DUOr interference from headphone outputs #eladduo #fdm-duor #duor

Ad Dieleman

This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.

The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.

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