On Thu, 4 Jun 2009 19:43:30 -0400, Joe Subich, W4TV wrote: >Unfortunately, the AL-82 and SB-200 both create a pseudo-neutral >by placing high voltage bypass capacitors from each line to the >chassis/ground. Unless the chassis is properly grounded to the >station common point ground along with every other piece of >equipment in the shack, the noise (hum) s still going to appear >in common mode on the shields/cases.
Actually, nearly ALL equipment powered from the AC line has capacitance between phase and ground, thanks both to stray capacitance in the power transformer and bypass capacitors that are part of line filters. This capacitance couples leakage current to the green wire (the equipment ground). The return path for this current is to the power system ground via the breaker panel, so there will be IR drop in the green wire. Because the coupling is capacitive, most of this current consists of the harmonics of 60 Hz, and is what we commonly call "ground buzz." As Joe and Paul have noted, some older power amps improperly connect 120V fans between one side of 120V and the green wire. In addition to the IR drop issues, there this current generates a 60 Hz hum field that can magnetically couple into the rig and/or interconnect wiring. This field tends to be pure 60 Hz, so is heard as HUM, not buzz. For a more detailed analysis of both mechanisms, see http://audiosystemsgroup.com/RFI-Ham.pdf and http://HamInterfacing.pdf Another source of HUM is the magnetic field produced by the transformers in big power supplies. The unshielded audio transformers used in the K3 make it susceptible to strong magnetic fields. The above tutorials show how to reliably interconnect ham gear, computers, and other audio gear without transformers. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
