LM386 Hum Problems (Solved)

[Meehah]

Hello all. I have recently built a LM386 guitar amplifier from the datasheet schematic (can be found here with the title "Amplifier with Gain = 20 Minimum parts") https://lowvoltage.wordpress.com/2011/04/23/lm386-audio-amplifier/ . It sounds nice hooked up to some headphones but there is a significant problem. It hums loudly constantly like a single coil pickup but worse and it doesn't go away when volume is turned down. It also faintly receives radio stations at a barely audible volume. I have checked all soldering but the culprit is nowhere to be found. I used a 0.1uf cap instead of 0.05 as the latter was unavailable and a 220uf as opposed to a 250uf for the same reason. I know this probably relates to grounding but can't work out in what way. Help would be very much appreciated.

[Phoenix]

How are you powering the circuit? Have you tried powering it with a battery? I'll bet you're using an unregulated power supply.
Also, that schematic is missing an important component, a local decoupling capacitor for the supply rail. You'll want something big, like 100uF or more from 9V to ground.

[GibsonGM]

^  Those, esp. the power supply, plus - this isn't in a metal box, right?  Any time you amplify a signal, any noise and junk riding along with it is amplified, too.  That includes hiss, hum, and radio signals.

If you put this in an enclosure when you're done, you might need a very small cap to ground from the input...like 50 - 100 pF.  That will shunt RF to ground and take it out of your signal.   

I have found it useful to check out what is in my room for 'signal generators', and to be aware if I'm near a local radio station and so on.  Knowing what SHOULD be causing you problems can help you rule out things quickly...like, a bar light in your electronics room, local radio, light dimmer...if you know it WILL interfere, you can sometimes just dismiss those things until you have boxed up a project.  If it still does this after it is enclosed properly...then you KNOW it is something else! 

[Meehah]

I'm powering it with a 9v battery. I'll try the capacitor from the power supply to ground today though and tell you how it goes.

[Meehah]

Success! The two new capacitors have removed all the hum and it is sounding excellent. Thanks a lot for all your help, you've been very kind and patient with my lack of proficiency.