AB Switch buzz/hum issue

[Sky]

Hi Guitar Pedal Friends!

I built this AB pedal for a friend (hence his face on the pedal).  It works!  However, it adds some noise when powered by a 9 Volt adapter like a 1 spot.  When used with a battery it works fine with no noise.

The slight buzz/hum that is added isn't terrible, but I would love to get rid of it entirely so that my friend can actually use this with his power supply and not keep replacing batteries. 

1. Does anyone have any suggestions for fixing this issue?   
2. Would you have any comments on why buzzes like this happen?
3. Maybe how to prevent situations like this from happening for future builds?
4. All tips are welcome. 

-Mark

SIDE NOTE:  Thank you so much for your time answering this question.  I really enjoy learning about this stuff, and the responses on this forum are extremely helpful for a noob like myself learning about electronics.  ...I just recently finished a successful Slow Loris Build, but I haven't boxed it yet.  I will post once I do.   Blessings.

[nzCdog]

You can try adding extra filtering between your +9V and Ground.  Try an electro Cap like a 10-100uF... You can solder it directly onto the DC jack if you like.  For an added measure, try a 1-100nF Ceramic Cap in parallel with the first filter cap as well. 

Sometimes AB pedals create ground loops when running between two amps, in which case you'll need to isolate the earths somehow.

[Sky]

HI NzCdog,

Thanks so much for this!  So to be clear...I need to  insert the electro cap in series between the 9V+ and the Resistor (that goes before the LEDs)? 

Also...any great suggestions for learning the electronics behind this.  I took a DC Theory class at a community college (got and A in the class), but I still don't fully understand all of this and don't really no where else to look.  Thanks again. 

-Mark