Weird hum from bass amp, but only when it's off ???

[drenwick]

Our bass player runs a line out of his Ampeg head to an input of our mixer. Last night after hooking everything up, we were getting a low frequency hum that I narrowed down to his input. Changed out the XLR cable and it still did it. This was with his Ampeg OFF. Turned the amp on and the hum went away and everything seemed fine. At the end of the night, he turned it off and got all kinds of weird noise - it was only for a second as he turned it back on immediately until I powered down the rack, but I thought I even heard radio interference. Any possible explanations?

 

Thanks in advance for your help!

[6Imzadi]

I have had this same situation happen to me as well.

 

It has happened with the keyboard sends from the key module through a DI.

 

I don't reallly know why, but turning on the unit stops the noise. Likely it is an open collector type thing when the unit is not on. Allows the mic cable to not really be grounded and the two signal wires don't have equal signals present.

 

Agedhorse, does this sound right?

[drenwick]

Got it... Kinda like-

 

Patient: Doctor, it hurts when I hold my arm like this... What should I do?

 

Doctor: Don't hold your arm like that.

[agedhorse]

If it's not a truely balanced output and is capacitor coupled, there will be different inpedances on the lines and there may be unexpected currents flowing that would be different if the op amp driving the hot lead in an impedance compensated unbalanced output is unpowered.

 

It's "similar" to the open collector circuit that Kevin mentions, in that when the op amp is unpowered the output goes high impedance.

[samkokajko]

I have experienced that with a lot of bass amps. Among them an ampeg and a $1500 mark bass (amazing amp BTW). I assumed it had something to do with an open ground when the unit is off. But just mute the input while the bass amp is turned off and it isn't a problem.