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Old 2nd October 2011, 09:54 PM   #81
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Originally Posted by CopperTop View Post
So is the preferred approach to break the connection to the speakers, or to short them out? I am not convinced by the series relay approach.

Looking in detail at a relay data sheet, DC breaking capacity is specified for a resistive load. It seems to fall off rapidly as the voltage exceeds 30V. For a typical relay, at 50V DC, breaking capacity is <10% of the nominal current rating (so a 20A relay is rated for <2A at 50V DC).

Just what voltage and current will be present if the amp goes wrong? I'm presuming that it could be 50-60V for a powerful amp, and >10A through the speaker coil. Can you be really confident that you're not going 'tack weld' the contacts even with a large relay and snubber across the contacts?
I believe there is not too much a choice. I don't like to use fuses on the power rails as their present 2 inconveniences: They introduce thermal distortion and they are slow and not reliable. i do not want my amp to be supplied only one one rail for some times or until i change a fuse.
On my 2X140 watts amp, with a huge power supply, i had made hundreds of short circuit demonstrations at different listening levels, at the time i wanted to sell this protection idea to manufacturers. And i'm still using the same relay, and the same amp !!! It is a good relay, i use one for each output, with 2 paralleled contacts. And my loudspeakers are high efficiency (high motional current) with a very complex filtering and lot of big selfs. Yes, it makes a big sparkle, both with the screw driver you use to short circuit and inside the relay, but it has always worked, and still work, as i said. And serial resistance is still Milli Ohms. In a way, this sparkle clean/ burn the pollution on the contacts ;-)
I believe a problem can occur if there is a delay before the protection cut the relay, as it is with most of the usual schematics. Not the case here, it is instant and the contacts do not get time to heat at all. When you use a relay to contact on, it has some bouncing. Not when it cuts.
Anyway, you can use power mosfet instead if you prefer, as for a PA system. At home, you do not short circuit your outputs, by habit ;-)
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Old 2nd October 2011, 10:03 PM   #82
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I believe too for those who are afraid about the opening capabilities of the relay they can add a fuse in the output line of the amp, before the CR loop or the power rails.
Like that, the loop will correct the distortion of the fuse, and the fuse will blow in case of short circuit and not opening of the relay. If the relay works as expected, the fuse will not have the time to blow, i presume.
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Last edited by Esperado; 2nd October 2011 at 10:13 PM.
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Old 2nd October 2011, 11:29 PM   #83
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It's clear that Quad have thought very hard about these problems. The Quad 405 has a triac-based crowbar and the negative rail to the input stage comes from before the fuse, while the positive rail comes from after its fuse - they may have found that this gave the best behaviour in fault conditions..? They also had current limiting SOA protection on the transistors, but still felt the need to fit the crowbar. (Does the crowbar triac have any heatsinking, or is it guaranteed that the fuse will always blow quickly? You could imagine a fault putting a constant few amps through the triac but not blowing the fuse and eventually burning out the triac..?).

And then in the later 606, 707, 909 they use the floating ground arrangement that provides a direct coupled output and yet the equivalent of series capacitor protection to the speaker. There's only a single fuse to blow in the transformer secondary. It seems fiendishly clever. In those amps, the design seems very clean and simple and they don't bother with the crowbar.

No relays!
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Old 10th October 2011, 09:26 AM   #84
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Solution, in case of short circuit to protect relays from cutting a huge amount of current was trivial, after some thinking about: to cut the input signal !
Just this cut has to be faster than the power one. choosing good parts.
Added in my protection circuit, together with paralleled two mechanical relay's contacts, a solid state relay if needed (you can chose the one you prefer or the two.)..
No one can argue anymore about any sonic or measurement degradation with those, i suppose.
And a very fast Speaker-off circuit when power off or AC failure.
http://www.diyaudio.com/forums/digit...ml#post2740494
Enjoy.
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Old 10th October 2011, 01:04 PM   #85
AndrewT is offline AndrewT  Scotland
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Fitting an input mute that activates before normal amp shutdown and output relay opening is a well worthwhile method of increasing the life of the output relay.
The same would apply if the relays were in the supply rails.

But, that input mute does not prevent an output DC fault from burning the contacts as they try to open.
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Old 14th March 2012, 12:02 AM   #86
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