"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

In article , Graham Holloway
wrote:
[snip]


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.

Yes. I ended up also deciding that the best place for fuses is in the power
rails rather than the o/p.

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

In article , Don Pearce
wrote:
On Mon, 29 Nov 2004 13:55:30 +0000 (GMT), Jim Lesurf
wrote:

Following on from some discussions in other threads I couldn't find any
existing data on the resistance-current properties of fuses, so did a
quick measurement as a simple experiment.

The results are shown on the graph at

http://www.st-and.demon.co.uk/temp/fuseplot.gif


[snip]

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.

Yes, I'd agree this may well be so, particularly for LF or LF+HF intermod
effects. Not seen measurements, etc, on this. But I avoided using o/p fuses
in the past precisely because I feared such things might occur.

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.

Yes. In fact IIRC some of our older sinewave oscillators in the teaching
labs use a small incandescent lamp to stabilise the gain of the oscillator.
(That makes me also recall that I think the old 'Sound Technology THD
analyser I used to use also did something similar.)

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

On Mon, 29 Nov 2004 16:24:29 +0000 (GMT), Jim Lesurf
wrote:

Yes. In fact IIRC some of our older sinewave oscillators in the teaching
labs use a small incandescent lamp to stabilise the gain of the oscillator.
(That makes me also recall that I think the old 'Sound Technology THD
analyser I used to use also did something similar.)

Slainte,

Jim

I have a home-built Wien bridge oscillator stabilized by a thermistor
- a tiny bead sealed in a glass tube. It is very good above a hundred
Hz, but its attempts to stabilize the oscillator down around 10Hz are
really pretty pathetic.

d

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

"Don Pearce" wrote in message

On Mon, 29 Nov 2004 16:24:29 +0000 (GMT), Jim Lesurf
wrote:

Yes. In fact IIRC some of our older sinewave oscillators in the
teaching labs use a small incandescent lamp to stabilise the gain of
the oscillator. (That makes me also recall that I think the old
'Sound Technology THD analyser I used to use also did something
similar.)

Slainte,

Jim

I have a home-built Wien bridge oscillator stabilized by a thermistor
- a tiny bead sealed in a glass tube. It is very good above a hundred
Hz, but its attempts to stabilize the oscillator down around 10Hz are
really pretty pathetic.

Been there did that, with the same results. I moved on to one based on a CdS
cell driven by a LED and precision rectifier/integrator.

"Jim Lesurf" wrote in message


Yes. In fact IIRC some of our older sinewave oscillators in the
teaching labs use a small incandescent lamp to stabilise the gain of
the oscillator.

Case in point most really old oscillators, including the amateur's fave -
the Heath IG-18

(That makes me also recall that I think the old
'Sound Technology THD analyser I used to use also did something
similar.)

The alternatives use either photocells (typcially CdS) or later on, FETs.

I published a CdS cell update for the IG-18 a few decades ago in Audio
Amateur.

Audio Magazine published a series of articles for construction of a modern
THD analysis rig that used a FET to stabilize its oscillator.