OK, we're getting closer now. A cathode follower has good power supply rejection, so the B+ quality isn't hyper-critical. How good is good enough? From an engineering perspective (as opposed to unsubstatiated anecdote), it's good enough when the noise/hum/ripple is low enough not to degrade the output signal at a level higher than the desired signal to noise ratio. This should take into account the PS rejection of the CF, which makes things easy.
For example, let's say you're designing a CF to handle signals in the 1V range, and you want the S/N to be better than -80 dB with respect to the 1V reference level. That means that any noise components due to the supply should be 80 dB below 1V. If the CF has a PS rejection ratio of 40 dB, that means that the supply's hum and noise should be 80-40 = 40 dB below 1V, or 10 millivolts for this example.
For example, let's say you're designing a CF to handle signals in the 1V range, and you want the S/N to be better than -80 dB with respect to the 1V reference level. That means that any noise components due to the supply should be 80 dB below 1V. If the CF has a PS rejection ratio of 40 dB, that means that the supply's hum and noise should be 80-40 = 40 dB below 1V, or 10 millivolts for this example.
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