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Electrical hum from topanga


mattpitts74

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In the last few months I have built a topagna following the recipe here on CA.

 

my only alterations are with a bigger case and the addition of a PPA usb card. I have recently noticed that there is quite a lot of electrical hum coming from the computer if I put my ear to the case :-(

 

i was wondering if anyone could make any recommendations as to how I might go about fixing this?

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If you're using a monitor or TV for both your computer and video entertainment, this may help you. I have an odd hum that only appears when I connect my computer source to my monitor. When I first noticed it, I thought it had to be one of my aging components, so I ran the usual troubleshooting measures for audio. The power amp was silent with open or shorted inputs. But when I plugged the preamp back into the amp, the hum returned and was present regardless of the input selected (with the DAC on or off). So I concluded (prematurely) that it was the preamp and ordered a new DAC-preamp (well it was a really great excuse...)

 

An hour after ordering the new unit, it occurred to me that I hadn't checked the entire system thoroughly enough to be sure of the noise source - and I'd have felt just terrible if I bought a new DAC/preamp that I didn't really need. So I unplugged everything from every input in the chain and started connecting sources one at a time, even running the DAC directly into the power amp and using my Nuforce iDo as the signal source. Long story short, the hum disappeared when I disconnected my computer's digital out (coax or USB) from my DAC, and I discovered that it was only present when my computer (at that time a PC) was connected to the monitor (regardless of video connection - HDMI, component and VGA all brought back the hum). So I put one of my old, small flat monitors on a shelf and used it instead of the monitor we use for TV viewing to display my music GUI.

 

When I built my BeagleBone, I thought I'd be fine again and connected the mini-HDMI out to the good monitor - and the hum came back. I still don't know why plugging the video output of whatever computer's sourcing my music into the monitor on my rack causes a hum, but it definitely does. As far as I can tell, there's no conductive path between the monitor and the rack rails or any component mounted to them. There's absolutely no metal-to-metal contact between the BBB and the rack - it's sitting on a hard rubber isolation pad at the back of a shelf.

 

Bottom line - see if the hum goes away when you disconnect your computer from your monitor.

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