Buzzing sound from speakers on amp

Thread Starter

john2k

Joined Nov 14, 2019
146
I have a pair of speakers 8ohm 40W ceiling speakers connected to a amp. The amps has a 3.5mm jack input which i've got a IP telephone plugged into using a cable that looks like the picture below. I use the Power Over Ethernet powered telephone to multicast stream audio to the speakers. Works very well. However, when there is nothing playing I am getting a rather annoying buzzing sound from the speakers. If I unplug the 3.5mm jack from the amp then it becomes silent again. So something being caused by the telephone. The telephone is being powered by PoE. One way to solve it is to reduce the output volume on the amp to a level where the sound is still there but very very low that it cannot be heard. But problem with this is that my overall volume level is then compromised. Is there a way to silence the silence?


 
Last edited:

crutschow

Joined Mar 14, 2008
27,708
Sounds like hum pickup from the power line.

The PoE is a likely suspect for that, which may be causing a ground loop.

Does the amp have a two-wire or three-wire power cable?

Can you power the telephone from a separate wall-wort supply?

An audio transformer at the telephone output may help by keeping the telephone ground isolated from the amp ground.
 

wayneh

Joined Sep 9, 2010
17,153
Depending on the quality of the audio you want to hear, a high-pass filter could be used to knock down the 60Hz hum. You'd lose content below 100Hz or so and that would be unacceptable in a home theater, but maybe that's not a big deal for ceiling speakers?

Killing noise at the source is better.
 

Thread Starter

john2k

Joined Nov 14, 2019
146
Does the amp have a two-wire or three-wire power cable?
It has a 3 wire cable.

Can you power the telephone from a separate wall-wort supply?
It can but the phones don't come with the PSU for it as they are primarily designed to run PoE. So hence I don't have a suitable PSU to power it.

An audio transformer at the telephone output may help by keeping the telephone ground isolated from the amp ground.
Any suggestions of what i'm to be looking for?

Depending on the quality of the audio you want to hear, a high-pass filter could be used to knock down the 60Hz hum. You'd lose content below 100Hz or so and that would be unacceptable in a home theater, but maybe that's not a big deal for ceiling speakers?
It won't be used to listen to home theater quality stuff, mainly annoucenments and melodies.
 

crutschow

Joined Mar 14, 2008
27,708
It has a 3 wire cable.
That's a likely source for the ground loop since your amp is grounded and so probably is the PoE supply, wherever it's located.
It can but the phones don't come with the PSU for it as they are primarily designed to run PoE. So hence I don't have a suitable PSU to power it.
A wall-wort supply of the appropriate voltage and current would work for that.
Any suggestions of what i'm to be looking for?
An audio isolation transformer such as this.
 

Thread Starter

john2k

Joined Nov 14, 2019
146
An audio isolation transformer such as this.
I had one lying around exactly like that and it solved the problem. And I bought a few different ones but the different ones I got I tried one and in one direction it makes the noise and in the opposite direction it doesnt. Are these directional? Below is a pic of the isolator that i bought and seems to cancel the noise if plugged in certain direction only.

 

Thread Starter

john2k

Joined Nov 14, 2019
146
Transformers are bidirectional, so I'm puzzled as to why you found that difference (?).
That's what I thought originally because my first one that looks like the example you sent works in both direction. But this one only worked in one. Maybe it was just a coincidence. Will try it out on another again.
 

Thread Starter

john2k

Joined Nov 14, 2019
146
Just out of curiousity, isn't there a way to remove grounding from the Poe cable that goes to the telephone without effecting it's power. Other option is to remove earthing from the amp but seeing as the amp is metal, I think the earth is needed.
 

crutschow

Joined Mar 14, 2008
27,708
isn't there a way to remove grounding from the Poe cable that goes to the telephone without effecting it's power.
It's likely the ground to the Poe power supply and you likely don't want to remove that.
Other option is to remove earthing from the amp but seeing as the amp is metal, I think the earth is needed.
Yes, for safety, that ground should also not be removed.
 
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