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Old 25th April 2004, 01:23 PM   #1
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Default How does clip protection work?

How does it work? I have the possiblility of buying a big pro amp, but one chan has failed and the clip light stays on. Logic suggests that the clip circuit is shutting down the input. So if I fixed it or bypassed it, the amp would work.

I searched the threads but all I found out was that you guys don't use them much.
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Old 25th April 2004, 02:42 PM   #2
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Usually the 'clipping sensing/protection' is done by 'monitoring' the current passing through a low value resistor (say 0.1 to 0.22 ohms or so) placed either on the output devices emitter (source when mosfet) or on one of the amp's rails.

So I guess if the clipping indicator light is on at all times then too much current is passing trough that particular resistance, this could mean ugly stuff like fried output devices and so on.
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Old 25th April 2004, 02:47 PM   #3
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clipping occurs when the output volts reaches the rail voltages, and analogy is when you jump and hit the ceiling, that condition is clipping...

so a monitoring circuit, uses a comparator to monitor tyhe output voltages, taking the rails as refference voltage....
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Old 25th April 2004, 02:50 PM   #4
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One channel is probably defective, with an output transistor shorted to the rail. This appears as a clip.

The repair could be simple or complex, cheap or costly. Unless you are an expert at repair, you might not be able to get it working. You have to make that evaluation.
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Old 25th April 2004, 02:57 PM   #5
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Hmmm. So I could narrow down the possibilities by disconecting the path out of the protection circuit?

Also a blown FET wouldn't kill the rest of the amp because the curent limiter would have shut it down before then?

I need to hit the books about dead FET diagnosys. I would ask for help on here but the audiophiles would kill me for being lazy!
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Old 25th April 2004, 03:26 PM   #6
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you HAVE to fix the outputs...or else you will get one side's voltage on the speakers~!!! make sure you determine if the other components are working properly before turning it back on...or else poof...u just killed the new parts...
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Old 25th April 2004, 04:07 PM   #7
K-amps is offline K-amps  United States
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In the old Adcom's Nelson used a simulated clip circuit that was driven off the input stages, meaning it was more accurate with 8 ohm loads etc.

Point is, the clip LED may be driven off the input stages and you might need to look there too...

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Old 25th April 2004, 05:15 PM   #8
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I take what everyone had said so far. Its a QSC USA 850. I'll download the repair manual before I start into it. I'm still up for suggestions though.
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Old 25th April 2004, 05:27 PM   #9
imix500 is offline imix500  United States
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Actually most professional audio amps use a comparator on the outputs and Crowns in particular use a distortion detector that tracks the waveform in and out of the amp, also a kind of comparator. Some are VERY good at stopping such problems as shorted output devices from taking out any anything else by turning off the HV rails if the comparators throw up a red flag.
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Old 26th April 2004, 01:53 AM   #10
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Default speaker time dalay ralay!

if your amp has a speaker relay, then chances are you have blown output devices on one channel...

you can spot it by checking the offset at the outputs, one channel has an offset voltage close to either rail...so that channel has blown ouput devices....
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