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

Klaus Brosche, DK3QN

As you wrote, you are loading the output with 4.7 ohms in series with cap. Which results, as your
indications say,  in an LPF at minus 3dB @ 50KHz.

I have currently no way of verifying that. I don't have my Ham Radio Laptop at hand right now
with the simulation software. It is just my guess.
So, maybe I'm wrong. In such case: forgive me ;-)

Klaus, DK3QN

Am 24.09.2020 um 22:36 schrieb Ad Dieleman:

I don't think that the interference comes from spurious switching signals from a class D amplifier: the 2.4 MHz signals disappears completely when loading the output with a low enough resistance, i.e. no high-frequency filtering at all. The filter I applied also doesn't really filter out HF signals, it just loads the output with 4.7 ohm for frequencies over roughly 50 kHz, this being the 3 dB cutoff point. So I still think that the output stage is oscillating under certain circumstances, be it a class D amplifier or something else.

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