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Posted to rec.audio.tech
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How safe operating an amp with no fuse?
wrote:
All I'm asking is for information on what conditions would
cause the amp to catch on fire
A shortut caused by a failed transistor. A transistor WILL fail if its
core temperature gets too high, CORE temperature. All known audiophile
designs aim for heatsinks larger than average consumer designs because
the cost is less of an issue.
(ie. what exactly can go wrong and what exactly is
flammable inside an amp)
Insulation, laquer, pcb, radiated heat that ignites nearby objects,
worst case a spray of melted copper that ignites nearby objects +
hazardous gasses emitted.
so I can do a risk assessment
I trust you to be void of any capability to so do.
and decide how likely the risk of fire is.
At a guess, 0.8 within 5 years of operation with the output heat sink
removed.
If you understand how a fuse works
yourself, you would be easily able to explain this,
A fuse prevents too large current from being delivered in case of a
shortcut somewhere after the fuse.
other than simply repeating what you've read on a printed
circuit board.
Erm, themn folks here are them folks as writes what is on them printed
circuit boards. Those that have written the "no user serviceable parts,
refer to qualified service personnel" are their lawyers, same lawyers
also no doubt put a full page of warnings in the manual for the amp.
"Replace Fuse only with Fuse of same spec and rating" already is there",
as is "Do not interfere with cooling of amp" and a full page of not-to´s
in drawings with large crosses over for the illiterate.
They have also, as I, seen an amplifier emit smoke for no obvious
reason, which is to say that a large power amp is perfectly able to
start burning, in which case the fuse will stop it by stopping the
current that delivers the heat.
TAke the amp to a qualified repairshop and get it made back into what it
was ex works. If you want to actually improve it, then ask THEM whether
it is possible to retrofit larger capacity, max 50 to 100 percent more,
in the psu and increase the quisciescent current so that the amp
actually gets a wee bit hot when idling with the cooling in place. Both
tweaks may be possible, but understand that it may also be with any
given amp that there are good reasons against them. Standard class AB
amp supposed.
Thermic runaway is a very real risk if quisciescent current is increased
too much, but if the amp can run without cooling while idling now, then
it is likely to be on the low side of specs ex works. Perhaps even
asking them to set it correctly as per the service manual will be what
gives you a real improvement effect. This based on the known fact that
quisciescent current increases when an output stage runs hotter and
giving you the benefit of the doubt.
For sonic improvement the default advice with a pre-existing
installation is: first get the loudpsekaers properly placed in the room,
next fix issues with the room, next look into transduxer quality, and
then, and only then look into electronics quality.
//Walt
Kind regards
Peter Larsen
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* My site is at: http://www.muyiovatki.dk *
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