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Same power supply from piano and amplifier

I have a digital piano with LINE OUT. I also have a mini amplifier named "pam8610" (cheap and very common) which has LINE IN. I have 2 power supplies, each one with 12V and 5A. When I connect each power supply to each device (one power supply to the piano and the other power supply to the mini amplifier) everything works just fine: when I play the piano the sound goes to the mini amplifier and then goes to the speaker.

HOWEVER, when I connect both devices (piano and mini amplifier) to the same power supply while keeping the LINE connected, my piano turns off immediatly, something is really wrong. If I disconnect the LINE and use the same power supply, no problem happens (but of course no sound is produced because the LINE is not connected).

Using a multimeter, I discovered that the GND of the LINE OUT is the same GND of the power supply (continuity test in the multimeter). I dont know if this is common practice, but for some reason my piano provides the LINE OUT with the same GND of the power supply. So, when I use the same power supply to power the mini amplifier and when I connect the LINE, some kind of short is happening. Using 2 separated power supplies, everything works fine.

Do you guys have any idea how can I fix this or what is going on?
 

batata004
batata004 OP
4 posts
 

Put a 2uF bipolar capacitor

@imhififan I did what you said and it worked!!!!!!! UHULL!!! Using just one power supply, everything worked. However now I have lots of humming/noise on the speaker connected to the mini amplifier.

 

I didnt have a 2uF bipolar capacitor, so I took 2x 4.7uF electrolytic capacitors and connected them in series with the GND of the line in and line out. I did this:

 

GND of LINE OUT of piano -> positive leg of capacitor (1) -> capacitor  (1) -> negative leg of capacitor (1) -> negative leg of capacitor (2) -> capacitor (2) -> positive leg of capacitor (2) -> GND of LINE IN of mini amplifier.

 

Is this correct? Any idea how I can reduce the humming?

imhififan's avatar
imhififan
2,181 posts
 

I think the humming is ground loop, maybe you can disconnect the bipolar capacitor and leave the negative leg open circuit. Hope it can eliminate the humming.

🤞🙂🤞

batata004 OP
4 posts
 

 

@imhififan I will try your suggestion of leaving the negative leg open, but if I do that, the circuit will be open! In the image below you can see how I connected the capacitors. I connected then in series on the GND line... if I leave any leg of the capacitors open, the circuit will be open. Am I doing something wrong? Thanks for your attention!

 

imhififan's avatar
imhififan
2,181 posts
 

@batata004 

Without schematics on hand, all I can do is guessing!

Since the piano LINE-OUT negative is connected to ground, and you're using single power supply to drive both the piano and the amp, so both unit the ground will be connected together by the power supply negative lead.

I also saw your last post at diyaudio, you mentioned disconnected the mini amplifier from everything (PSU and line in cable) and the GND of the line in is connected to GND of PSU (0 ohm resistance). If that is the case, you don't need another ground lead from the piano line-out to the mini amp line-in and the capacitor at all.

Just simply disconnect the capacitor from the ground and see if the piano signal can pass thru.

If disconnect the capacitor didn't work, maybe you can try solder the bipolar cap one lead on the mini amp line-in negative, the other lead solder to the ground of the mini amp board, and see if the humming noise reduced.

 

 

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