I just ran new wires for mics and speakers in plastic conduit.

did you place the "different cord for the speaker" inside the same plastic conduit or did you have the cable outside of the conduit. if outside, test it inside perhaps if it is a different brand or type of wire.

most likely the wires you are using aren't shielded so they are picking up interference. shielded wires or running them away from eachother should solve the issue.

maybe the resistance of the wire is excessive in one direction.
maybe the plastic is not enough to shield the wires from eachother
maybe the plastic conduit is amplifying the electrical leakage and causing the speaker wire to absorb the energy.

i'd say yank the wire out of the conduit and use it again to check for the hum.
you need to have the mic wire and the speaker wire on top of eachother as they would be in the conduit.

if you can wire both wires as they lay on top of eachother without hum, its the conduit.
if you wire up both wires and the hum is there.. seperate the mic wires from the speaker wires and try again.. if the hum is gone, the wires are too close to eachother (inferior shielding of one of the wires)
if you wire up both wires and the hum is there after you have seperated the wires, the hum is probably caused by a bad cable.


if you use a different cord.. the cord should be the same length, but that doesnt mean the cord is the same internal resistance.

if the spool of wire is extremely poor quality.. you might find different resistances throughout the entire spool.
meaning, if you cut one length of 20 ft .. another cut of 20 ft might be different resistance.
and that could mean the entire spool of wire is trash, or the difference is simply just enough to cause a hum.

it could also be a connector on the wire, if any of the connectors are loose or the wrong resistance (impedance) then that could mean the wire is good, but the connector needs replaced.

Maybe you used unshielded wire instead of shielded wiring? Powered Speakers (amp built into speaker cabinet) MUST have shielded wires from the audio source to the speaker. Likewise microphones MUST have shielded cabling from the mic to the audio mixer.

G.J.