When I hook a device to the monitor input of my digital piano, I get a very noticable 50Hz hum.
I understand that the monitor input doesn't have to be particuarly good quality, but it really is annoying.
I'd know what to look for if the attached device had a ground connection, but it happens even with battery powered products such as an ipad.
The cable is a cheap 3.5mm-jack to 3.5mm-jack thing.
I've done some experimenting:
ipad attached to piano, electrically insulated from everything else --> hum
ipad attached, touching ipad screen --> even more hum
Unplug the cable from the piano--> no hum
Cable pluged into piano, other side unplugged -> no hum
Cable pluged into piano, touch exposed jack parts for left and right channel -> hum (on the according side)
Cable pluged into piano, touch exposed jack ground part -> a little hum
Cable pluged into piano, shorten all three terminals on exposed jack with a bit of wire -> no hum
Cable pluged into piano, shorten all three terminals on exposed jack with a bit of wire and touch the wire -> hum
I first suspected the cable to act as an antenna, but the cable alone is quiet.
Looks like it happens only if an antenna is attached to the cable. Both ipads and humans seem to be effective antennas.
What I don't understand how this can work with everything shortend to the ground part of the jack. With a low-impedance path to ground, how can I still inject the 50hz signal?
edit (more investigation)
Piano has a built-in class II power supply with no ground connection.
Piano is connected to AV receiver via 6.35mm jack - cinch cable.
AC voltage between 3.5mm jack ground and mains ground measured with multimeter--> 33mV
Resistance between 3.5mm jack ground and mains ground -> 3 Ohm
Unplug the connection to the receiver -> lots of extra hum
The 3.5mm to 3.5mm cable is defect. Ground has a loose connection.
I'm pretty sure the faulty cable is not the culprit as the connection seems to be intact with the cable hanging from the jack. We'll see, replacement should arrive tomorrow.
The only ground connection of the piano is through audio cables and the AV receiver. I also get the hum without that connection, that is with no ground connection at all.
If it is caps in the power supply, shoudn't it hum all the time? I only get the hum through the monitor input.