amp hum. school me...

[dougazbass]

I started getting a small amount of hum out of my Old US ampeg rocket bass amp. Wasn't sure what causes it.
I thought I'd go to Guitar Center and try some amps.
They had a neat used EDEN 264. Tried it with my bass and there was hum. then went thru the following:
acoustic amp: quieter hum
swr amp: louder hum
GK amp: loud hum
Mark Bass amp: no hum
EDEM nemesis: dead quite

I'm not trying to compare brands here. But I am very unknowledgable about amps. Why are some quite? Do they all start to hum with age? some of these were used.

[floridagatorfan]

might be that some have tweeters and that makes the hum louder

[Jerrold Tiers]

OK.....

If the amp does not hum until you plug in, the amp circuits are probably not to blame.....

It would seem to be either your bass, or a combination of your bass with the amps.

You can also try comparing with the cord plugged in and no instrument, or better yet, with a new all-metal phone plug....

Some amps actually mute when nothing is plugged-in, and naturally that would make them quiet. Plugging in the all-metal jack opens the input, but the metal handle shields the open input. What's left should be what the amp has as hum.

For hum that is picked-up, yes, the EQ settings, as well as the amp's basic "flat" setting, and having or not having a tweeter can make a huge difference in what you hear.

[dougazbass]

Thanks for the comments. I will add that I used the same cable for all and tried 2 other basses with the EDEN 264 and got the same hum level.

[B-string]

Try a new cable.

[fdeck]

GC may have had a mis-wired outlet or an extension cord with an open ground.

Any amp that has a mainstream line-frequency power transformer will leak some AC magnetic field into the surroundings, to be picked up by your bass. Amps that had the old EH core transformers would orient them so the fields were mainly picked up around the sides of the amp, and not in front. Even toroidal transformers will leak to some extent if their windings are not totally uniform.