Can I filter noise from power supply?

[Peabody]

I have 5VDC wall wort power supply, which is a switching supply, and
it has a typical barrel connector. But I rigged up a small one-inch
square pc board with a USB socket and a barrel socket mounted on it.
So, I can use the power supply as a USB power source for charging my
MP3 player and other things.

But, when I used this power supply to play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

I can't regulate the supply because I need the full 5V, but I
wondered if I can add things to the little circuit board that would
reduce the noise - such as a series inductor, or a large capacitor
to ground, or both. Will those things work, or do they screw things
up?

Pics of this setup are here:

http://drop.io/owhsijhen

By the way, for future reference, a dumb USB power supply should
have its data pins shorted together.

 

[Tom Biasi]

"Peabody" <waybackNO784SPAM44@yahoo.com> wrote in message
news:3KuHl.56035$LA7.44986@newsfe20.iad...

I have 5VDC wall wort power supply, which is a switching supply, and
it has a typical barrel connector. But I rigged up a small one-inch
square pc board with a USB socket and a barrel socket mounted on it.
So, I can use the power supply as a USB power source for charging my
MP3 player and other things.

But, when I used this power supply to play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

I can't regulate the supply because I need the full 5V, but I
wondered if I can add things to the little circuit board that would
reduce the noise - such as a series inductor, or a large capacitor
to ground, or both. Will those things work, or do they screw things
up?

Pics of this setup are here:

http://drop.io/owhsijhen

By the way, for future reference, a dumb USB power supply should
have its data pins shorted together.

What you say may work, it would help to know more about the noise but that's

a little more advanced.

Tom

 

[Eeyore]

Peabody wrote:

I have 5VDC wall wort power supply, which is a switching supply, and
it has a typical barrel connector. But I rigged up a small one-inch
square pc board with a USB socket and a barrel socket mounted on it.
So, I can use the power supply as a USB power source for charging my
MP3 player and other things.

But, when I used this power supply to play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

Was the equipent the mp3 player was connected to grounded / earthed ?
That'll be the problem. Most low power SMPS wall warts have a Y
capacitor between primary and secondary sides that gives rises to a
leakage current at mains frequency and harmonics thereof. That current
will run to ground along your audio cable and add a nice buzz ( worse
still if the audio equipment has poor / old fashioned attention to
internal grounding ).

Graham

 

[westom]

On Apr 21, 9:50 pm, Peabody <waybackNO784SPA...@yahoo.com> wrote:

But, when I used thispower supplyto play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

Filtering is the power supply's job. Why do you want to build what
should already exist in the supply? Simpler and probably just as much
are power supplies from electronic supply houses such as newark.com,
mouser.com, digikey.com, jamesco.com, alliedelec.com, etc. Either buy
a 5 volt wall wart that says the filtering already exists, or buy the
inductors and capacitors from the same sources to construct a filter.

Anyone with minimal electrical knowledge can build this filter.
Simply create a chain of capacitors between the plus and minus wires;
with coils (inductors) between each shunt capacitor. Inductors are
in series. Capacitors connect in parallel. Every "parallel capacitor
and series inductor" chained together means even better noise
immunity.

Need more information? Any basic power supply primer will show this
filter.

 

[Eeyore]

westom wrote:

On Apr 21, 9:50 pm, Peabody <waybackNO784SPA...@yahoo.com> wrote:
But, when I used thispower supplyto play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

Filtering is the power supply's job.

You can add more as required for the task. Dead easy.

Graham

 

[whit3rd]

On Apr 21, 6:50 pm, Peabody <waybackNO784SPA...@yahoo.com> wrote:

I have 5VDC wall wort power supply, which is a switching supply, and
it has a typical barrel connector.  But I rigged up a small one-inch
square pc board with a USB socket...
But, when I used this power supply to play music, rather than just
charge the player, it was clear there's a good bit of noise in the
supply which doesn't show up when batteries are used.

Try another wall-wart (even if it's an old analog type).
They're cheap, widely available, and your immediate
friends and family probably have dozens in the garage
for things that broke years ago.

Pickup or ground loop problems can account for this behavior, too; can
you hear a difference with the ground disconnected at the wall-wart,
or with the plug reversed in the socket? Can you find a USB cable
with those ferrite-bead clamp filters on it, and see if that helps?