No input amp hummmmm

[Jack.Straw]

Hey guys. I'm trying to figure out a way to get rid of, or at least reduce, the hum of an amp i built when there is nothing plugged into it. It's an little LM386 based amp. Here are the symptoms:

Battery powered = no hum at all.

Wall-wart powered with guitar or media player plugged in = no hum, or very little hum at high volume.

Wall-wart with no input device attached: major hum! Touching the common/ground with my fingers reduces the hum significantly, but by no means gets rid of it.

All wires are shielded & the amp circuit itself is now enclosed in a grounded housing. Is there anything else i can do to help reduce the no-device attached hum?

Also, the amp and all it's controls/inputs are going to be installed in a conductive metal container. The shell of the DC input jack will ground to the container, as will the input jacks (i think/). Will the "grounded' metal container act as a shield around the circuit as a whole, or will it increase the hum?

Thanks for your time and help!
-Jack

 

[Chippie]

I hope you aint English and a politician in a current/previous life....


What voltage does your walwart output?

What voltage does your amp require?

You'll probably find a voltage regulator will filter out most of the the ripple....

 

[Jack.Straw]

Hi Chippie. The walwart i'm testing with is a multi-voltage unit, currently set to 9 volts. The amp requires 9 volts. I've tested with the walwart in question, as well as a couple other 9 volt ones.

 

[crutschow]

Grounding the shell of the DC input to the metal container should reduce the hum with no input. You can also try connecting the container to the 3rd prong safety ground of the AC plug.

 

[millwood]

the input impedance may be too high. Ground the input with a resistor < 47k (I usually use a 10k or 22k resistor for that). Make sure that it doesn't upset any dc nulling - check the datasheet for input bias current.

 

[Jack.Straw]

Thanks guys. Millwood, do you mean the DC input?

 

[millwood]

Jack, check to see if your signal input is grounded. posting a schematic would be quite helpful.