Persistent 60-cycle hum I can't get rid of

[Rad]

Hi folks,
I'm recording on a DAW from a Mackie Onyx preamp. As of late I have noticed a low-level 60-Hz hum coming into the recording irrespective of how I vary cable positioning.

The hum does not vary if I change the cable, mic, or particular preamp used (the Onyx has 4) or the position of the cable. However, I have noticed that in late hours (after 12 pm or so) the peaks I see on the spectrometer are much smaller relative to usual daytime hours. My first guess was that there is AC voltage contamination, but the power going into the Mackie is already being filtered by a Furman M-8 power conditioner which should be taking care of precisely that, including grounding et al.
Also, I have noticed that if I switch off the phantom power of the mic and listen just to the pre _with a mic cable attached_ but the mic not powered, the hum gets much stronger. If I switch the phantom on and listen to the live mic signal, the hum gets smaller but is still audible (my guess the balanced line doing its job to reduce the noise). This looks to me like power line effect of some sort coming through the mic cable, but I don't understand why the usual methods don't work to remove it, and in particular, why moving the cable doesn't help. W/o cable the hum disappears even on the spectrometer. So, I am now digging into the literature to look for something I haven't tried... help if u can.

[DAS]

I still think its possible you have an AC supply issue. The M8 is a very low cost, basic power conditioner. It filters RFI and has a surge suppresser built in. There is no ground treatment at all (that I know of), nor anything to address other types of EMI.

Normally this type of problem is best tracked down by disconnecting things until the hum goes away. Often, I start with nothing connected but a pair of headphones and then work from there until I hear the hum.

[dpd]

60 Hz has an extremely long wavelength, so I don't think moving the cable around the room will do you any good. When you remove the cable you have removed the 'antenna' that appears to be inducing the signal into the front end of the Mackie. Is your room near an of electric motor of any kind? Fluorescent lights?

Another guess is that you may be hearing the residual AC on the Mackie power supply that is bleeding into the 1st amplifier stage of the preamp - is it purely 60 Hz or are there the typical harmonics at 120, 180, etc? If so, that tends to tell me it's from the Mackie power supply circuitry at a base level.

Perchance, is your DAW and Mackie plugged into outlets that are on different circuits? That could cause a small offset in the grounds and induce hum.

I've had to get very good at removing hum in recordings using a very flexible EQ plugin for those times I can't eliminate it at the source.

[Rad]

Hi dpd,

No, it's purely 60Hz without any harmonics, so my guess is that the Mackie power supply is probably Ok. All gear is plugged into the same Furman power conditioner, including the DAW.
I hadn't thought about the 60Hz wavelength that way, but now that you mentioned, it makes sense that shifting the cable doesn't change things. I tried a couple of different cables too to make sure it's not caused by a broken shield, to no effect, so this looks like magnetic interference of some sort. I have no idea of how the wiring in this room has been done, but now I recall that a friend of mine in his studio had to completely rewire all electrical connections in the walls before getting rid of the same problem there (in fact they put all cables in metal pipes with grounding and removed all old wires). S