Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

In article , Patrick Turner
wrote:


Stewart Pinkerton wrote:

On Mon, 22 Nov 2004 17:26:15 +0000 (GMT), Jim Lesurf
wrote:
[snip]

1) The fuse resistance changes as the fuse warms up. Hence under
dynamic conditions with music it places a time-varying resistance in
series with the amp-speaker. This tend may introduce time-varying
modifications of the frequency response.


The fuse at the output requires about 4 Kg of pressure to clip it with
end on force.. Unlike many fuse holders, the ones I have in there hold
the fuse very tightly.

But you are right about soldering, its good.

I had the fuses in there while I tested and the thd caused by the fuses
seemed to be negligible.

Unfortunately, you may find that conventional distortion measurements may
not show up the effect of the fuse in series with the output. The effect
will depend upon how the thermal time constant of the fuse compares with
the time of a cycle of the test waveform. The effect shows up more clearly
if you either carefully measure the apparent gain (into say 8 Ohms) or the
output impedance, each as a function of signal level.

FWIW when I personally first encountered this effect it was as a result of
listening, then going back and investigating with a friend's design for an
amp we were testing. The midband distortion levels were low. But the amp
sounded slightly odd. Investigation showed the output impedance due to the
fuse was being modulated by the medium-term rms current level. This then
affected the gain, and the frequency response into the speakers, in the
same time-varying manner.

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

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

On Wed, 24 Nov 2004 23:25:49 +1100, Patrick Turner
wrote:

Stewart Pinkerton wrote:

On Wed, 24 Nov 2004 01:52:31 +1100, Patrick Turner
wrote:

There simply isn't any reason why an amp must have a flat open loop voltage response.

Bull****, there are very good reasons.

It does mean that if the peak in OLR is at say 300Hz, then at
30 kHz, it might down 40 dB.

Therefore when global NFB is applied so that 60 db is in effect at 300 Hz, only 20 dB
is acting at 30 kHz, and at 300 kHz there is almost none.
This is what one wants.

No it bloody well isn't! You have very little feedback at 30kHz, so
you have a very good chance that there's more than 1 % HF IMD on say
29/30 kHz tones, which you might very well find on SACD, or in
distortion products from stupidly wideband vinyl carts. This may very
well be audible. This is definitely *not* what one wants!

But there are no significant signals from microphones at 29 or 30 kHz.

So, you don't believe that DVD-A and SACD have anything to offer? :-)

BTW, I was referring to the massive RF *noise* on SACD, rather than
anything microphone-derived.

One does not want much NFB acting at seriously high frequencies, since
maintaining low distoprtion and low Ro
at 300kHz is useless, and very prone to making an amp very unstable.

Agreed, but maintaining good figures out to 30-50kHz *is* important,
not for THD but for anharmonic baseband IMD products, which are much
more audible than low-order THD - and can be very unpleasant.

Can't say I have ever heard of problems with 29 kHz and 30 kHz tones causing
serious 1 kHz tones to appear in the audio...

Ever tried it?

The first harmonic produced by a 20 kHz signal is at 40 kHz.
So NFB isn't needed at all at 20 kHz, were it not to keep the Ro
nice and low, but nearly all amps do have a varying Ro.
This can't matter much if the Ro is less than 1/10 of the speaker impedance.

See above. HF IMD most certainly *is* a problem, so good linearity to
at least twice the 20kHz audio bandwidth *is* important if the highest
sound quality is your aim.

20 dB of NFB at 30 kHz would be enough to tame most garbage at that F.

Indeed, and it will all work much more nicely if that same 20dB is
applied across the entire audio band, avoiding the need for excessive
phase shift..........

Tube amps can't accept so much NFB because of the phase shift caused by leakage inductance,
shunt inductances, and shunt capacitances, and miller C in the tubes ( also in bjt circuits ).

20 dB in a pentode amp is all one should aim for.

Actually, with any tube amp, the garbage can is what one should aim
for........ :-)

Well, I knew you'd say that, hope the can is big enough for a man about your size...

As a matter of fact, they are! Fekkin' great 'orrible wheelie bins.
Yup, you could get at least a dozen tube amps in there!
--

Stewart Pinkerton | Music is Art - Audio is Engineering

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

On Wed, 24 Nov 2004 12:49:52 +0000, Kurt Hamster
wrote:

On Wed, 24 Nov 2004 23:16:17 +1100, Tat Chan used
to say...

Kurt Hamster wrote:
On Wed, 24 Nov 2004 14:30:04 +1100, Tat Chan used
to say...

Kurt Hamster wrote:

On Tue, 23 Nov 2004 17:05:33 +0000 (UTC), Stewart Pinkerton used
to say...

You think bull**** technical claims should *not* be corrected?

It all depends one the reasons for the correction doesn't it. The
regulars in here are well aware of your reasons.

Sorry, I am new here. What reasons would those be?

Simple really!

His own self-importance and desire to be noticed.

You are kidding, right?

Nope

He never kids - he's a sad and spiteful loser.

UseNet is hardly the place to be noticed among audiophiles. I don't have the
stats, but I reckons web based forums would be the place to where most
(ignorant?) audiophiles hang out.

It's a perfect place to attempt to be someone you aren't. I also made no
comment about how successful he might be.

You're certainly making a very poor fist of attempting to be a normal
human being, that's true enough.

Self importance? Well, I have seen much worse behaviour on UseNet where the
knowledgable techies belittle and insult the technical ignorami (is that the
plural?).

Again I've said nothing about his success, just his desire.

My desires are nothing to do with you! OTOH, you do invite belittling
and insulting.........................

Sure, one might get an ego boost by giving out advice or helping others learn,
but if the final result is a gain in knowledge by the people asking questions,
it can't be that bad, can it?

Pinky's helpfulness is eclipsed by the sheer quantity of hot air.

The short rodent clearly doesn't understand that keyboards aren't
air-driven.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

"Kurt Hamster" wrote in message
...
On Wed, 24 Nov 2004 18:13:36 +0000 (UTC), Stewart Pinkerton used
to say...

On Wed, 24 Nov 2004 12:49:52 +0000, Kurt Hamster
wrote:

On Wed, 24 Nov 2004 23:16:17 +1100, Tat Chan used
to say...

Kurt Hamster wrote:
On Wed, 24 Nov 2004 14:30:04 +1100, Tat Chan used
to say...

Kurt Hamster wrote:

On Tue, 23 Nov 2004 17:05:33 +0000 (UTC), Stewart Pinkerton used
to say...

You think bull**** technical claims should *not* be corrected?

It all depends one the reasons for the correction doesn't it. The
regulars in here are well aware of your reasons.

Sorry, I am new here. What reasons would those be?

Simple really!

His own self-importance and desire to be noticed.

You are kidding, right?

Nope

He never kids - he's a sad and spiteful loser.



Oh dear Kurt, these whiners can dish it out, but they sure as hell can't
take it.....


(Ya *do* hafta fekkin' larf......!!! :-)

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

"Jim Lesurf" wrote in message
...
In article , Patrick Turner
wrote:


Stewart Pinkerton wrote:

On Mon, 22 Nov 2004 17:26:15 +0000 (GMT), Jim Lesurf
wrote:
[snip]

1) The fuse resistance changes as the fuse warms up. Hence under
dynamic conditions with music it places a time-varying resistance in
series with the amp-speaker. This tend may introduce time-varying
modifications of the frequency response.


The fuse at the output requires about 4 Kg of pressure to clip it with
end on force.. Unlike many fuse holders, the ones I have in there hold
the fuse very tightly.

But you are right about soldering, its good.

I had the fuses in there while I tested and the thd caused by the fuses
seemed to be negligible.

Unfortunately, you may find that conventional distortion measurements may
not show up the effect of the fuse in series with the output. The effect
will depend upon how the thermal time constant of the fuse compares with
the time of a cycle of the test waveform. The effect shows up more clearly
if you either carefully measure the apparent gain (into say 8 Ohms) or the
output impedance, each as a function of signal level.

FWIW when I personally first encountered this effect it was as a result of
listening, then going back and investigating with a friend's design for an
amp we were testing. The midband distortion levels were low. But the amp
sounded slightly odd. Investigation showed the output impedance due to the
fuse was being modulated by the medium-term rms current level. This then
affected the gain, and the frequency response into the speakers, in the
same time-varying manner.

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, do you have details of any research on the temperature coefficient of
low amperage cartridge fuses in audio use i.e. both quick acting (F) & anti
surge (T) ?

Thanks,

Mike

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

I wrote this several days ago, but held back ... it being early am, and I
wasn't sure of what I'd written, especially the paragraph beginning
"Perhaps again" ... and I'm still not ... but I want to release it anyway

Verbatim ...

Patrick Turner writes in
:
:
: Jim Lesurf wrote:

..

: interested to see your definitions for some of the terms you use like
: "middle of the switching region". Also for why it has to be "10 times"
: as opposed to some other value, etc....
:
: You may do your own research on the issues raised. I picked arbitary
: figures.
:
: I was assuming you claim was based upon evidence. Do you not have any
: evidence for what you said? You have given no real reason so far for the
: values you now say are "arbitrary" should be taken as validating the claim
: you made.
:
: Yes, I got the evidence. I don't have time to present it to the readers of the
: group for the umpteeth time.

I think here that Patrick forgets that he has only quite recently started
posting to uk.rec.audio, and that many here may not be aware at all of
any of his previous presentations, or debates, in say, rec.audio.tubes.

: Its time you read the info that's out there to be read.

...

: Then don't believe me if I have no evidence.
: I suggest the evidence exists to support the claims I make, but I don't have
: time to
: argue yet again and again

Yet, for many here, Pat, your "voice" may be quite new ... it may be a
mistake to assume that they have read anything you have written earlier.

...

: I have not the time to devote to addressing all issues raised.
: I can't provide bookfulls of info now that you won't read about elsewhere.
:
: Patrick Turner.

Perhaps at least you could list your website ...

Perhaps again, looking through posts you have made elsewhere and
previously, you may be able to point (via a Google reference?) to where
you have discussed these, similiar, or closely related issues ... then at
least relative strangers may be able to build up some understanding of
your POV and experience ... and even, similiarly, vice versa ...

If, of course, it would save time ... or aid in clarification ... at all.

Evidently time is (or appears to be!) short, at the moment, but that is
no reason to be short in your responses ... ;- ( brief, short, terse? :-)

I think Jim is genuinely searching to understand your POV, and his gentle
questions for clarification, and honest feedbacks, might be considered
edifying, rather than disputatious. It may be useful to revisit them ...

I believe such questions and answers are a valuable teaching process.

IOW. Time may be short, but at least be aware when you are starting anew
in a new forum that you may have no history but the present ... [again!]
Similiarly, what may feel like oppositions may be useful engagements ...

Smooth pebbles, after all, are formed from collisions of rough edges ;-)

Best regards, (and please excuse my frank if not georgeous remarks;-)

RdM
no website at this time ... ;-

:
: 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

--

I have learned silence from the talkative, toleration from the
intolerant, and kindness from the unkind; yet strange, I am ungrateful to
those teachers.

Kahlil Gibran

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

In article , Mike Gilmour
wrote:

"Jim Lesurf" wrote in message


Unfortunately, you may find that conventional distortion measurements
may not show up the effect of the fuse in series with the output. The
effect will depend upon how the thermal time constant of the fuse
compares with the time of a cycle of the test waveform. The effect
shows up more clearly if you either carefully measure the apparent
gain (into say 8 Ohms) or the output impedance, each as a function of
signal level.


Jim, do you have details of any research on the temperature coefficient
of low amperage cartridge fuses in audio use i.e. both quick acting (F)
& anti surge (T) ?

Alas, no. :-/

I did do some measurements on this 20+ years ago, but at that point decided
to avoid them simply to avoid extra o/p resistance, etc. I then did some
quick measurements on the fuses in my friend's amp (this was about 5-10
years ago). But in each case I didn't keep results, and only did
measurement on a few types and examples. so don't know how general the
results would be.

I was thinking about this again this morning when I went in to 'work' and
asked another colleague who had been involved for the amp tests. But he
could not recall details, either. I could re-do some measurements sometime,
but these would then just be on a few 'example' fuses, so the results might
not apply to all types.

In principle, you just use a bench supply to run in constant-current mode,
and a couple of meters to plot out the quasi-static IV curve. However this
does not tell you the dynamics of the response time if it is quick. For
that pulsed measurements are needed.

I assume someone *has* done this, but I don't know where the results may
be. Can't recall seeing a paper on it anywhere. I suspect that many
engineers had the reaction, "Oh, that might be a problem, so I'll avoid it
rather than study the details!" :-)

Fuses do have some frustrating/surprising properties. FWIW I use 5A fuses
in the power rails for the 732 and this cheerfully delivers peak currents
well over 30A on music[1] with no signs of fuse failure. Yet when I checked
some fuses, some blew in given conditions when others did not, despite
being the same rating, and out of the same packet...

[1] The music means the current is in brief peak spikes, and each fuse only
sees the current for that 'half' of the waveform. This seems to make a lot
of difference and the fuses I've used seem to survive this despite a d.c.
current much less than 30A probably causing the fuse to blow in less than a
second. IIRC I used 'Fast' fuses as the Slow ones never seemed to be any
use as they seemed to take far too long to be of value in protecting
anything. However since this is mostly recalling 20+ years ago, I can't
recall details, and for all I know modern fuses differ in some ways.

The snag is that the behaviour is highly non-linear. As you increase the
current, the teperature rises, and the resistance rises with it. This, in
turn, tends to increase the heating effect for a given current. As the fuse
heats up the heat loss due to radiation varies as T^4 (-ish). So you end up
with a non-linear and time-varying behaviour. Below a given level the
situation stabilises. Above some level the system may go into thermal
runaway, thus producing a fuse-blow time that varies in a complex way with
the current level.

My reaction was along the lines given above - to simply avoid them in the
signal path. I have assumed that other people do the same. However if they
do not, it is a problem we may have to re-visit. Does anyone know of data
on this? If not, I, or someone else, could perhaps do some measurements of
some standard example types and report the results.

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

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

RdM wrote:

I wrote this several days ago, but held back ... it being early am, and I
wasn't sure of what I'd written, especially the paragraph beginning
"Perhaps again" ... and I'm still not ... but I want to release it anyway

Verbatim ...

Patrick Turner writes in
:
:
: Jim Lesurf wrote:

..

: interested to see your definitions for some of the terms you use like
: "middle of the switching region". Also for why it has to be "10 times"
: as opposed to some other value, etc....
:
: You may do your own research on the issues raised. I picked arbitary
: figures.
:
: I was assuming you claim was based upon evidence. Do you not have any
: evidence for what you said? You have given no real reason so far for the
: values you now say are "arbitrary" should be taken as validating the claim
: you made.
:
: Yes, I got the evidence. I don't have time to present it to the readers of the
: group for the umpteeth time.

I think here that Patrick forgets that he has only quite recently started
posting to uk.rec.audio, and that many here may not be aware at all of
any of his previous presentations, or debates, in say, rec.audio.tubes.

: Its time you read the info that's out there to be read.

...

: Then don't believe me if I have no evidence.
: I suggest the evidence exists to support the claims I make, but I don't have
: time to
: argue yet again and again

Yet, for many here, Pat, your "voice" may be quite new ... it may be a
mistake to assume that they have read anything you have written earlier.

...

: I have not the time to devote to addressing all issues raised.
: I can't provide bookfulls of info now that you won't read about elsewhere.
:
: Patrick Turner.

Perhaps at least you could list your website ...

Perhaps again, looking through posts you have made elsewhere and
previously, you may be able to point (via a Google reference?) to where
you have discussed these, similiar, or closely related issues ... then at
least relative strangers may be able to build up some understanding of
your POV and experience ... and even, similiarly, vice versa ...

If, of course, it would save time ... or aid in clarification ... at all.

Evidently time is (or appears to be!) short, at the moment, but that is
no reason to be short in your responses ... ;- ( brief, short, terse? :-)

I think Jim is genuinely searching to understand your POV, and his gentle
questions for clarification, and honest feedbacks, might be considered
edifying, rather than disputatious. It may be useful to revisit them ...

I believe such questions and answers are a valuable teaching process.

IOW. Time may be short, but at least be aware when you are starting anew
in a new forum that you may have no history but the present ... [again!]
Similiarly, what may feel like oppositions may be useful engagements ...

Smooth pebbles, after all, are formed from collisions of rough edges ;-)

Best regards, (and please excuse my frank if not georgeous remarks;-)

RdM
no website at this time ... ;-

:
: 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

--

I have learned silence from the talkative, toleration from the
intolerant, and kindness from the unkind; yet strange, I am ungrateful to
those teachers.

Wisdom is lost on thee...



Kahlil Gibran

Thanks for pointing out some salient facts about my posting history,
and how I may be seen to be not quite fitting in with the this group.

But I have contributed bookfulls of postings at rec.audio.tubes,
and also a lot at the much more uncivilized aus.hi-fi, where I laboured long and
hard to explain to a mainly
unappreciative audience who rarely used a soldering iron.

I did make some remarks here earlier about the non flat and peaked response the open
loop response of
a typical SS amplifier, one which I am completing as we type.

When measuring an SS amp for open loop response, it is necessary to shunt
the AC signal NFB with a large value cap from the NFB port of the amp
to 0V, so the DC voltage at the output can still be fed back to maintain
CD stability at least, necessary because passive trimming of the DC gain
is impossible because in fact the gain is around 250,000 times, or over 100 dBv.

So when a cap is used to shunt the DC, the cap does have some impedance,
even if it is 300 uF, and so the response does not measure flat below the
open loop HF pole, which is around 1 kHz.

SS amps are usually all direct coupled, which means their response *should*
extend down to 0.0 Hz, but in fact this is impossible, unless we have
infinitely low impedance of the PS, which is almost possible if we used
good active regulation, or about 50 truck batteries..

When NFB is connected normally, its easier to estimate the gain and the open loop
response
by placing a CRO on the output of the diff pair.

In my amp this voltage is nearly flat from 5Hz to 800 Hz, and there is 4 mV to make
35v at the output into
8 ohms.
Thus the gain of the VAS stage is 8,750 times.

I had already measured the diff pair gain to be 30 during the testing stage,
and so total open loop gain is 30 x 8,750 = 262,500 times.

The output stage is emitter follower, and has about 40 dB of NFB applied there.
At 400 Hz, and when global NFB is applied, the open loop gain is reduced from
262,000
to 33, a reduction of nearly 8,000 times.
Since thd at 34vrms of output is around 0.005%,
we could have expected thd to be 8,000 times worse with no NFB,
and that would mean it would be 40%.

When i did test the amp with no NFB, thd sure wasn't 40%,
more like 5%.
so the use of NFB in SS amps does not always seem to follow the laws of
gain/feedback equations like it generally does in tube amps using far less NFB.

Perhaps the fuse at the output really is contributing all that thd;
my oscillator which makes about 0.002% at the 1v level.
And the wiring of this re-engineered amp isn't state of the art, and so star
earthing not entirely optimal, but then 0.005% at 144 watts isn't too bad a result.
At 10 watts, the oscillator distortion seemed to be dominating my measurements...
Maybe I can wheel out the 1 kHz LC filter with 30 dB attenuation at 3 kHz,
and that may improove the thd measurememnts, along with placing a
shunt across the fuse, or using a 40 amp auto fuse, which should have less effect
than the
10 amp one I have their now.
The fuse is outside the NFB loop.

When the CRO was connected to the signal from the collector output of the diff amp,
the response showed a rise in the signal at 6 dB/octave above 1 kHz for about 24 dB,

then a couple of peaks and troughs, before the signal rolled away before 1 mHz.
This rise in signal is the amp trying to keep the output signal flat because the
open loop
response would normally have a roll off at 6 dB/octave after 1k.
The tiniest drop in output level is corrected thousands of times by the OLG.

I estimated there was 20dB of NFB actrive at 50 kHz.
and at 1 kHz, its about 77 dB.

So with the NFB of the output stage, a total of about 117 dB is in effect where OLG
is maximum.

I have yet to finally optimise the HF OLG of the VAS by using a variable radio
tuning cap and a pot in series
between the VAS collector and its base.

When that's done, time for the amp to be introduced to Motzartian test signals.

After having spent the time on this project, I won't be repeating it;
I will tell a client I either start from scratch and build him a new amp,
preferably using mosfets, and leave such blown up Phase Linears for the dumper bin
that someone here said was near full of tube amps.

I may appear to not be willing to discuss every widget and twadget in the thread;
I have to earn a living, and I am time poor.

But I hope I have been of use to those actually building something, rather than
spending time all night
picking holes in people's arguments and building nothing, observing nothing,
measuring nothing,
and understanding less.

And BTW, I do have a website, and one due for re-vegetation after all the browsers
have passed
over the last 4 years, and I will get around to it, along with my tax return, and
all the other work.

There is little at my website regarding solid state design and build methodology.
The master at that appears to be Douglas Self, from whom I have learnt a lot.
Ben Duncan is another who inspired me to consider the considerables.
Fritz Langford-Smith taught me more about tube craft than anyone else.
he wrote the Radiotron Designer's Handbook, along with his merry team of helpers.

I hope I can learn more than I forget.


Patrick Turner.

http://www.turneraudio.com.au

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

"Jim Lesurf" wrote in message
...
In article , Mike Gilmour
wrote:

"Jim Lesurf" wrote in message


Unfortunately, you may find that conventional distortion measurements
may not show up the effect of the fuse in series with the output. The
effect will depend upon how the thermal time constant of the fuse
compares with the time of a cycle of the test waveform. The effect
shows up more clearly if you either carefully measure the apparent
gain (into say 8 Ohms) or the output impedance, each as a function of
signal level.


Jim, do you have details of any research on the temperature coefficient
of low amperage cartridge fuses in audio use i.e. both quick acting (F)
& anti surge (T) ?

Alas, no. :-/

I did do some measurements on this 20+ years ago, but at that point
decided
to avoid them simply to avoid extra o/p resistance, etc. I then did some
quick measurements on the fuses in my friend's amp (this was about 5-10
years ago). But in each case I didn't keep results, and only did
measurement on a few types and examples. so don't know how general the
results would be.

I was thinking about this again this morning when I went in to 'work' and
asked another colleague who had been involved for the amp tests. But he
could not recall details, either. I could re-do some measurements
sometime,
but these would then just be on a few 'example' fuses, so the results
might
not apply to all types.

In principle, you just use a bench supply to run in constant-current mode,
and a couple of meters to plot out the quasi-static IV curve. However this
does not tell you the dynamics of the response time if it is quick. For
that pulsed measurements are needed.

I assume someone *has* done this, but I don't know where the results may
be. Can't recall seeing a paper on it anywhere. I suspect that many
engineers had the reaction, "Oh, that might be a problem, so I'll avoid it
rather than study the details!" :-)

Fuses do have some frustrating/surprising properties. FWIW I use 5A fuses
in the power rails for the 732 and this cheerfully delivers peak currents
well over 30A on music[1] with no signs of fuse failure. Yet when I
checked
some fuses, some blew in given conditions when others did not, despite
being the same rating, and out of the same packet...

[1] The music means the current is in brief peak spikes, and each fuse
only
sees the current for that 'half' of the waveform. This seems to make a lot
of difference and the fuses I've used seem to survive this despite a d.c.
current much less than 30A probably causing the fuse to blow in less than
a
second. IIRC I used 'Fast' fuses as the Slow ones never seemed to be any
use as they seemed to take far too long to be of value in protecting
anything. However since this is mostly recalling 20+ years ago, I can't
recall details, and for all I know modern fuses differ in some ways.

The snag is that the behaviour is highly non-linear. As you increase the
current, the teperature rises, and the resistance rises with it. This, in
turn, tends to increase the heating effect for a given current. As the
fuse
heats up the heat loss due to radiation varies as T^4 (-ish). So you end
up
with a non-linear and time-varying behaviour. Below a given level the
situation stabilises. Above some level the system may go into thermal
runaway, thus producing a fuse-blow time that varies in a complex way with
the current level.

My reaction was along the lines given above - to simply avoid them in the
signal path. I have assumed that other people do the same. However if they
do not, it is a problem we may have to re-visit. Does anyone know of data
on this? If not, I, or someone else, could perhaps do some measurements of
some standard example types and report the results.

Slainte,

Jim

--
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In effect a postive temperature coefficient resistor in series with the
signal path. I assume L can be generally ignored as straight piece of wire.
It's interesting to observe the 'element' of a clear glass cartridge fuse of
the fast blow variety near to its I rating, you can see the wire 'element'
bow slightly only to return when the current is reduced (the element seems
too short for this to be a result of expansion). I understand this to be
the reason why some older fuses just fail below their rating because they've
been through this heat-cool cycle once too often. But is there an ongoing
change to the fuse's resistance due to these heating-cooling cycles? Does a
new fuse have the same resistance as an old fuse when heated? Too many
unanswered questions....
Experimenting with a fuse in the signal path bypassed using a relay contacts
controlled by the operator/listener may be a way to highlight changes.


Mike

Valve amp (preferably DIY) to drive apair of Wharfedale Diamond II's
 

On Fri, 26 Nov 2004 02:17:32 +1300, RdM wrote:

Smooth pebbles, after all, are formed from collisions of rough edges ;-)

Nice thought............ :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering