| diegot |
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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| Leolabs |
| What's the size of the heatsink??? |
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| MikeBettinger |
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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| gmphadte |
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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| diegot |
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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| MikeBettinger |
| 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.
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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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| gmphadte |
| 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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| diegot |
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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| gmphadte |
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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| AndrewT |
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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| diegot |
Andrew,
1. The space is for a polypropylene.
3. The cap is a 22uF one, as recommended at datasheet
4. There is one in the new pcb attached.
Now I ask for everybody opinion.
I've made two new pcbs, only scratchs. I think that these ones have better grounding. What you think?
Which is better, the one where the output zobel cap go into the power ground or the another one?
Thanks for the replies. |
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| Leolabs |
| Wondering where it's your speaker ground go to??? |
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| diegot |
| Together with the power ground, in the PCB. |
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| AndrewT |
Hi,
I prefer mk1, not mk2 nor mk3.
What are your time constants (R*C)? in the input filters? NFB loop? and PSU?
22uF could be right or too low depending and the R that goes with it. |
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| diegot |
Here is the values.
The PSU id for four channels.
What do you mean with mk1, mk2, mk3? |
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| AndrewT |
Hi,
assuming you propose to use 8ohm speakers then the following RC values apply to your amp.
Input = 1uF * 22k = 22mS
NFB = 22uF * 680r = 15mS
PSU= 10500uF * 8r = 81mS.
The NFB time constant is far too low for the input RC.
It needs to be at least 1.4* input RC i.e. NFB>=31mS and could usefully go up to 57mS.
At the moment your bass response is controlled by the NFB time constant at -1db @ 21Hz. But this is a bad way of doing it.
The designer has not done his job!
If you change the NFB RC to 56mS by changing the cap to 80uF then you can raise the input RC to about 40mS by changing the input cap to 1.8uF and get your -1db frequency response down to 8Hz, along with lower phase errors in the low frequencies.
You have posted three PCB layouts. mk1, then mk2 & mk3 and asked for comments. |
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| diegot |
Thank very much for the tips. I gonna increase the input impedance and the two feedback resistors. This way I can maintein the gain and aim the values suggested by you without changing any cap.
Regards. |
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| AndrewT |
Hi,| quote: | | increase the input impedance and the two feedback resistors |
a perfectly acceptable alternative.
Bear in mind that many sources already have a DC blocking cap on their output.
The source output cap and power amp input cap are in series.
Thay should be considered as a paired system to calculate your input RC filter (similarly check HF). |
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| diegot |
The problem is really related to the tranformer voltage. I spent all my day playing with the grounding and with the power supply. Only lowering the supply voltage makes the amplifer work fine.
Damn, I need another power transformer. |
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