For older receivers with a fixed (crystal?) audio filter for CW, what is the usual center frequency of the audio bandpass of the filter? Or cut-off frequency for low-pass CW filters?
For newer receivers (and external audio DSP boxes) with configurable narrow audio filters for CW, what is the usual default or factory configured center frequency for the narrowest audio filter?
Do experienced operators more commonly increase or decrease the narrow CW filter factory frequency setting?
But I’m especially interested in old radios with fixed frequency (crystal or ???) audio CW filters.