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Amplifier sharing same GND as LINE IN

batata004

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Jul 15, 2021
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State of Santa Catarina - Brazil
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?

IMPORTANT: I am pretty sure I am not overloading the power supply, because my piano turns off even without playing any sound... as soon as I connect the power supply to the mini amplifier, the problem happens.
 

Speedskater

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Mar 5, 2016
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In well designed units:
a] the audio circuit common is connected to the metal chassis
b] the DC supply common is connected to the chassis
c] a RCA unbalanced LINE out (or in) common (shell) is connected to the chassis

* * * * * * * * *
matbe one of your units does not treat either DC supply lead as common.
 
OP
batata004

batata004

Member
Joined
Jul 15, 2021
Messages
10
Likes
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Location
State of Santa Catarina - Brazil
I think the GND paths are all connected right... Maybe it's not common sharing the same power supply accross amps and pianos or line out devices? But I still cant understand what is causing this...
 
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