Feedback from electronics alone would indicate you're looping an input to an output. If it's just an input, you may have a ground loop, or have lifted the ground entirely. If this is the case, you've potentially removed the ability of the cable to reject noise.
It's quite possible that the power supply of the laptop is generating interference, but it's not the first ting I'd be looking at. Does it still hum if you remove the XLR cable and plug the mic straight into the adaptor?
Keep in mind as well that most laptops have horrible sound cards built in, and if you don't have enough signal to noise, you might be hearing more noise. than anything. Any chance you've got a preamp lying around that you can push the signal with? Or,do you have any other device that you might use to test for interference hitting the cable?
You mentioned it's a 1/4 cable, your laptop has a 1/4 input? That would be unusual. Or perhaps you have an external card/breakout you're using?

Spot,

I neglected to add that the 1/4" is plugged into a 1/4' to 1/8" adaptor as well. Unfortunately the setup is at my brother's house so I'm unable to troubleshoot in real-time. I'll have him plug the mic directly into the adaptor and see what happens. He haid he tried the adaptor on a 1/4' input on his amp as well and the humming was even worse (perhaps I should've mentioned that too). I suggested an 1/8" extension between the laptop and the adaptor but it sounds like your thinking the proximity of the adaptor is not the likely culprit.

The laptop is expecting to see a low impedance source. By using the 600 ohm to 30k ohm transformer, you are feeding the low impedance laptop mic input with a source impedance that is WAY too high.

The impedance matching transformer is the problem. You just need a straight XLR to 1/8" adapter cable, not the transformer that you are using. Pins 1 and 3 of the XLR are ground and should be connected to the "sleeve" of the 1/8" plug. Pin 2 is "hot" and carries the unbalanced audio signal, which should go to the tip of the 1/8" plug.

John

Thanks John. Any ideas where I can find such a critter? All I've been able to find are the XLR to 1/4".

HOSA makes one:

HOSA XVM-105M

Pretty much every music store on the planets carries HOSA cables.

John

Hosa, AT, Tascam, all offer the XLR straight to 3.5mm cables with no transformer built in.

here's the first one Google showed.

You guys are awesome as always!

Thanks again!

"He haid he tried the adaptor on a 1/4' input on his amp as well and the humming was even worse "

ASSUMING that the amp referred to is a guitar amp then the impedence mismatch theory is negated because all standard guitar amp inputs are high impedence.

Too many assumptions would have to be made in this case for an accurate diagnosis. For example; not all mics with XLR connection are LO-z and not all LO-z mics require phantom power.

I have used the setup you describe using the LOz-HIz transformer into a laptop mic input with excellent results when all of the components are right for it.

The straight-thru cable suggested may solve the problem given a certain set of circumstances but more information could help to confirm it starting with the brand name and model number of the mic being used.

Blues

I had a similar noise problem using a M-Audio Quattro(a 4 channel USB device). It was finally traced to the power supply for the laptop. Charger plugged in - noise, unplugged - clean. Try unplugging the charger and see if any change, real quick and easy fix if it works.