Hello

Can anyone suggest an easy circuit with maybe a few caps and resistors to limit the line noise from a 9V 500ma DC wallwart that I am hooking up to a pre-amp ( http://www.paia.com/proddetail.asp?prod=K98 )?

Thank you so much!
curtis p.

 

I'm afraid there is no easy answer to your problem. The hum is due to to ripples of the rectified DC from the walmart, possibly with a very low value of smoothing capacitor inside.

You can parallel a large value smoothing capacitor of 2,200uF to the DC 9V and see if the hum problem now becomes acceptable.

Failing that there are three possible solutions, either:

1. You measure the output voltage of your existing walmart and if it is more than 14V, then you can fit a 7809 to get regulated 9V output. There is the slightest chance that this will not work if your preamp takes too much current.

2. A better and proper solution is to buy a 12V walmart and fit a 7809 voltage regulator to get a high quality 9V.

3. Or you buy a 9V walmart which has the word "regulated" clearly marked on the body.

 

thank you Mr. Chung...excellent work.