Isolate or float ground on USB 5v power supply

[ExtraCredit1]

I have a problem that must be common. When I use a phone as a audio source and then charge the phone from the same power source that the audio system is powered from I get really bad noise.

I tried to isolate the ground on the signal side but lost audio performance.

When I plug the phone into a separate USB power source like a USB portable battery the noise goes away 100%


So how do i isolate the ground on the USB power to stop this ground loop?

Thanks
Mike

 

[AndrewT]

what type of noise?
Hum , buzz, white noise, whistles?

 

[ExtraCredit1]

Noise and whistle that changes and buzz. It's a mess.

Totally not something you can ignore

Mike

 

[Mooly]

It sounds as though the phone PSU (internal in the phone) and the audio ground aren't one and the same. If a ground lift resistor doesn't help (rather than just isolating the audio ground) then your options might be limited. A DC-DC convertor to supply the 5 volts would be the obvious choice I suppose.

 

[ExtraCredit1]

I also hooked up a Bluetooth adapter and it makes the noise also.

How do I out a ground loop isolator on the 5v power from USB?

Thanks
Mike

 

[Mooly]

I don't think you could add any sort of ground loop isolator on the PSU side, just a low value resistor (say 10 ohm) in series with the audio output ground but I'm doubtful that would do anything. Or lets say if it were possible, then it would be a unique solution for that one phone model and would need a lot of testing and so on to see whats really happening and how the circuitry is configured.

A DC-DC convertor would give total isolation but you would need to determine the current needed. In theory that's a simple solution, in practice cost might be an issue if you need say 1 amp output.

 

[ExtraCredit1]

Mooly

I actually have the same problem with my battery powered amplifier I built and I am using a DC to Dc 48v to 5v to power the USB charge port but it has the same noise as it does in my boat system. Both with a iPhone and a Bluetooth adapter as source.

So at least the dc to dc converter that I have on my battery system does not isolate the ground.

Basically if I am using the same power to power the usb audio source as the audio amp I get the noise.

Mike

 

[Mooly]

You would need to check for continuity between the DC input and output of your convertor. The convertors I'm thinking of are like this and often isolated up to a couple of KV.

12PURB4805MP - POWERPAX - 12W REG DIP 48V IN / 5V OUT | CPC Products&MER=e-bb45-00001002

It sounds like your issue really needs a detailed investigation using a scope etc to see what is really going on.

You say its OK on a battery powered USB charger ? Have you tried connecting the ground of that charger to the amp ground (first check there is no voltage differences) and see if any noise appears then.

 

[ExtraCredit1]

I actually used one of those battery devices that has a USB output so its then powering the audio source off a separate power source all together. I tried to plug the battery into a 12v outlet with a USB adapter to charge the portable battery and the noise came back.

The crazy thing is that this must be a huge problem out there. I cant be the only one using a headphone output to power a car audio system? everything is great until I try and charge the audio source.

Mike

 

[ExtraCredit1]

I was just reading in the car audio forums and people are not having problems with Bluetooth adapters that take a 12v power source. Its all with adapters and phones that need 5v for the power from USB. So it must be something with converting 12v to 5v that creates the imbalance and noise?

 

[Mooly]

Yes, it would seem so from what you say. Afraid I've no real answers based on all that.