Same power supply from piano and amplifier

@imhififan you are right, the GND of the power supply pin on the mini amplifier is connected to the GND of the LINE IN - there is continuity when using multmeter. 

@imhififan I only have ceramic capacitors and eletrolytic ones at hand now, but I will try to buy the bipolar one that you said. Just to make it clear: I should take the signal wire that leaves my piano (Line out) connect it on one leg of the capacitor and the other leg I must connect to the singal line input of the mini amplifier. Is that right?

@erik_squires and @yoyoyaya I am pretty sure the problem is not current, because the problem happens even when I have no speakers attached to the mini amplifier. Also, the power supply is 5A which is plenty for the piano (which uses no more than 1A) and for the mini amplifier (which should use way less than 1A without speakers).

@jasonbourne52 I would like to be able to use only one power supply, and at least understand what is the problem!

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?

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.

🤞🙂🤞

@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!

@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.