Hi all and everyone is OK I hope.

I have a problem with my Chinese mini MP3 player. Power supply is 220 v in and 24 v out switch mode type. There are one subwoofer amplifier ( Chinese type ) and hand made LA4270 stereo amplifier powered by this power supply.
Both amplifiers line in ( L & R ) are connected parallel then MP3 player line out ( L & R ) are connected to amplifiers line in.
To power up the MP3 player I made a voltage regulator using by 7809 which is powered with 24 v.

When MP3 player is off the amplifiers give no hmmm or buzzzz.Very pure and smooth. Works perfect.

The story begins when I power the player on. Even if there is no signal on the line out a hmmmm sound starts.
I powered the MP3 player by an external battery and it is OK. Pure stand by sound. So I decided that the problem is related with 7809 voltage out.
MP3 player has an embedded 78M05 on it. It wakes up minimum 6 volts so I decided to use 7809. My regulator had a 100 uF / 25 in and 47 nF out in the beginning. Later on I put 470 uF to V-in and 2 x 10.000 uF parallel to V-out. The hmmm sound has been decreased almost 80 % but you now I still hear the 20 % when the music ends between the songs.
Yes maybe there is a way to remove it 100 % but I am not an engineer and just a hobby maker. Circuits are mounted in a metal box and well grounded to the chassis.

Your suggestions will be highly appreciated for me. Thanks in advance.

 

Possibly a ground loop? Does the humming sound like it's 50Hz or 60Hz?

 

How long is the cable between the MP3 source to the Amplifier?
Does the 9V DC have proper decoupling on both sides?
Is the signal cables properly screened?

 

Replace the defective 9V regulator.

 

Possibly a ground loop? Does the humming sound like it's 50Hz or 60Hz?

Thank you all for your kind and prompt replies. I solved the problem.

I don' t know why and how but I supposed that the 24 v DC voltage which comes to 7809 is not a perfect DC so I put a bridge diode and just one 10.000 uF instead of 20.000 to the 9v output side.

And the result ; hmmm sound completely removed.

The most interesting thing is both amplifiers were working perfect with this 24 v and no any distortion when the mini player was off.

Anyway let Murphy thinks about it Thanks to all again.

 

Murphy knows that a DC voltage regulator needs the AC to be rectified and filtered properly.
18VAC from a transformer has a peak voltage of 25.4V which is reduced to 24V DC peaks by a bridge rectifier. These peaks must be filtered so that the input voltage to the regulator never drops below 2V higher than its output voltage.

Most amplifiers and opamps have a spec'd "power supply rejection". An OPA134 audio opamp reduces its power supply hum 100000 times. Then 2VAC power supply hum produces only 20uV of hum at its output.