ETF and a 60 Hz problem

ctviggen · « **on:** 25 Apr 2005, 11:02 pm »

When I run ETF, it appears that there's 60 Hz getting into the measurement somewhere. For instance, on the waterfall plots (spectral decay plot), there's a huge peak at 60 Hz that does not decrease over 1000 ms. I can think of three sources for this -- the microphone/amp, the sound card (though one would think this wouldn't be the cuprit), and the 30 feet of cable that goes from the computer to the preamp. Is there anyway to test what it is? Or, if not, is there any way to get rid of it? Or should I just buy the test CD?

Thanks!

mgalusha · « **Reply #1 on:** 26 Apr 2005, 03:44 am »

Are you using a desktop or a laptop? If a laptop, make sure to unplug it. If you are using a desktop it sounds like you may have a ground loop. If you can, use a shorter cable or try plugging the PC into the same outlets as your gear. You could also try running a ground wire from the PC to the preamp.

mike

ctviggen · « **Reply #2 on:** 26 Apr 2005, 12:59 pm »

Those are good points, Mike. I also realized last night that I'm using a Behringer (sp?) microphone and a preamp for the mic that's DC. So, the AC has to be, as you say, though a ground loop or the extremely long run of cable I'm using. Over the weekend, I moved all my HT gear onto a completely separate circuit, although I can't remember whether I did the ETF measurements before or after I moved the circuit. This weekend, I'll give it another try, and if I still have 60 Hz getting in, I'll move the computer (a desktop, sadly) over to where the HT is plugged in.

ctviggen · « **Reply #3 on:** 26 Apr 2005, 01:02 pm »

Oh, the ground wire is a great idea, too. I could use a three prong to two prong plug to get rid of the ground, or I could run a wire over to my monster power center, which has a location for a ground (although I'd have to figure out how to do this so that I don't make the situation worse instead of better).

PEB · « **Reply #4 on:** 26 Apr 2005, 01:53 pm »

I would absolutely keep the PC on earth ground. Lift earth ground for the entire stereo during testing.

ctviggen · « **Reply #5 on:** 27 Apr 2005, 01:00 pm »

Lifting the earth ground on the system shouldn't be bad -- I can lift the ground at the breaker box for the duration of the test, then reinstall.

ctviggen · « **Reply #6 on:** 3 May 2005, 06:37 pm »

Apparently, a USB sound card or the PSD (power spectral density?) option will fix this. I ended up purchasing the PSD option.