This may be off topic but I know you guys are pretty clever and helpful !! (Have I buttered ya up e'nuf?)
I have a small amp that I use to power my MP3 player with my under the pillow speakers for "rain and thunder" sounds for sleeping - when I use a wall wart I get that darn 60 cycle hum - anyway to eliminate that??  It sounds real weird when the batteries start to go in the middle of the night!!!

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Best way is to change to a "regulated" adapter. The most common adapters are unregulated, and do not have a voltage regulator inside.  But most important, and relative to your 60Hz problem, is that most unregulated adapters do not produce what is known as "clean DC current".

In a tube guitar amp, for example, the power transformer converts the 117-120v AC from your wall outlet to a much higher voltage. That higher AC voltage is then converted to high DC voltage via a rectifying tube or diodes. At this point the DC current, when portrayed in a drawing, would look like a bumpy road. The tube amp circuitry then adds filter capacitors to help "smooth out" the DC current, thus eliminating that irritating noise before the high voltage continues through the circuit.

You could always try experimenting with your amp circuit. But I cannot help there. Personally, I would move toward a proper adapter that matches the needed 9v DC output, and matched or better mA rating based on the load draw of your amp.

Best answer is in your question. Get batteries

You could use a small portable Radio/Tape/CD Player with a MP3/Aux. input for the MP3 player. No unwanted noise and they can be had for 20 to 30 dollars.

Hey i apologise if my last answer seemed flippant, you're running this all night eh? 

1 try flipping the phase of the dc output from the wall wart (clip the low voltage end off, reconnect with the 2 wires flipped)..

2. consider a shorter/ better quality audio cable, especially if its a long one and you have the excess coiled up. (I'm referring to the cable between mp3 player and amp)

A little low voltage amplifier like this will be fine, it may not work but it certainly won't damage anything :)

Really depends on what components are used in the amp. Example: film caps  and resistors can work with electric flow in either direction, but electrolytic caps will not. Some chips/FETS will work, some will burn out.

I ruined my 386 chip amp by accidentally getting the polarity wrong when plugging in the wawart for less than a minute.

Hey guys - thanx for the replies!! (and Kid - No worries!!  ;-)) - I will try your "fixes". This amp is a nice little Burton that takes 4 AA batteries which I figured added up to 6 volts but maybe I will try a 9v wall wart. I like this thing cause it is real small. 
The next thing I gotta figure out is where can I get ear plugs that block the wifes snoring but allows the soothing sounds of the frogs and rain???? I can't get my brain to convert the "snores" into "thunder"!!!! Go figure!!  I may try earbuds inserted into a head band - they hurt my ears otherwise.

dude its simple, file divorce and marry a frog in equador

Oh, no way am I going through that inter species thing again !!!! They say "3rd times a charm" but I'm not buying it !!! ;-)

Hi. First time poster, but I am an amateur radio operator (KE7RYM) and hum and noise are a worse problem now than 20 years ago. Most wall wart transformers use a high frequency to get around the need for lots of iron. While more efficient, these little goodies transmit all over the place-and through the power line to anything else on that circuit as well. CFL bulbs hum, routers hum, darned near everything not on a small battery can generate hum.

Besides using batteries, make sure you aren't allowing hum to enter the equipment. Google "Pin 1 Problem" and "High fidelity" for detailed information.

Hey guys - before I even got to try anything, at least I don't think I screwed anything up - the battery side won't power it up any more, so I'm stuck with wall wart - but on the bright side, the little mp3 dealy will power the speaker, so, the amp was a bit redundant, I guess. I wonder if the little gizmo that shuts off the battery side when you plug in the ac side somehow is malfunctioning - I took the back off but then chickened out - I know how this usually ends for me!! At least part of it works, I better leave it at that!!

The amp won't power up if I reverse the polarity both on the wires or switching how I plug it in, Oh well, I probably got bigger concerns that I should deal with, wait, is it too late for Xmas shopping?? Oh Oh...

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