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Old 6th October 2005, 12:59 AM   #1
pjpoes is offline pjpoes  United States
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Default oscillation

How would I know if my amp is oscillating. Someone mentioned that one reason it might be running hot all the time is that its oscillating. I get no weird noises, I see no oscillation in the speaker unless its supposed to be, such as with music. I dont know much about amps oscillating, but I always thought you could see or hear it.

As for how hot, I still dont know exactly. When the amp is on and plugged into a preamp and 8 ohm speakers but no music is played, the amp us warm to the touch. At the temp I would have expected it to run at if it had been playing all day. If I then play music for an hour or two, the amp gets quite hot. Not so hot that it shuts down, and not so hot that I can't touch the heatsink, but hot enough that I wouldn't want to hold my hand on it for a long period of time. Given that skin burns at around 170 degree's, I want to say that the amp must be less than 170, infact probably less than 150 degrees, but not much less if any. The heatsink might be too small, but looking at what some people have used, I would have thought its enough. I think I may have misjudged, I never bothered to do the math with it. Anyway, if anyone could let me know if oscillation does sound like a possibility and why it might be happening so I can find it, I would appreciate it. Thanks.
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Old 6th October 2005, 01:20 AM   #2
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What chip amp is it?
Which circuit diagram did you use?
What are the dimensions of the heatsink?
A picture would be quite helpful also.
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Old 6th October 2005, 01:45 AM   #3
pjpoes is offline pjpoes  United States
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I used the Audiosector LM3886 premium kit. Only differences is that the powersupply has the caps bypassed with some small value BG's.

350va 25-0-25v transformer. The heatsink is one from MPJA.
http://www.mpja.com/productview.asp?product=14674+HS
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Old 6th October 2005, 02:28 AM   #4
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Your amplifier may be oscillating, even if you canīt hear it.
The amplifier can oscilate at frecuencies not audible (50 to 100 Khz or more).

If possible try, to use an osciloscope to see the output.
Other way may be a frequency counter or multimeter with frequency function.

Your "hand thermometer" indicates that the chip is not too hot.

Try with the input shorted, to see if the temperature goes down.

hope this can help
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Old 7th October 2005, 05:01 AM   #5
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hey tla
amp chips have a freq band of operation.so how can there be ultrasonic oscillations.
hook up amp o/p through solid power res. to an oscilloscopes and see for oscillations.
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Old 7th October 2005, 12:26 PM   #6
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Hi pjpoes,

If you don't have an oscilloscope then you could try this simple circuit that was suggested to me. http://www.diyaudio.com/forums/showt...175#post680175

It won't tell you definitively whether you have oscillations, but will probably give you an indication.

I didn't think Peter did an LM3886 kit???? Do you have long wires from your input jacks to the amp pcb, or long power supply wires?

Tony.
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Old 7th October 2005, 01:10 PM   #7
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if the amp is getting hot because of oscilation then u probably never gonna hear it .
the reason the amp is gettin hot during oscilation is because the output transistors (of the chip amp) are unable to swith of fast enough with a wery high oscilation frequency ,so they stay in "on mode" and getting very hot

plus , the oscilation can accur where there is input signal and stop when there is no signal .

the only oscilation u can hear is a low freq oscilation that wont heat up your amp , just make anplieasent noise
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Old 7th October 2005, 01:49 PM   #8
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I mis-spoke, its an lm3875 kit. Does that change anything that was said so far.

My input leads are kinda long, about 10-12 inches right now. I made them up long because I wasn't sure how long they would need to be for the inclosure, so I did that.
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Old 7th October 2005, 02:32 PM   #9
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no I don't think it matters too much, just checking .... are your input leads sheilded cable or just plain wires?? I was thinking that if they are long (and unshielded) they may be picking up some rf, potentially causing oscillation.

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