[Awk.Pine]

This isn't DIY, but I thought maybe you guys could help--or at least might find this curious. My headamp hums (in the headphone output; the unit itself is silent), but not all the time: if it is upside-down or held vertical, front-side down, there is no audible noise; if it is oriented any other way, it hums. This is with no interconnects, just power (16V AC) and recording out of the headphone jack with my M-Audio Firewire Audiophile. This happens whether near my audio equipment or away from it; whether sharing an outlet with other electronics or not; whether the chassis is grounded or not; whether the case is open or not. The hum seen at the end of my recording, below, is representative of what it sounds like after being left on all night--if it is rightside up. The level looks to be about .003, or 0.3%, or -50 dB, and is certainly a 60/120Hz tone.

Let me describe:

Amp fully warmed up. Starting upside down. (Times in m:ss, and correspond to red stars in the image.)
0:00 - no noise.
2:00 - turn correct-side up. About fifteen seconds later, noise starts to appear. Hum peaks at about 3:30, and holds steady.
5:00 - turn upside down. Noise tapers until, at about 6:15, it is nil.
7:00 - turn right-side-up. Noise builds after about 10 seconds, and plateaus at 8:15.
9:00 - turn upside down. Noise tapers to nil.
11:00 - turn left-side-up. Hum builds to plateau at 12:15.
13:00 - turn upside down. Noise tapers to nil.
16:00 - hold face-down. No noise.
17:00 - hold face-up. Some noise.
19:00 - turn upside down. Noise tapers to nil.
22:00 - turn left-side-up. Hum builds.
28:00 - turn correct-side up. Hum stays about the same.
31:00 - turn upside down. Noise starts to taper...
~31:20 - Audacity crashes, ending my experiment.

So, for now, I'm listening to it upside down. But ... this just weirds me out. Any thoughts?

 

[rb67]

sounds like something is picking up an rf signal. Maybe a grounded shield is in order.

 

[Awk.Pine]

This didn't generate much interest, but I'll follow up on it for the records.

I added a Faraday shield and the--thanks rb!--hum has gone away. There could easily be a confounding variable here, but I made a change, went overkill, and there's no more problem.

I wrapped aluminum foil around the circuit board. When wrapped around the board and the internal frame, there was no change; when wrapped around the board, between the frame and the board, everything went perfect. I used a bit of chips ahoy wrapper (mylar?) to insulate between the foil and the board. The foil is grounded to the chassis through friction and one mounting screw; directly grounding the foil didn't make a difference.

Oh, I also added some insulation between the volume pot shaft and the chassis. The shaft contacts the chassis, which could intermingle audio ground and chassis ground. A bit of electrical tape solved that. I don't think this issue relates to the hum discussed above.

Cheers!