power amp clicks, speakers shut off

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TapeOpHillary
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power amp clicks, speakers shut off

Post by TapeOpHillary » Wed Apr 08, 2009 7:28 am

what's it all about when your monitor amp clicks and the monitors stop working but the amp stays on? is this a dead resistor? loose/cold solder point?
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Post by Nate Dort » Wed Apr 08, 2009 7:47 am

Thermal protection maybe? Without pulling it apart on the bench, it's hard to diagnose something like this.

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Post by CurtZHP » Wed Apr 08, 2009 7:54 am

A lot of power amps have some sort of thermal protection built in. When the temperature inside the box reaches a certain point, it triggers a relay which switches off some stage of the circuit. That's likely the clicking sound you're hearing. Once the temperature returns to normal, the relay opens and the amp behaves. The amp stays powered on so the control circuit can still operate. It's also better than having the amp just shut off and cause a huge THUMP in your monitors.

Where is the amp located? Can it get enough fresh air to stay cool? Do you have it sandwiched between other pieces of gear in the rack? Try putting some space between it and other gear. If it's properly ventilated and still misbehaving, then something inside is likely out of tolerance.
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Post by TapeOpHillary » Wed Apr 08, 2009 7:55 am

i thought about that too. is it common for most amps to have different circuits for headphones than for monitors? because the headphones will still work.
(actually have this issue with my crest power amp and a friend's stereo receiver)

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Post by TapeOpHillary » Wed Apr 08, 2009 7:59 am

CurtZHP wrote:A lot of power amps have some sort of thermal protection built in. When the temperature inside the box reaches a certain point, it triggers a relay which switches off some stage of the circuit. That's likely the clicking sound you're hearing. Once the temperature returns to normal, the relay opens and the amp behaves. The amp stays powered on so the control circuit can still operate. It's also better than having the amp just shut off and cause a huge THUMP in your monitors.

Where is the amp located? Can it get enough fresh air to stay cool? Do you have it sandwiched between other pieces of gear in the rack? Try putting some space between it and other gear. If it's properly ventilated and still misbehaving, then something inside is likely out of tolerance.
what if when it's turned on it's side the problem happens but when it's seated properly it doesn't happen (with proper ventilation on all sides)?

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Post by CurtZHP » Wed Apr 08, 2009 8:45 am

A thermal protection circuit will still allow the headphone amp portion of the unit to work. The headphone circuit doesn't generate any heat and runs at a very low voltage. The heat is generated by the high current/high voltage final amplifier stage.

If it only does it when the thing is stood on end, it could just be a flaky relay. Relays are electro-mechanical devices (IOW, they have moving parts), so it's quite possible that good old fashioned gravity is playing a role here. It's a stretch, but not impossible.

It could have to do with where the large heat sinks are inside the unit. Since heat rises, if the orientation of the unit is such that the heat is rising right into the thermal protection circuit, that could be a factor.
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Post by TapeOpHillary » Wed Apr 08, 2009 8:55 am

CurtZHP wrote:A thermal protection circuit will still allow the headphone amp portion of the unit to work. The headphone circuit doesn't generate any heat and runs at a very low voltage. The heat is generated by the high current/high voltage final amplifier stage.

If it only does it when the thing is stood on end, it could just be a flaky relay. Relays are electro-mechanical devices (IOW, they have moving parts), so it's quite possible that good old fashioned gravity is playing a role here. It's a stretch, but not impossible.

It could have to do with where the large heat sinks are inside the unit. Since heat rises, if the orientation of the unit is such that the heat is rising right into the thermal protection circuit, that could be a factor.
excellent... i will do further investigation!
thanks!
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Post by nclayton » Wed Apr 08, 2009 10:16 am

Some amps have thermal protection, but 90% of the time the relay at the output is actually used on DC coupled amps to protect speakers from output offset voltage. It's timed to come on within a few second of when you turn the amp on so that the DC settles before the speakers are attached, and then there's usually a servo and comparator which shuts it off again if offset goes beyond some preset level.

I don't know if this info really helps you though, since it doesn't get you any closer to fixing your amp. But at least if you can find the amp's outputs PRIOR to the relay, you can measure that point with a voltmeter and see how much DC offset there is. If there isn't any, or at least hardly any, then the problem is most likely in the protection circuit itself rather than the amplifier.

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Post by TapeOpHillary » Wed Apr 08, 2009 10:20 am

nclayton wrote:Some amps have thermal protection, but 90% of the time the relay at the output is actually used on DC coupled amps to protect speakers from output offset voltage. It's timed to come on within a few second of when you turn the amp on so that the DC settles before the speakers are attached, and then there's usually a servo and comparator which shuts it off again if offset goes beyond some preset level.

I don't know if this info really helps you though, since it doesn't get you any closer to fixing your amp. But at least if you can find the amp's outputs PRIOR to the relay, you can measure that point with a voltmeter and see how much DC offset there is. If there isn't any, or at least hardly any, then the problem is most likely in the protection circuit itself rather than the amplifier.

Ned
thanks ned,
i'm not sure what i'd be looking for with measuring the dc offset - my voltmeter only gets used for continuity ;)
but it's good to know!

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Post by nclayton » Wed Apr 08, 2009 10:22 am

Then again, if the amp only shuts down when it's on its side, it could be a ventilation problem, as someone already said. So......

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Post by TapeOpHillary » Wed Apr 08, 2009 11:45 am

nclayton wrote:Then again, if the amp only shuts down when it's on its side, it could be a ventilation problem, as someone already said. So......
just got more info about my friend's receiver - there are surround o/p's that still work. assuming the surround o/p's are different circuitry than the main o/p's??

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Post by CurtZHP » Wed Apr 08, 2009 6:14 pm

That's possible. Could be the manufacturer skimped on the surround amp circuits.

Not knowing much more about the amp, this is all just speculation though.
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