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Old 23rd December 2001, 03:14 PM   #1
Super is offline Super  United States
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Angry Can overheating be the cause?

This morning, I continously heard a popping sound coming from my speakers, even when no material was playing, but the components were on.

Typically when having this problem, the pop or hum occurs from either the refrigerator or the microwave, but this was not the problem. After playing some material, I noticed that the midrange on my right speaker, which is bi-amped, was cutting out, and then slowly coming back on.

So the pop I was hearing was definitely coming from one of my monoblocks powering the right channel. After feeling the heatsink of the left channel, it was hot, but no hotter than usual. But, after feeling the right channel heatsink, well, I have this lovely pink stripe along my right index finger now.

I checked the fuses, they're fine. Can overheating perhaps trigger protection circuitry, causing the channel to cut out? I've never had this problem before. Should I try simply increasing the ventilation around that amp? (The amps were designed to be stacked, and there is no power switch, it is always plugged in)

Any advice would be greatly appreciated.
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Old 23rd December 2001, 03:40 PM   #2
paulb is offline paulb  Canada
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Default Re: Can overheating be the cause?

Quote:
Originally posted by Super
Can overheating perhaps trigger protection circuitry, causing the channel to cut out? I've never had this problem before. Should I try simply increasing the ventilation around that amp?
Yes, and yes. Although it's unusual that it cuts out so frequently, typically overheating protection has a delay (hysteresis) built in so that it should be something like a minute or two before it recovers.
Hot enough to burn your finger is too hot. Given the fact the other channel isn't, there's got to be something wrong. Unless the bad channel's amp is poorly ventilated.
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Old 23rd December 2001, 05:26 PM   #3
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There are several types of protection used in amplifiers. Some will cut mains power in the event of heatsinks getting too hot, many IC-based amps have thermal protection built into the amp device. Both will create audible pops when they activate.

How rapidly are the pops occuring?

As a simple test, it might be worth your while unplugging your speakers (at the amp end), and seeing if the same amp still gets very hot. This will eliminate a marginal cable short or speaker problem (though you'd probably have noticed reduced levels from your mid-range if this was the case).
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Old 23rd December 2001, 06:17 PM   #4
Super is offline Super  United States
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This morning, the pops were occurring every 5 minutes or so. I've left the amps unplugged all morning, and I've just plugged them back in. I've separated the two monoblocks, so they have the same amount of ventilation now. If the problem persists, I'll try the suggestion given by Jamie. I'll report back and let you know how things are going. Thanks for the help.
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Old 23rd December 2001, 09:14 PM   #5
Super is offline Super  United States
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After plugging the amplifier back in and some mild playing, the same amp is severely overheating once again. The monoblock has its gain devices and PS in separate chassis, so I'm positive that its the PCB+gain devices causing the problem. (The second amp's PS is fed from the first amps PS, and that one is functioning fine)

The monoblock uses four MOSFETs for the gain stage, and the area of the heatsinks closest to the them is where the most heat seems to be coming from. Also, 2 of the mosfets are Motorola devices, and 2 are RCA, so I'm thinking that they were replaced at one point in time as well. Would excessive amounts of heat be generated if the devices are bad? Is there anything that I can check electrically?

Any help would be greatly appreciated.
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Old 23rd December 2001, 09:40 PM   #6
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Did you recently added a new source i.e. cd player/recorder,MD, pre-amp?

Consider uplugging all your sources, connect the input of your power amp to ground and see if the overheating remains. If it does try to use a scope to find any oscillation, or excesive ripple.

If it doesn't the problem is with at least one of your sources. Try to connect one source at the time.
Make sure there are no wiring, or grounding problems. Check interlinks etc.

Hope you find and fix the problem soon.

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Old 23rd December 2001, 10:13 PM   #7
Super is offline Super  United States
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No, I have not recently added any new sources. I've been using my current configuration flawlessly for months.

I checked all the wiring (minimal components in the gain stage, simple PCB), and that of the PS, there are no grounding problems. After doing as you said, the overheating continued. I will try and have my neighbor use his scope to check for oscillation or excessive ripple. Does it seem feasible that the MOSFETs may be the problem?

Thanks for your help.
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Old 24th December 2001, 12:58 PM   #8
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Bryan,

Here are some other suggestions:

I suppose your power amp is an AB or B type amp.
Compare the quiescent current (bias) for both channels. If the overheating channel is having an higher then usual quiescent current overheating will occur. Look for faults in the part of the circuit that takes care of the biasing of the mosfets.
Comparing is probably easy, if there are source resistors in the circuit. Simply measure the voltage drop over these resistors. This should give you similar readings for both channels.

If you have 4 ouput mosfets in a push/pull config, one of the fets could be faulty and could get rather hot. Measure Vgs for each mosfet in both channels and compare!

Hope this helps a little.

P.S. Don't forget the possible oscillation problems!
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