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Old 4th March 2010, 04:08 PM   #1
opor is offline opor  Thailand
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Default My UcD start oscillation

There is something wrong with my amp.
When I connected power rail to only one channel it seem OK even it has some noisy . But when I connect same power rail to another channel .I turn it on both there is so much noisy and when I volume up by turn R23 it start oscillate and this oscilation is noticeable from another channel too . So how to fix it?
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Old 4th March 2010, 04:12 PM   #2
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The problem you are having seems to be something related to carrier tracking between both amps. I'm sorry I didn't look closely at the circuit, but you would probably do best to start looking into how this could be happening. If the two amps share one sink, is the sink grounded. If that isn't it then look around for other ways two high power square waves could couple between two circuits. It could have something to do with your signal routing too.

Last edited by Andrew Eckhardt; 4th March 2010 at 04:17 PM.
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Old 4th March 2010, 04:27 PM   #3
opor is offline opor  Thailand
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Two channel has its own sink. So how to check even if there is couple.
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Old 4th March 2010, 04:45 PM   #4
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Well I think your symptoms indicate that there is definitely coupling, so there is no need to check that. Keep in mind that under no circumstances will this problem completely go away. It is a serious problem with switching amplifiers and the only thing you can do is try to minimize it through every trick you ever heard of. Make sure that you can't slow down switching time in the interest of reducing noise any further before affecting other performance parameters. It's a balancing act there. Think of the leakage field around the filter inductors. Try to minimize drain capacitance to the heatsinks. Make sure ground, power, and signal routing is absolutely optimized. After all that, try putting each channel in it's own cage. Like I said, the trouble never ends. You can even put shielding between the power and signal sections of each channel.
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Old 4th March 2010, 05:06 PM   #5
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I forgot to mention. Before you go absolutely insane trying to make a noiseless PWM amp, check to make sure you don't have output stage cross-conduction occurring. That is probably the most common PWM amp gotcha and of course it will generate very high current ringing far above the normal switching frequency. If you have that going on it'd be the place to start fixing.
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Old 7th March 2010, 02:39 AM   #6
opor is offline opor  Thailand
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Thank you Andrew, I have changed R46 to 390R so everything is OK.
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Old 7th March 2010, 01:55 PM   #7
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What is the circuit based on 555?
Current regulator at overcurrent?
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Old 7th March 2010, 02:46 PM   #8
opor is offline opor  Thailand
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Yes,it is overcurrent protection.
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Old 28th July 2010, 08:09 AM   #9
stewin is offline stewin  Kenya
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hi opor pls post the pcb of your schem and are your output irfp 250 heating ?
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Old 4th August 2010, 12:13 AM   #10
stewin is offline stewin  Kenya
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try increasing r54 & r55 or try to use irf540z instead of irfp250
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