Cambridge A75 Power Amp
 

In article . com,
solwisesteve wrote:
So I've just had it running for an hour at 80% volume and the 'scope
shows a perfectly clean output on both channels! :-)

You possess a scope but ask about getting a power amp repaired?

--
*He who laughs last, thinks slowest.

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A75 Power Amp
 

On Mon, 16 Apr 2007 07:29:57 +0100, Steve Swift
wrote:

You remind me of one of my favourite quotes. Some HiFi magazine was
reviewing an amplifier with staggering (at the time) output power of
several hundred watts per channel which claimed to have complete output
protection, even against shorting. So they decided to test it by taking
the leads out of the speaker while running at full volume and brushing
them together.
"The result was large frightening sparks" has stuck in my memory ever since.
I'm glad your amp is back in business.

Well, those several hundred watts WOULD make some nice sparks. But
was any harm done?

Cambridge A75 Power Amp
 

Well, those several hundred watts WOULD make some nice sparks. But
was any harm done?

Only to the reviewer's fingers, apparently.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk

Cambridge A75 Power Amp
 

On Mon, 16 Apr 2007 07:29:57 +0100, Steve Swift
wrote:

I tell you what though, the resisters load I was using got f**king
hot! :-)

You remind me of one of my favourite quotes. Some HiFi magazine was
reviewing an amplifier with staggering (at the time) output power of
several hundred watts per channel which claimed to have complete output
protection, even against shorting. So they decided to test it by taking
the leads out of the speaker while running at full volume and brushing
them together.
"The result was large frightening sparks" has stuck in my memory ever
since.
I'm glad your amp is back in business.


I remember Crown (or Amcron as they were known in the UK at that time)
demonstrating the robustness of one of their large amps at a hifi show by
connecting the speaker outputs to two parallel bars and putting a
screwdriver across the bars. This gave a shower of sparks whenever the
screwdriver was moved.

Cheers

James.

Cambridge A75 Power Amp
 

In article , James Perrett
wrote:


I remember Crown (or Amcron as they were known in the UK at that time)
demonstrating the robustness of one of their large amps at a hifi show
by connecting the speaker outputs to two parallel bars and putting a
screwdriver across the bars. This gave a shower of sparks whenever the
screwdriver was moved.

FWIW I more than once spent a day doing this with the amp I was designing.
The result wasn't only sparks. It meant I went though a boxes of rail
fuses[1], and had to replace the o/p binding posts as there were so many
burns and broken spot welds on the threads that I could no longer wind down
the spinners. The scrwdriver tended to look 'abused' as well... :-)

[1] The distinction being that I relied on dc line fuses for 'protection'.
Less convenient than current limiters, but allowed the amp not to be
bothered with current limiter effects. ;-)

The 'crown' test is easy enough for amps that have current limiters. The
real test is to determine what current or voltage the amp can actually
deliver without limiting.

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

Cambridge A75 Power Amp
 

In article , Bob Latham
wrote:
In article , Jim Lesurf
wrote:

The distinction being that I relied on dc line fuses for 'protection'.
Less convenient than current limiters, but allowed the amp not to be
bothered with current limiter effects. ;-)

I applaud your intention to not use limiter circuits but may I ask, were
your designs single supply rail and AC coupled or were they dual rail?

Dual rail. Fuses in each rail, plus an ac fuse.

The reason I ask is probably obvious, if one rail fuse blows is it not
very likely the other rail will appear across the speaker as 50v of DC?

Not into a short circuit. :-)

FWIW No speaker load ever blew the fuses in my experience.

The design didn't/doesn't behave as you assume. Once one fuse had blown the
amplifier gain evaporated, and the output fell to very little. i.e. nothing
was then 'driving' the output to seek the other rail. So what you ask about
wasn't "very likely". It had been designed not to act as you fear. :-)

When testing with a screwdriver, it was a matter of 'chance' if one fuse
blew, or the other, or both. In each case once the screwdriver was removed
the o/p voltage wasn't at a rail even with no load.

Of course, if the speaker had already developed a fault and its impedance
had dropped to the order of an Ohm, then the amp might have then fried the
result, simply due to its willingness to assert the audio waveform into low
impedance loads. But I wouldn't blame the amplifier for that. ;-

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

Cambridge A75 Power Amp
 

FWIW No speaker load ever blew the fuses in my experience.

Nor in mine, but that counts for naught as I've never had speaker fuses.

However, I've had sufficient mains fuses blow on me for no apparent
reason to believe that fuse wire tends to evaporate over time (like a
tungsten light-bulb filament, but slower).

I'm particularly worried about this effect in the case of the fitted
fridge/freezer that the builder installed in our new house - the 13A
plug is somewhere behind the fitted units, so if that fuse ever
evaporates then we'll have to dismantle the kitchen to get at the plug.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk

Cambridge A75 Power Amp
 

In article , Steve Swift
wrote:
FWIW No speaker load ever blew the fuses in my experience.

Nor in mine, but that counts for naught as I've never had speaker fuses.

Nor have I. I was referring to fuses in the dc lines of the power amp. :-)

FWIW I'd avoid a fuse in the speaker lead as it would be a nonlinear
element as well as adding series resistance.

However, I've had sufficient mains fuses blow on me for no apparent
reason to believe that fuse wire tends to evaporate over time (like a
tungsten light-bulb filament, but slower).

I'm not sure of the physical reasons, but I also would expect fuses to
alter over time. I assume the risk of eventual failure would depend on the
typical current level relative to the rating, and the details of any
current 'surges'.

FWIW In the power amp I was referring to, I've used one for 20+ years, and
a second for about 10 years, with no fuse failures.

Above said, I have experienced the other end of fuse behaviour. I've also
used a pair of Armstrong 626's for many years. These have a dc line fuse
for their pre-amp and tuner which was rated at only just above the steady
current. Hence unless you uprate these they tend to fail ever few years.
The unit then seems 'dead' as no lights come on, etc. But the giveaway is
that you can play music if you put the signal in via the tape monitor input
which bypasses the preamp, 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

Cambridge A75 Power Amp
 

In article ,
says...
FWIW In the power amp I was referring to, I've used one for 20+ years, and
a second for about 10 years, with no fuse failures.


I was always taught that a transistor was an expensive device designed
to protect a fuse..

T.
--
Do Binary Tripods have 11 legs ?

Cambridge A75 Power Amp
 

Tony Gartshore wrote:
In article ,
says...
FWIW In the power amp I was referring to, I've used one for 20+ years, and
a second for about 10 years, with no fuse failures.


I was always taught that a transistor was an expensive device designed
to protect a fuse..

T.

My first proper transistor amplifier (I don't count the Sinclair
Z-12)was a 30 watt design published in (I think) Practical Electronics
which had a stabilised power supply in which the series regulator
transistor would blow to protect the fuse.

S.



--
http://audiopages.googlepages.com