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Re: OT: feedback resistors in PAs

jacques: ""
To avoid burning the PCB, the manufacturer often lifts the resistor a few centimeters above the PCB, on standoff if he's serious, or with the resistors' wires if he's clueless about mechanical vibration tolerance.""
Tis an unfortunate move. Most axial resistors are rated to dissipate power through the leads onto the pc traces. A typical rating for an axial diode, for example, has 3/8 inch long leads, with a specific pad size. It would be better to just dedicate a larger pad for the resistor connections.

Jacques: ""
But anyway the open area under the resistor's body creates a loop, in which any induced voltage will be ...in serial with the feedback...Jeeeezzz. Often, this loop is vertical, so that it can get flux from the Earth's magnetic field if the PA is "correctly" set. So, any vibration will translate into a signal. ""

Loop, whatsa loop? :-)

I would also be concerned with the magnetic fields that are being generated internally within the chassis, by the xfmr, the power leads, and all the high current leads around the zistors, the output binding posts, everything. I do not see good poweramp wiring design practices much, and the magnetic hash within the chassis will indeed be coupling to the FB loop, especially where it will do the most damage. The half gauss of the earth's magnetic field is small taters.

Cheers, John



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