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Switching amp hash filter


Frank Koenig:
The power amps in my life these days, both large (Powersoft K) and small (Parts Express hobby boards), are switch-mode. They all exhibit a few hundred mV to several V of switching hash on their outputs. This is of no consequence in normal operation but makes it pretty much impossible to see any meaningful low-level output on a scope.

With nothing better to do yesterday I cobbled together a simple second-order Butterworth low-pass and put it in a little box with banana/binding-post input and BNC output. It uses an ancient, 88 mH, air-core, basket-weave inductor, a 440 pF capacitor and a 10 kOhm load resistor which yields a cutoff frequency of ~27kHz.

At 20 kHz it's .6 dB down and introduces -72 deg of phase shift (lag?  ;) ) so cannot be used for critical frequency response measurements but it cleans up the hash and I hope will be useful for looking at low-frequency noise, DC offset, low-level distortion and other problems. We'll see.



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