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i´ve just build a prototype speaker from a TDA7293 (http://www.ebay.de/itm/TDA7293-85W-100W-Mono-Audio-AMP-Amplifier-Board-AC12-50V-Update-vision-TDA7294-/201478109342), the circuit is quite straightforward. To avoid the noise of a switching power supply, i bought a toroidal transformer. While the music sounds astonishing good, in the quiet parts, or when there is no music, the system is humming in a quiet way but it is loud enough to be heard. I think, it is the 50hz.

So, how to filter the hum, since it originates from the toroidal transformer? But the transformer is giving AC, so i can´t filter it away. Does someone know, if this is a drawback of there cheap china rectifier? Should i buy a better one? And if yes, which one?

First trial

Or is there a possibility to filter the hum? With soldered connections (I appended a photo of my prototype)


Ok, can´t attach a photo to a comment, (a little confusing because it seems, if i rather should edit my question than to add another answer), so i attach a new photo, i´ve already answered a question to stephen, maybe it is now all upside down...)

Hmm, it seems, if my comment were not added, so i type it here again. I´ve changed the setup but i can´t attach more than 1 photo (under 10 points), so i attached only the screenshot. I rearranged the setup, soldered most of the connection, but the hum is still there if i connect something to the input. I´ve feed some frequencies with a signal generator, and it is a little curious, the input signal seems to have more noise than the output. The hum is still there, but the music is rather good, if i can judge it on 30 year old blaupunkt car speaker chassis...

enter image description here

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    \$\begingroup\$ You say you built a speaker, but the thing you linked to appears to be a audio amplifier. It has the power supply built in, so probably little you can do about the hum other than keeping the transformer far enough away so that its magnetic fields don't bother the circuit. It's probably just a crappy circuit. \$\endgroup\$ Feb 20 '16 at 18:31
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    \$\begingroup\$ Why every second word is "quite"? \$\endgroup\$ Feb 20 '16 at 18:33
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    \$\begingroup\$ Disconnect the audio input that comes from the CD player by taking the wires out of the terminal blocks, both left and right channels. Do you still have hum? If the hum clears then use some better screened cable and / or a better CD player. Still have hum? Move the mains wiring well away from everything, you actually have it running under the speaker which is not a good idea. If that doesn't stop the hum then the amplifier is at fault. It is probably a poor design. You could try replacing the large electrolytic capacitors but it could be a waste of time and money. What do you expect for €6? \$\endgroup\$
    – Steve G
    Feb 20 '16 at 18:57
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    \$\begingroup\$ Switching supplies are usually easier to filter because the switching noise is outside of the audio band. And of course batteries have no switching noise. \$\endgroup\$
    – mkeith
    Feb 20 '16 at 20:04
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    \$\begingroup\$ Ground loops is the likely cause - your wiring looks a mess and the signal comming from the CD player is probably contaminated by 100 Hz. It could even be induction from the AC wires into the signal wires. \$\endgroup\$
    – Andy aka
    Feb 20 '16 at 21:43
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The toroidal type mains transformers do not have a lot of magnetic leakage.

The hum you hear is more likely at 100Hz.

The hum that remains with no input connected can be reduced by increasing the filter capacitor but adding a proper voltage regulator (linear or switched mode) would help more.

The hum when CD player connected is probably due to differential ground noise from the two separate mains supply points, sometimes if you can use a common power-supply ground you can reduce it sometimes this is NOT possible (such as when analogue and power grounds are not tied together in the devices).

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Your hum must be at 100 hertz, not 50 Hertz. Its nothing wrong with your transformer or rectifier. If anything is seriously wrong with any of those, you will hear the hum louder.

Your problem is improper grounding. Do some studies on star grounding, ground loop and ground current in amplifier circuits.

You can solve your problem simply by shifting the ground point.

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The humming is caused by a ground loop or a poor connection of grounding system of the board. Also, try to increase the value of your filter capacitor in your power supply.You can achieve more filter capacitance by placing extra electrolytics in parallel with your existing ones .If you keep your scope probe on and the hum is indeed power supply based you will see a volt reduction that is in proportion to total system capacitance .The larger C will make your amp sound better at high power so it is worth doing even if it does not cure the hum.

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