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Possibly one of these low power class-D
Am 24.09.2020 um 20:52 schrieb Andy
something like that. I guess the Duo might use a Class-D
switch-mode audio amplifier chip for speaker and headphones
no real reason NOT to use one in that position, they're very
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.