Quick note on line level output currents

Consumer audio standard line level output is -10 dB, 0.316 V rms [dB = 20 * log (V/1V)]. Some devices like computer sound cards can boost that at the max volume settings, my Asus Xonar can do 6 dB or 2 V rms. Quite a lot of digital audio produces 2 V rms output, DACs and CD players and not just computer sound cards.

The amount of output current required by the line driver is the signal level divided by the load impedance, so to estimate the worst case scenario we have to consider the smallest practical load and the largest likely signal. The input impedance of consumer audio is typically 10k to 100k. 10k is the lowest design point, but sometimes people do strange things like drive two components at once which halves the value, or headphones, or pro audio gear with 600 ohm inputs.

The long and short of it, though, is that consumer audio inputs are never normally going to draw more than 1 mA. For pro audio the maximum is meanwhile 3 mA. 5 mA bias current through the driver output stage will be enough to keep it in class A under all foreseeable circumstances short of driving headphones.

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To be brutally honest, outside of the little exercise above with currents, I don't know how to put this knowledge to much other practical use. Mostly our interest in signal levels is about gain, and, at floor level, loudness: "How much gain should my phono stage have to play at the same level as my CD player?" "Do I need an active preamp?" "How do I match the amplifier sensitivity to the preamp gain?" The main difficulty as I see it is the relationship between the peak digital signal level and the typical music signal level is not defined, just as the relation between the "line level" -10 dB signal and the typical music signal level doesn't seem to be fixed. So a 2 V CD player could sound 16 dB louder than a "line level" tuner or phono stage, but as far as I can tell I have no basis for assuming it will be.

For phono stages with MC (0.5 mV, -66 dB typical signal) inputs, 55 dB gain is common, so that implies the output is about -10 dB or "consumer line level". MM (5 mV -46 dB typical signal), 35 dB gain, works out to the same -10 dB output. For "digital level" you'd need to boost that about 16 dB, though this is rarely done - leaving me to wonder if the real loudness difference is so large as that. It's a bit hard to get a handle on without test recordings on both LP and digital formats, which I don't have, so instead I'm having to go with the same album on both LP and CD and looking at where the volume control is for what I estimate is the same loudness...