That's easy to answer in general -- if a computer runs on line power, then it has at least one AC -> DC conversion step. If it has a small, light, high-efficiency power supply that doesn't require large 50/60Hz transformers, then it converts twice:
120V AC -> 170V DC -> 20 KHz switching inverter -> 20 V DC.
So with respect to frequency it's 60 Hz -> DC -> 20 KHz -> DC. Yes, complicated, but much more efficient than the old iron-core transformer days, even though the latter involved fewer steps.
The step from 120 VAC to 170 VAC is an unavoidable increase because 120 VAC is RMS voltage, not peak voltage, and the DC conversion yields the peak value:
Peak V = RMS V * sqrt(2)
It wasn't very long ago that 170V transistors were difficult to acquire and notoriously sensitive to voltage spikes, but this problem is essentially solved.
A computer that only ever runs on batteries may have one, or no, conversions, depending on whether the battery voltage is exactly what the computer needs.
I speak as someone who has been designing electronics so long that my first designs used vacuum tubes, and whose 20 KHz inverters flew on the Space Shuttle.
120V AC -> 170V DC -> 20 KHz switching inverter -> 20 V DC.
So with respect to frequency it's 60 Hz -> DC -> 20 KHz -> DC. Yes, complicated, but much more efficient than the old iron-core transformer days, even though the latter involved fewer steps.
The step from 120 VAC to 170 VAC is an unavoidable increase because 120 VAC is RMS voltage, not peak voltage, and the DC conversion yields the peak value:
Peak V = RMS V * sqrt(2)
It wasn't very long ago that 170V transistors were difficult to acquire and notoriously sensitive to voltage spikes, but this problem is essentially solved.
A computer that only ever runs on batteries may have one, or no, conversions, depending on whether the battery voltage is exactly what the computer needs.
I speak as someone who has been designing electronics so long that my first designs used vacuum tubes, and whose 20 KHz inverters flew on the Space Shuttle.