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Why do I hear hum when using a cheap, low-quality condenser mic?

The problem here is that the mic does not have a very high CMRR (noise rejection). The hum resulting from the phantom power circuit in the Octopre LE will be rejected by better designed, more expensive mics which have a higher CMRR but not by a lower quality condenser mic.

A parallel earth connector (PEC) can be used to increase CMRR. A PEC is a low resistance piece of wire which is bonded to the outer chassis of the piece of equipment at either end, thus offering a connection in parallel with the screen of the audio cable but of lower resistance. If you want to try something which may reduce the hum then use a fairly thick copper wire and make sure one end is in contact with the Octopre LE casing and the other in contact with the Mic casing (exposed metal).

NOTE: we have not tested this and so cannot guarantee that this will completely solve the problem.