Minimize noise on electromagnet using audio frequencies
You might be missing a trick here - try series resonating the inductor with a capacitor of precisely 3.18 uF. With the 500uH coil (and series resistance of 0.8 ohms), you'll get a voltage magnification across the coil of about 15:1.

In other words, you apply a 1V RMS sine wave at 4kHz and you get 15V RMS sinewave across the coil. There's another benefit too - it's a fairly tight filter and rejects distortion/noise from the amplifier that's driving it. Your amplifier needs to be able to drive a small load resistance because now all the load the amp will see is the losses in the inductor i.e. the 0.8 ohms but, if the amp's minimum load is 2 ohms then insert an extra 1.2 ohms in series - the magnification will only be about 6.3 now but still pretty reasonable.







simulate this circuit &ndash; Schematic created using CircuitLab

Resonant frequency is \$\dfrac{1}{2\pi\sqrt{LC}} = \dfrac{1}{2\pi\sqrt{500\mu\cdot 3.18\mu}} = 3991.4 Hz.
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