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Old 26th October 2006, 01:30 AM   #1
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Angry Weird Humming problem

I'm prototyping a TDA2050 amplifier, when I turn it on everything is fine, dead quiet and no oscillations, but after 5 minutes a hum appears on the speaker.
I did everything that I now to kill the problem, but it don't go away. It disappears only when I shut down the amplifier and still playing with the capacitors charge.

Anyone can help me? Please, this is really giving me a headache.

The attachment have the amplifier and psu pcb layout.
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File Type: jpg tda2050dt.jpg (46.0 KB, 372 views)
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Old 26th October 2006, 02:43 AM   #2
Leolabs is offline Leolabs  Malaysia
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What's the size of the heatsink???
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Old 26th October 2006, 02:49 AM   #3
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Hi Diegot,

Are you sure you don't have one of the electrolytic caps in backwards. Is the amp hot when the hum sets in? Possibly a cold joint that changes as the lead heats up... It's definitely either loading down the supply or the filtering is becoming ineffective over time, because the hum goes away when the ac is pulled yet the amp still plays. From your pictures it looks like you have a few supply caps paralleled. You have a number of clues to help , Good luck.

Regards, Mike.
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Old 26th October 2006, 10:31 AM   #4
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Default heatsink

Leolabs wrote
Quote:
What's the size of the heatsink???
I will go back still and ask ' have u installed a heatsink ???'

Gajanan Phadte
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Old 26th October 2006, 03:11 PM   #5
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Leolabs, the heatsink have 6 x 6 cm, it was from a old AT power supply, but it's only a temporary one.

Mike, thank you very much for your tips, the problem is really related to the power supply. I took the power supply of the first chipamp I had made and plug it in the new one. Everything is fine now, but I cannot use this old supply, so I will fix the new one, maybe the caps that I bought are failling.

Gajanan, thanks for your reply, but I'm here to get some help not a joke.

Regards
Diego
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Old 26th October 2006, 04:52 PM   #6
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Quote:
Originally posted by diegot
the problem is really related to the power supply. I took the power supply of the first chipamp I had made and plug it in the new one. Everything is fine now, but I cannot use this old supply, so I will fix the new one, maybe the caps that I bought are failling.

The clue here is that the hum builds up overtime. This sounds like a leakage path that gets worse as it heats up. I don't know if you've had it on long enough for one of the caps to appear warmer than the others.

Good luck getting to the bottom of it.

Regards Mike.
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Old 27th October 2006, 03:57 AM   #7
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Quote:
Gajanan, thanks for your reply, but I'm here to get some help not a joke.
It was meant to help only and no pun intended. I somehow thought u ignored the need of the heatsink.

Gajanan Phadte.
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Old 28th October 2006, 04:02 PM   #8
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Finally I discovered the source of the problem. It's related to the transformer. The PSU caps and the amplifier layout are OK.
My transformer is a 20+20V 240VA unit, when the amplifier get warm the PSRR decreases and the hum appear.
I plugged a 12+12V transformer and the problem doesn't occur.
I believe that this occur because of the idle voltage of the power supply, higher than the maximum voltage specificated at the datasheet.
It's a behavior of the TDA2050 that I wasn't expectating.
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Old 30th October 2006, 02:57 AM   #9
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If u had exceeded the absolute max ratings of the IC, then better u replace the IC with a new one as it may not work to specs.

Gajanan Phadte
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Old 30th October 2006, 07:39 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
four points I see on the PCB.

1. the input cap shows an axial electrolytic. The space looks ideal for a large polypropylene or polyethylene metallised film type.
Aim for a good RC time constant to match your NFB time constant and PSU time constant. A value between 1uF and 10uF could fit.
Is there a DC bias on the electrolytic?

2. the internal link between the input ground and the power ground should be removed. Instead take the input ground from a new connection right beside the RCA grounding point to the Central Star Ground. You may want to try fitting a 10r resistor into the space left by removing the link.

3. the NFB cap looks quite small. What RC time constant is there?

4. is there a resistor preceding the RF capacitor on the input?
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