1st things 1st, this is an extremely common problem, a minor buzz coming from the amp when you're not playing, not touching any metal part on your bass/guitar and the volume & gain is up quite high. There are A LOT of factors that may be causing the problem, sometimes more than one at once. Try playing the bass at more than one place. Eg at home and in the shop where you bought it. And more places still. Try plugging your amp in the kitchen and see if its still there. There are many possibilities, but below are the most common.
I suspect 3 things:
1) CHECK THIS FIRST! Does your amp have a ground pin on the power cable? If not, and if your amp's power cable has 3 wires inside, get a 3-pin plug and fix it on. (Ask someone who knows how to do this if you're not sure, but its actually very simple... But I will not be responsible for any electrocution, thus this disclaimer)
2) Shielding /ground loops. refer here:
http://www.guitarnuts.com/wiring/shielding/shield3.php
Most cheap (even expensive ones sometimes) instruments have bad shielding. Eg on the insides of the cavities they claim to use 'conductive paint' but it doesn't conduct. The ultimate solution is to do it yourself, but it can be quite a lot of work. If it solves the problem, its worth every little step of the way...
The link above shows its for a strat but you can follow the sticking of metal foil on the insides of the cavity part. Its worth doing, but the copper shielding tape is hard to find i think. Try Art Friend, they sell copper sheets that you can cut up (warning sharp edges!) but you have to stick using glue as they are not self adhesive.
As for ground loops, its a bit more complicated to explain. I shall not try to do it here. Try referring to the link I gave above, I think they did mention a bit about it.
3) Sources of interference
Ok this actually would be irrelevant if you do step 2 above coz the full shielding would (most probably) not make interference a problem. I'm talking about stray electromagnetic noise, RFI and some others (like alien radio signals) that are in the air all around us. While not much can be done for the alien communication frequencies, you can try these:
- Stay away from computer screens and fluorescent lights (like some table lamps that use those energy saving white light bulbs... and don't play too near the ceiling lamp as well, keep the bass at least 1m away. Just checking to be sure. Actually just kidding)
- In fact stay away from anything that you think might be causing the buzz/hum (TV, bomb timers, HIFI, table fan etc etc) and see if it stops. If it does, then you've nailed the interference source. You can try moving closer to the thing that you might think is causing the problem to see whether it worsens. And then try to switch off the appliance (like switch off the lamp) and see if it goes away.
- Try to plug your bass amp somewhere else, definetly not on the same wall outlet socket as a computer/lamp/tv etc. Try plugging the amp in by itself. There might be a small possibility that your house grounding might be 'dirty' as well (it hums everywhere) so do try to go around (friends house etc) and see of the problem is still there.
You might be surprised. There might not even be a problem with the bass, it could be just a lamp!
