"Phil Allison" wrote in message ...


"Gareth Magennis"
"Phil Allison" "Gareth Magennis"
Arfa

Not seen one of those before.

As for the magents, I believe they might be something to do with
affecting
the arcing between contacts under various fault conditions?

Phil will know the complete details as I remember him posting on here
before about it.


** Someone has a good memory.

Similar magnets are attached to the relays in certain BGW amplifiers (
1980s, US made heavy weights) and no doubt for the same reason.

The purpose is to enhance the DC breaking capacity of the contacts - with
DC
supplies above about 60V, ordinary relays cannot break arc that forms upon
opening IF the amp " goes DC".

The magnetic force causes arc to form into a U shape, making it longer and
far more likely to break.

IME, an 80V DC supply feeding into a 4ohm load via a large relay will
break
this way - but *just* and probably only once !!!!!

It would have NO chance without the magnetic field.


Phil, you have to also tell us the extremely relevant story of grounding
the
speaker relay's normally off terminal, so a "DC fault" amplifier doesn't
fry
the speakers via the arc that doesn't die.


** Some power amps use that idea ( ie diverting the arc to ground while
shorting the speakers ) but there has to be fuses in each DC supply rail -
or you will soon have a fire on your hands.



Yes, I thought the idea was that proper fusing protects both amp and
speakers.

I note that some Peavey amps, for example, do not have any speaker relays at
all, but use a SCR/Triac to short the output in the event of DC, saving the
speakers, but blowing the amps, which dont appear to be fused properly for
such an occurrence.


Gareth.



Gareth.