Help Identifying Where the Hum is Coming from!

[acoles1619]

Hello everyone,

I apologize if I am not posting this in the right area, it is supply related though. So I am having an issue with a little circuit I built to power two different devices at the same time. One uses 12V, the other uses 5V. Pretty much I am using a battery as an input, it then runs straight to another connector for 12V output. The same power line also runs to a 5V regulator to supply my 5V device. When using this set up I get a lot of hum when listening through a pair of headphones. Not sure why this is, but I noticed that both devices have the ground pin for the power, and the ground for the XLRs tied together. Is this causing some kind of loop?

[stratus46]

Originally Posted by acoles1619

Hello everyone,

I apologize if I am not posting this in the right area, it is supply related though. So I am having an issue with a little circuit I built to power two different devices at the same time. One uses 12V, the other uses 5V. Pretty much I am using a battery as an input, it then runs straight to another connector for 12V output. The same power line also runs to a 5V regulator to supply my 5V device. When using this set up I get a lot of hum when listening through a pair of headphones. Not sure why this is, but I noticed that both devices have the ground pin for the power, and the ground for the XLRs tied together. Is this causing some kind of loop?

3 terminal regulators have a habit of oscillating when used without input / output capacitors as specified in the data sheet. The oscillation would likely not be what you're hearing but it should be checked - preferably with a scope.

You almost certainly won't have ground loop problems when using battery supplies. What exacly are you connected to and what is the input configuration of the amplifier driving the headphones?

G²

[wintermute]

When you say "Pretty much I am using a battery as an input" is there any part of any of the circuits which are attached to the mains or the mains earth? Or is it automotive? if automotive if the engine is off is the hum still there?

Hum is usually associated with the residual ripple after rectification 100Hz (or 120Hz) , or sometimes the primary mains freq (50Hz or 60Hz) and harmonics thereof. If you are running off batteries this should not be present...

I've never considered what the symptoms would be if you have ground loops with a battery supply. Certainly currents could flow through multiple paths just like they do with a mains power supply but what the result of these undesired currents would be I don't know.

Are the devices in shielded boxes? Does the hum change if you move the boxes elsewhere? Are they sitting close to mains wiring? I've seen hum induced from mains leads running close to interconnect leads before.

Tony.