.

Agreed.

What happens to the voltage at the speaker output then?

Depends, but its possible and often done so that the output of the amp does
not swing to the other rail. This may take some care, as I've definately
seen amps with the output firmly stuck on one of the power supplies.

And could it toast the speaker?

Yes, hence the desireability of some additional other kind of protection -
often a relay or a thyristor.

In article , Fleetie
wrote:


One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

I have wondered about this. However whenever I tested by driving an amp
into a low impedance until fuse failure *both* fuses blew almost at the
same instant. (i.e. I didn't see any time delay) This may well depend upon
the amp design, though.

One point to bear in mind is that you have to use surprisingly low-value
fuses. Thus you tend to end up with a situation where you can get quite
high music and sinewave powers through line fuses that then blow quickly
with a comparable steady level, etc.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html