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Old 28th October 2007, 12:15 PM   #1
d3imlay is offline d3imlay  United States
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Default Output relay success stories

Does anyone have any success stories where the output relay on a failed amplifier disconnected the speaker and prevented speaker failure? How about relays that welded.

It seems to me that the time delay for the sensing circuit to trigger plus the 10-20 millisecond delay that it takes for the relay to drop out would allow a failed amplifier to damage the woofers. Even if the voice coil doesn't burn, I'd expect the coil to jump the gap.

Very few relays have contact ratings greater than 28 VDC. Certainly the relay contacts would weld shut on most any amp good for 100 watts or more (50VDC or more rails).
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Old 28th October 2007, 12:40 PM   #2
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Over the years I have experienced many failures that caused the activation of the protection circuit. The relay always prevented the sacrifice of a speaker.

I took in a SAE2200 one time that had failed and had its relay contacts welded together. Can't help but think some damage had resulted.

Most of the time it works and works well but nothing is 100% in this life. Better a relay than nothing at all.
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Old 28th October 2007, 12:43 PM   #3
d3imlay is offline d3imlay  United States
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What relay seems to work well?
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Old 28th October 2007, 12:53 PM   #4
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Default I for one instance, and now have to have them

I now only use amps with an output relay. It saved me from damaging a very fragile driver once and I was a believer.
Used to use only Lowther speakers and had a set of 15 ohm, silver voice coiled PM-7A drivers in my caibinets. The amp had a cold solder joint in the feedback loop, and after months of use opened up. This dumped 33 volts dc on the output.
I was not looking at the speaker when this happened, so I did not see how far the driver jumped. But my ear facing the speaker rang for two days. The 100 dB efficient driver made an unbelievable noise. It upstair in my listening area. My wife was downstairs in another room and came running. She thought I had pulled the trigger on a 12 ga shotgun or some bomb.
The relay saved the driver, and I used it for a long time with no aftereffects. If you value your speakers, you should have a realy on the outputs.

George
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Old 28th October 2007, 01:03 PM   #5
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I like Magnecraft and Potter&Brumfeld.

You can always double up on the contacts.

The Magnacraft is a w78CSX-4

The other is a R10-E1-Y4-3.5K
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Old 28th October 2007, 01:04 PM   #6
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Another shot
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Old 28th October 2007, 01:57 PM   #7
Hi_Q is offline Hi_Q  England
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I would always recommend a relay protection circuit with SS amps.
Despite their problems with release times, they are a good first line of defense. Nowadays you can buy 10 Amp contact rated relays with safety breaking, i.e. designed to force open contacts that may be stuck together. My friend still has nightmares of the day his unprotected Cambridge amplifier blew the right hand channel vintage Tannoy speaker and in his ignorance, thinking it was the speaker, proceeded to wire the faulty amp channel to his left Tannoy!
He cried all day until I was able to find him a Tannoy expert to rewire the cones. Oh Boy what a day that was!
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Old 28th October 2007, 02:40 PM   #8
Javin5 is offline Javin5  Switzerland
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Default Is electronic protection a preferable alternative to relais?

Very interesting. How would you rate the reliability of electronic protection circuitry compared to relais? They certainly can be made to react quicker than relais. Any practical experience?
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Old 28th October 2007, 04:16 PM   #9
CBRworm is offline CBRworm  United States
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I can't answer your question, but in my mind a mechanical disconnect (relay) is preferable. Assuming the circuitry driving it is good.
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Old 28th October 2007, 05:02 PM   #10
Mooly is offline Mooly  United Kingdom
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Quad in their 405 power amp used a triac as a crowbar across the speaker outputs.A 15k resistor & 10mf cap set the time constant, but a diac in the gate meant around 30volts dc offset would be needed before it would operate.Seem to remember it was really to provide protection for their elecrostatic speakers.Doug Self's articles in "Electronics World" mag back in the 90's investigated relays, and I think he covered such things as drop out time.How saturated the magnetic core of the relay is has a large effect on how quickly the relay will "let go".Run it at as low a current as it will reliably hold.Also set your time constants on the safe side, for e.g. ask yourself do you really need to produce 100watts at 5Hz.Design your offset detection circuit to react as fast as possible.
Karl
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