Mains hum without ground loop

Question

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.

Answer 1

It was the cable after all.

It has a loose connection. Unless manipulated, it connects and sound gets through. That is why I didn't notice initally. Even then, I ruled it out as the cause for the hum as it worked most of the time (of course I got a replacement anyway).

I measured the resistance of the cable:

• left channel: 0.9 Ohm
• right channel: 0.9 Ohm
• ground: 3-200+ Ohm fluctuating, occasionally disconnects completely.

I went a bit CSI on that cable. The shield is twisted together and soldered to the jack by a few strands. It broke of there. The assembly is molded in rubber, the strands stayed in place and still made a lousy connection.

Sorry for the red-herring hunt.

Now that this is settled, what's with those 88 white and black buttons? They aren't labeled. Which one does "Flight of the Bumblebee"?

Answer 2

Looks like the "ground" of your digital piano is floating above the actual ground that you are standing.

Reading from your experiments, it suggest the ground of your piano might be at very high AC potential referenced to the earth. You didn't say how your piano is powered but i assume it operates from mains voltage. It may have poorly designed power supply which capacitively couples high voltage to the low voltage side amplifier section of the piano.

In addition to that, its internal circuitry may have been designed poorly which causes 50 Hz hum picked up by high gain amplifiers when ever the capacitively coupled leaking current find its way to the earth.

You should try these...

• Find the way to earth the star ground of the low voltage side of your digital piano. This will drive all leakage current away from the sensitive audio input sections.
• Feed the audio input to the piano thru an isolation transformer. This will eliminate the audio ground shorting to earth, however AC coupling might still be a problem.

Good luck.

Answer 3

Look for a (power filtering) electrolytic capacitor that leaked , or in any case, is possibly dried up and no good. Look for one on the input power, of either device, and replace.