[superdragon]

I made a simple audio circuit powered by a 12V AC adapter with a two-prong plug. It's plugged into the same power strip as a clock radio and an electric razor, but I get the same hum (actually more like a buzz) if I plug the adapter into a separate wall socket. The linked audio is what one of the speakers sounds like when I power on the amp. Is this 100% a ground loop problem, or does the frequency of the sound indicate a different problem?

If it means anything to the answer, when I start the circuit with a Bluetooth receiver, as soon as I power on the receiver (doesn't need to be paired or even in pairing mode, just powered on) the buzz goes away. While I'm on the subject, I'd love to be enlightened as to why the Bluetooth receiver eliminates the unwanted noise.

On some platforms, imgur auto-mutes content, so you may have to find the volume icon to unmute.

https://i.imgur.com/zozrbFy.mp4

 

[msing539]

It sounds like a ground loop to me but you said a two prong plug directly into the wall picks up the same hum? Different outlet? Are they powered speakers or passive?

 

[castleofargh]

I won't be of much help beyond this but unless the free online converter played a joke on me, this spectrum really doesn't look to me like the result of a ground loop issue:

 

[71 dB]

Well, the harmonics of that buzzing noise are at least multiples of 60 Hz. Sometimes unpowered sound sources can create these sounds for some reason, especially if the impedance levels of the connection are high.

 

[superdragon]

It sounds like a ground loop to me but you said a two prong plug directly into the wall picks up the same hum? Different outlet? Are they powered speakers or passive?

Passive loudspeakers. Different outlet, but on the same breaker. I have to move the unit tonight and will try it in a different room, but have a feeling that the hum will persist.

Well, the harmonics of that buzzing noise are at least multiples of 60 Hz. Sometimes unpowered sound sources can create these sounds for some reason, especially if the impedance levels of the connection are high.

@castleofargh Thanks for using the tool to analyze the frequency.

Early this A.M., while researching ground loops, I found some info on how some ground loop noise is actually twice the frequency of AC noise. The speakers are 8 ohm.

 

[superdragon]

I solved the floating input problem:

I split the main output of power negative and wired up the ground prong of a 3 wire grounding plug with on end. With that plug in an outlet, neither speaker buzzes when powered on without the Bluetooth receiver being powered on.

 

[redrol]

yes ground loop, 60hz with extra crap on top

 

[superdragon]

yes ground loop, 60hz with extra crap on top

Appreciate the analysis. Please see my grounding solution two posts up.