Noise in audio when charging USB

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ExtraCredit1

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Joined 2012
#1
OK so I just finished adding a USB charge port to my battery powered amplifier. but when you plug in a phone to the USB to charge it I get pretty nasty noise in the audio signal.

Is there easy way to filter this out?

My system is:

48v battery power source
Icepower amp
Stereo to Mono adapter sending signal directly into the amplifier
Then I just added a TI 48v to 5v DC to DC convertor for the USB charge port powered off the same battery.

Ideas?

Thanks!!
Mike
 
#5
ExtraCredit1 said:
If I charge another phone that is not providing the audio then there is no noise. So its something with charging the same phone I am using as the audio source.

Mike
Click to expand...

One issue with charging a handphone or MP3 or other portable device and playing from it at the same time is that the audio signal "ground" will be at a potential roughly halfway in between the battery V+ and 0V. Let's call this thing to be 2V.

When not charging, or charging and playing to headphone or other speakers, no problem.

Problem comes when you are powering both the amp and the phone with the same power supply and with the same 0V connected together. If you're using single-ended connection, which you have to be since you're using a handphone, then on the amp's input side the connector's ground pin would be connected to the 0V. However on the handphone the connector's ground pin is 2V. So the devices fight and you hear the outcome of this power supply arrangement instead of music.

Solution 1 - If your amp allows it, use differential input. Just 1 or 2 more resistors. Connect the handphone's signal ground to the negative differential input of both channels. You can still use TRS cable/connector for this.

Solution 2 - DC-blocking capacitor on signal ground.

Solution 3 - Input transformers, instant +$200 to cost. Doubt it achieves much more than solution 1.
 
#6
wwenze said:
One issue with charging a handphone or MP3 or other portable device and playing from it at the same time is that the audio signal "ground" will be at a potential roughly halfway in between the battery V+ and 0V. Let's call this thing to be 2V.

When not charging, or charging and playing to headphone or other speakers, no problem.

Problem comes when you are powering both the amp and the phone with the same power supply and with the same 0V connected together. If you're using single-ended connection, which you have to be since you're using a handphone, then on the amp's input side the connector's ground pin would be connected to the 0V. However on the handphone the connector's ground pin is 2V. So the devices fight and you hear the outcome of this power supply arrangement instead of music.

Solution 1 - If your amp allows it, use differential input. Just 1 or 2 more resistors. Connect the handphone's signal ground to the negative differential input of both channels. You can still use TRS cable/connector for this.

Solution 2 - DC-blocking capacitor on signal ground.

Solution 3 - Input transformers, instant +$200 to cost. Doubt it achieves much more than solution 1.
Click to expand...


Ok this makes sense. Thanks for taking to the time explain.

I am feeding the amp a balanced mono signal. I have a stereo single ended to balanced mono converter. Not sure how this effects what's happening?

If I wanted to do solution 2 and add a capacitor would that effect signal output or quality? What size capacitor and how would I wire it?

Thanks again for your help.

Mike
 
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