Hello. I've been using a Roland Quad-Capture to record with a standard Dell laptop for a while now. Until recently, I've only been recording with a self-powered condenser mic and direct inputs, and haven't had any problems.

I now have a condenser microphone which requires the Quad-Capture's 48V phantom power, and I've noticed while recording that there's a constant low-frequency hum intruding. Slapping on a high-pass filter eliminates the worst of it, but it's a nuisance, and kills the low end of what I'm recording.

Is this something I can fix, or is it an inherent limitation with phantom power in USB interfaces? Should I be looking for an electrical/grounding fault in my laptop or interface?

My preliminary troubleshooting seems to rule out a fault in the mic itself; the hum continues with the mic unplugged. Also possibly relevant: I've noticed occasional minor interference when the laptop's power supply is plugged in, though this seems a separate issue. I've googled around, and the only suggested workaround is to connect the Quad-Capture to a powered USB hub rather than drawing power from the laptop, which seems cumbersome and sort of defeats the purpose of a mobile laptop setup.

I'm not too savvy in this area, so I'm hoping I'm overlooking something obvious. Any advice much appreciated.

Well, I've now tried running the Quad-Capture via a powered USB hub, but it didn't alter anything.

Still keen to hear from anyone who has used a phantom-powered microphone through a USB interface, so I can tell if this hum is a fault or just standard.

Are you saying the hum is there ONLY when the phantom power is in use? If not, is it there when running without the power supply connected to the laptop?
Sounds more like a ground loop issue than anything else.

Only there when the phantom power is switched on, yes.

Disconnecting the laptop power supply doesn't make a difference either way. (I've noticed some chattering noise when the power supply is connected and my laptop battery is full, but that appears to be unconnected to the hum.)

If the hum is there whether or not the mic is connected I'd say it's a fault with the interface. The chattering noise is probably caused by a ground loop.

Cheers, tcc.

I suspect it's now out of warranty, so I suppose it's sort of inevitable that the interface is faulty. Will have to dig out the receipt. Many thanks for your help though.

I hope you can get it sorted out. Was wondering if you tried disconnecting the mains and your monitors and listening with headphones. Is the hum still there?