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Old 14th October 2011, 10:02 AM   #21
AndrewT is offline AndrewT  Scotland
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The schematics look OK.

But I suspect your ground lines in the build do not follow what you show in the schematics.
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regards Andrew T.
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Old 14th October 2011, 11:04 AM   #22
DF96 is offline DF96  England
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The caps across the rectifiers might increase noise, but they won't affect hum.

You have two points marked common ground. How are these connected? The two channels of a stereo system normally have their grounds connected together somewhere, and this can introduce a loop.

What is your measured and calculated supply ripple, circuit PSRR and output hum? Simply saying you have hum does not tell us very much. We need numbers.

If the problem is supply ripple, then adding capacitance might help. If the problem is induction into a loop, then adding capacitance could make things worse.

As Andrew has asked, is this now exactly what you have built?

BTW you have used more bridges than necessary.
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Old 14th October 2011, 11:27 AM   #23
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All the above are mounted on a big PCB. So no wires. When you say that the capacitors will increase the noise what do you mean? Also to me the problem is a noise listenable only from the woofer and not from the midrange. It is clearly a 50 - 100 Hz. Maybe for the amperes I need more capacitance than 130000uf. I can not measure ripple sorry
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Old 14th October 2011, 11:48 AM   #24
DF96 is offline DF96  England
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In the past people made perfectly good systems with 5000-10000uF caps, so it is very unlikely that you have too little capacitance. By my calculations, assuming 4A load, your PSU should provide about 15mV ripple - very low!

What test equipment do you have? You can't debug a circuit just using your ears.
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Old 14th October 2011, 11:55 AM   #25
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I am repairing my scope so right now only a multimeter.
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Old 14th October 2011, 12:45 PM   #26
DF96 is offline DF96  England
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I have just reread the thread, and noticed that you have an SE Class A amp. Does it have negative feedback? Does it have poor PSRR? Circuit?
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Old 14th October 2011, 01:03 PM   #27
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Yes it have negative feedback and it is JLH SO POOR PSRR. By the way the feedback point has to be where in the PCB?
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Old 14th October 2011, 01:57 PM   #28
DF96 is offline DF96  England
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I have found this. Is that what you have built? Note that fig 3 is wrong - see comment underneath about C4. JLH says 33mF is fine for a simple PSU, although with some hum. He recommends a regulated PSU.

The negative end of C4 is the reference point for the NFB, so it must go to a clean reference point such as the signal ground of whatever comes before the amp.
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Old 14th October 2011, 02:06 PM   #29
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it is done correctly but??
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Old 14th October 2011, 02:15 PM   #30
DF96 is offline DF96  England
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JLH says about 3mV hum from his simple supply. Your supply shouldl have much less hum than his, so PSU ripple is probably not the cause of your hum. Maybe time to start a new thread about a hum problem, not a PSU problem. However, with not much test equipment you might not get very far. Can you fix your scope - much easier then.
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