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Re: OT: feedback resistors in PAs

Jacques: ""
Yes, but appreciable anyway: Lorentz induction through an area of several cm2 varying about one thousandth by mechanical vibration at 1,000Hz in a field of 0.5gauss would give something in the tens of microvolts, so less than -100dB.
OK, just appreciable ;-) ""

500 Wrms into 4 ohms, sqr(500/4) = sqr(125) = 11.18 ampsRMS.

Peak I 11.18 * 1.414 = 15.8 amperes peak.

B(tesla) = (μ0 I)/(2 pi r)

μ0 = 4 pi 10-7
1 Tesla = 10000 gauss = 104 gauss

B = 2 10-7 104 I / r

B = 2 10-3 I / r Gauss
at 500 wrms into 4 ohms,

B = 2 10-3 15.8 / r Gauss

at the wire surface (r =1mm, or .001 meters 10-3 meters ):

B = 31.6 10-3 / 10-3

B = 31.6 Gauss.

at 1cm, B = 3.16 gauss.

at 10 cm, B =.316 gauss.

Mind you, this field it time varying, and proportional to the output current. AND, if the pos and neg rails or output devices are not equidistant, they will couple differently..meaning, feedback error coupling will be dependent on the quadrant of operation of the amplifier/load.

And, also? The haversine charging of the supply caps. The output current I used? The charging currents through the bridge are a lower duty cycle..at 10% duty cycle, your talking 3 Gauss fields at 10 cm...and they will also depend on which cap is being pulled.

Cheers, John

PS..hope I did the math right, this html is a pita..:-)

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