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That's exactly was I was wondering
The average designs around this class-D
chip family go w/o any LPF at the output.
I do not know how this is implemented in the DUO(r).
In case the outputs (stereo) are not
LPF-limited, the PAM-xxxx devices turn
into 'transmitters', as is true for any class-D amp, especially if
cables are longer than a few inches/centimeters!
Thus, a proper designed LPF at the amp
output is a real 'must' with such kind
The 'PAMs' are typically used in low
power one-box FM receivers with internal
speakers and a very short cable connection between PAM-xxxx out
(internal) speaker chassis.
Am 24.09.2020 um 22:01 schrieb Andy
Possibly one of these low power class-D PAM-xxxx chips?
Am 24.09.2020 um 20:52 schrieb Andy G4JNT:
saw something like that. I guess the Duo might use a
Class-D switch-mode audio amplifier chip for speaker
no real reason NOT to use one in that position,
they're very efficient.
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.