Smoking amp

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A fiend of mine phoned and said his stereo amp started smoking and his speakers made a loud noise. He switched it off and found that the tweeters on the speakers were burned.The power supply uses dual rectifiers bridges and powers both channels. When he opened the amp he found that the same resistors on each amp were burnt. Can the problem come from the power supply ? Both channels broken in the same way ? Can it be one of the rectifier bridges that have gone short and have sent AC through that burnt the amps. If the dc protection have switched then the tweeters would not burn. What else can cause such a failure, one rail missing ? :confused:
 
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Violent oscillation or severe clipping will burn out the tweeters and cause overheating of the output stage without necessarily tripping either DC or over-current protection circuits. It was once a common failure mode for small-medium stereo amplifiers played too loud at parties but now I think it's more likely an issue with DIY designs where stability or safety are seldom tested properly. There is a trend toward minimal output stability networks and this means increased risk with many newer designs.

So the question then, is what sort of amplifier, speakers, cables you refer to? Are they known commercial products with brands and model numbers or are these unknown DIY designs? It's a bit hard to offer any help with no details other than what happened. I'd like to think that a conventional power supply would have nothing to do with the problem but your details are too scant to rule almost anything out as a contributing factor.
 
Thanks guy's, The amp is a Quasi NMOS 350. https://sites.google.com/site/quasisdiyaudiosite/home/nmos350-mk2

It was working fine for 3 weeks daily without any problems. The protection is Quasi's protection circuit as well. R3 (10R) has burned on both channels from what I can see on the photo's he send me. I dont have the amp with me now, so it is difficult to test what all is faulty. He also said it made a loud vibrating noise. DC will not do it, maybe oscillation , but on both channels at the same time ?? The powersupply is common to both channels , maybe thats where the problem is ? That why I asked if a bridge goes short and the amps get AC instead of DC, what could be such outcome . I have build it on an aluminum plate, he want to build his own chassis.
 

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Both channels oscillating in a DIY build could just as easily be due to the same construction or component type errors, assuming this amplifier is not old and hasn't been put through a range of load tests - perhaps until now?

I have friends too who, because they buy from uncertain sources, have similar problems and I've been caught out with fake (or rather out of spec.) Mosfets too and surprise, surprise - they can be unstable in operation without any help from my other errors.

If you want to help your friend, why not get all the relevant details, like whether this fault just appeared after years of reliable operation, on initial testing, first hot day of the season, with signal, at high or low power - that sort of info that helps others to understand what initiates the problem. It seems most likely to be severe oscillation and this can be seen easily on an oscilloscope, probably beginning with small wriggly lines on a sinewave test but builds rapidly until it bursts into full, destructive oscillation. Mosfet amplifiers usually self-oscillate at very high (typically 20-60 MHz) frequencies which means even small circuit reactances become significant conductors and tweeters in conventional speakers are particularly vulnerable.

Assuming the 10R resistors are R35 in Quasi's schematic, these are the Zobel stability network and form a HF shunt to ground. This will overheat and burn with strong oscillation present, if the wrong resistor or capacitor type or rating is used and obviously, when the resistors burn, there will be no protection any more. Kaboom! Need I say, a wire wound resistor here would be a bad idea.

I don't follow your suspicion that the power supply is necessarily faulty. Let's look at the simpler idea that the power supply wiring is common and any oscillation, that will certainly be present on the supply to the oscillating amplifier, will be there in the other too. If the amplifiers are similar in all respects and oscillation is strong enough, it will likely destabilise the other and bring about the twin disaster.

Tracking oscillation in Mosfet designs is hit or miss without a 60-100MHz 'scope. Simple indicators aren't much help for diagnostic work and I still use an old TEK 2235 but it proves superb for the job and even now, similar models are a lot cheaper and easier to interpret than a modern digital type. A good 'scope for serious DIY like this isn't a luxury, it's essential.
 
Thanks for that info Ian, The amp is about a month old and I have tested both channels into clipping with a 9 ohm resistor. It clipped at 36v average( no more RMS:)) running from 62V+- rails. The mosfets are not irfp240 but Irfp250, they are a bit stronger as I remember. The resistors that burnt are R3 , not R35. I will see for myself if I get it back what else is buggered. I have got a Philips and Tek scope, but they are old and only 20Mhz. The components, well since we became a third world country you never know what you get. The only place is RS where you might not get fakes but when you buy from them you have to buy bulk and that inconvenient. Want 4 trannies then you have to buy 60. So I will wait till I have it in my hands. Maybe its just better to start importing the components.
 
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R3 according to the schematic, is 3k3 - that's not 10R and shouldn't, unless there was a high voltage indeed at the base of the input transistor, ever burn out. I would be looking at replacing just about every input stage transistor if that was burnt. R2 is actually 10R and is the "ground lift" resistor.
 
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OK, thanks for the confirmation. So, if the earth lift resistor is burnt, there are a few different matters to consider. One would be that this resistor should be supplemented by reverse parallel power diodes or a diode bridge when the resistor blows. This may be seen as a belt and braces approach but when there is lots of power and PA is the obvious intended use, there is obviously a need for the safety of something better than a simple resistor between protective chassis earth and power ground. Here an illustration:

earth-f4.gif


I suspect this may be Rod Elliott's (ESP Audio Pages) design but I found this post on another forum: Ground loop hum eliminator
 
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