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Old 3rd November 2007, 10:24 AM   #21
jnb is offline jnb  Australia
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I think Rod Elliott sums it up well. In my own words he says that the relay should not be used in the normally closed position for normal listening. This is because a loss of power to the relay or relay fault should act as a fault condition in the protection scheme, i.e. the relay shouldn't be counted on to switch in to protect the amp.

Rod also suggests the normally closed contacts be grounded. This is because if DC is coming from the rails and going to the speaker, and the relay tries to disconnect this, there may be an arc that would cause the current to continue through the speaker. With the NC grounded, the speaker is now grounded. So too the arc might continue to ground which is unfortunate but if so, the rail fuses should blow.

IMO, the protection is worthwhile and if desired, a high quality relay can be used and changed every year if desired. A relay cradle is a handy thing to use regardless.
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Old 3rd November 2007, 11:54 AM   #22
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I haven't come across any amplifier yet that would take the output to ground thru a relay. Instead they pass the signal thru the relay which makes sense. Grounding the output of the amplifier is only asking the amplifier to self destruct which shouldn't be the goal here. In my understanding the goal is to protect the speakers with minimum effect on the signal.

So, how does one protect the speakers if the amplifier develops a problem and willfully tries to pass DC along? Two ways are possible from the way I see it.....1.) before the dawn of protection circuits in amplifiers we relied on capacitors to block any DC that might go out. 2.) When relay/ptotection circuits came along they provided the necessary protection to see that no DC made its way to the speaker.
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Old 3rd November 2007, 11:09 PM   #23
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Quote:
Originally posted by jnb
I think Rod Elliott sums it up well. In my own words he says that the relay should not be used in the normally closed position for normal listening. This is because a loss of power to the relay or relay fault should act as a fault condition in the protection scheme, i.e. the relay shouldn't be counted on to switch in to protect the amp.

Rod also suggests the normally closed contacts be grounded. This is because if DC is coming from the rails and going to the speaker, and the relay tries to disconnect this, there may be an arc that would cause the current to continue through the speaker. With the NC grounded, the speaker is now grounded. So too the arc might continue to ground which is unfortunate but if so, the rail fuses should blow.

[...]
Those are two very good points, though I have never seen an amp which didn't follow the first one. The second one is not always implemented, but is a very good insurance because finding relays that will reliably break over 28VDC is not easy. To clarify, NC goes to ground, NO to the amp and common to the speaker. So the amp will only be shorted if there is an arc in the relay.
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