Electricity is in the air: the 50 Hz epidemic (“The Big Hum”)
Who hasn’t heard the slight humming sound caused by unplugged jack plugs or unearthed and switched-off devices? You don’t even have to touch the contact surfaces with your fingers, it is often enough that they are lying around open. We are constantly worried about microwaves, cell phone radiation and radio mice, but the 50 (or 60) Hz world hum (“the big hum”) is often ignored. Yet these rather long waves are really omnipresent in every household! Every piece of cable acts as an antenna and in this case it is bad luck that the frequency falls into the audible range. The first graph shows a 100 ms section from a measurement on a PC that is not switched on and is also physically disconnected from the socket, which also eliminates the earth via the (common) protective conductor on the PC.
And please don’t be surprised that it’s not a nice sinusoidal wave, because this ideal case is rather the exception in the wild with the wave salad from all possible sources. There’s a really nasty hibble here and I can still measure almost 5 mV “antenna voltage” at 1 kOhm input impedance. If I were to switch off all running, mains-powered devices in the room (and unfortunately there are far too many of them), this measured value would drop significantly. In my audio lab, where only a 48V DC mains supply is available in the larger area, I then measure almost nothing. A feat.
All well and good, but you can hear this quite clearly on an analog amplifier, even though the ground wire of the audio cable (also used here as a shield) actually has contact with the PC housing and the ground plane of the mainboard. But such operation with the plug pulled out is not the norm. So I simply plug the PC’s mains plug back into the socket. The difference is measurable and audible, because it is only 0.0003 instead of 0.005 volts. You have to turn the volume control on the amplifier way up to hear anything disturbing. But even here there is still something you don’t really like. Albeit very quietly.
Ok, now let’s run the PC and wait until the Windows desktop becomes visible. Now I measure with a good 2 mV, i.e. 0.0021 volts at the 1 kOhm connection, the usual “PC noise” that results as a sound carpet as a sum from all possible sources. This is also one of the problems with the onboard sound, but there’s another chapter for that. For now, this is about the hum, which has now disappeared. But as we can see, something always gets in the way.
99 Antworten
Kommentar
Lade neue Kommentare
Urgestein
Veteran
Veteran
Urgestein
Mitglied
Veteran
Urgestein
Veteran
Urgestein
Veteran
Veteran
Urgestein
Veteran
Veteran
Urgestein
Urgestein
Urgestein
Alle Kommentare lesen unter igor´sLAB Community →