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Old 29th December 2001, 10:38 PM   #21
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Quote:
Originally posted by GRollins
A constant magnetic field (such as the Earth's--if you're going to worry about 'DC' magnetic fields, don't forget that one) will not have an effect on the current in a wire. Only a varying magnetic field will create a current in a wire.
A current carrying wire travelling thru a magnet field will have a force act upon it .... the wire is now moving and this will then cause interactions with the DC current inside the wire. OK, this doesnt really follow in relay because of the mechanical structure .... BUT ..... But the signal travelling thru the relay is AC and therefore the magnet does effect it.

Quote:
Originally posted by GRollins
Crowbar circuits aren't perfect, either. For that matter some people seem congenitally unable to drive a car safely. Does that mean no one should drive a car?
I agree they aren't perfect but for DC protection they are usually safer than relays when designed properly.
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Old 30th December 2001, 07:14 AM   #22
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Quote:
Originally posted by AudioFreak
What do you have to lose? how bout the amp and speakers.
Of course... i meant by giving the circuit a try...

Quote:
The arc is potentially huge and can/will easily weld the contacts which can then lead to a totally dead amp and fried speakers.

The relays are great for turn-on/off muting but this is where their use should finish ... a crowbar circuit should be used for DC protection as a relay will easily be arc welded when used in the protection circuit.
For starters, i use a slow burn fuse in series with the relay output, just to be absolutely safe. But again, if you find a good quality relay with the appropiate DC ratings, why should you feel so worried? The arc is not "potentially huge" either... a 50v arc (let's say), regardless of the current, can only exist when the contacts are about 0,5mm close. This is with the relay moving, and the dropout time is measured in milliseconds... Yes, the current CAN be high, specially if the loudspeaker shorts, and YES, it can get hot. Enough to melt the contacts? It depends. I'm not saying it can't happen; but don't feel you'll kill your loudspeakers/amp just by placing a relay there. Good relays are very very rugged.

I checked other muting/protection alternatives, but they all compromised sound quality....
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Old 30th December 2001, 07:25 AM   #23
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the voltage is by no means limited to the supply rails and can be many many times this because of the inductance of the woofer.
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Old 30th December 2001, 07:41 AM   #24
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How high can the voltage get? well the back emf from the inductive woofer can approach infinity .... i think that should be enough for arc welding

In case you didnt realise, your relay + fuse solution adversely effects sound quality.
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Old 30th December 2001, 08:41 AM   #25
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Quote:
Originally posted by AudioFreak
How high can the voltage get? well the back emf from the inductive woofer can approach infinity .... i think that should be enough for arc welding

In case you didnt realise, your relay + fuse solution adversely effects sound quality.
I know, but again, the high induced voltage arc doesn't carry the whole amperage the smaller voltage arc does. What makes the arc destructive is the current flowing through it; voltage gives it it's minimum forming lenght; on air that's about 1000v/cm. And don't forget that the relay IS designed to take the DC current (well, in my case at least); this means that the contact area is not just a sheet of metal, but a "cushion" of some other material designed to take the heat of the arc.

And yes, by placing ANYTHING in the signal path i'd be affecting it, but what i'm i doing? Increasing damping? I even cared to use REGULATED dc on the coils to avoid any flux variations on the coil disturbing the signal.

What i mean with all this is that relays are far from perfect, but i see some people dissing them because they beleive the first time they'll work will end up useless. It's not the case.
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Old 30th December 2001, 10:48 AM   #26
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I agree Lisandro but for me a protection circuit is there to protect the system in catastrophic failure conditions and a relay circuit doesnt do this very well.
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Old 31st December 2001, 01:31 AM   #27
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Quote:
Originally posted by AudioFreak
I agree Lisandro but for me a protection circuit is there to protect the system in catastrophic failure conditions and a relay circuit doesnt do this very well.
Mine is designed to take a full short on the power supply... how more catastrophic than that can it get, i dunno But, to each one it's own, thats the beauty of DIY!
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Old 31st December 2001, 01:41 AM   #28
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usually the best is to mute inputs but for power amps you cant avoid relays.
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Old 31st December 2001, 02:20 AM   #29
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Because I've been fiddling with the Alephs a lot recently, I cobbled together a circuit using a three pole relay (large current ratings on the contacts for those who care about such things, came out of computer power supplies). One pole shorts the input to ground, the second drops the output to ground via a 10 ohm resistor out of the junk box. Thus the amp is completely isolated from the external world, front and back.
The third pole?
The third pole puts DC into an LED so that I can see at a glance whether the silly thing's in Mute or Operate status, as I live in sleep deprivation mode and often can't remember whether I've switched the confounded thing to let the music play, and I'm often too exhausted to get up and look to see what position the switch is in...
No there's no provision to sense DC at the output. I did design in a 5 second delay on turnon, immediate turnoff when the AC gets switched off.

Grey
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Old 31st December 2001, 07:37 PM   #30
haldor is offline haldor  United States
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Quote:
Originally posted by GRollins
If I have seen further, it is by standing upon the shoulders of Giants...
Sir Isaac Newton

Grey
Hi Grey,

This is the most commonly misused quote in science. The truth is that when Isaac Newton made this statement he was in the middle of a academic tussle with another researcher (Robert Hooke of Hooke's law fame) who had accused him of plagiarizing Hooke's work on optics.

Newton was generally a pretty unpleasant person, but he got downright nasty when it came to sharing credit for scientific work. After his dispute started with Hooke he called in some favors with the head of the Royal Society and got Hooke properly squelched. The poor guy ended up offering an apology to Newton.

The line about “Standing on the Shoulders of Giants” was in Newton's letter in reply to Hooke’s apology. Instead of being a humble acknowledgement of others achievements it was really another slam against Hooke (he was a dwarf). This was Newton’s friendly way of twisting the knife, he was saying in effect “My work stands on the shoulders of giants, not dwarfs”.

http://www.wordsmith.org/awad/awadmail18.html

http://www.uwe.ac.uk/fas/wavelength/wave21/ramsey.html

Phil
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