How much power can be drawn from the MacBook Pro's 3.5 mm port? [duplicate]

How much power can be drawn from the MacBook Pro's 3.5 mm audio port?

Is it possible for a device to draw power from it while playing aloud any audio the MacBook may be outputting through it?

The exact amount of power depends on the model of MacBook Pro.

You can usually expect to be able to draw something in the small milliwatt range at best - i.e. something like 10-20 mW for a smart phone audio output, and a laptop perhaps 50-100 mW.

If you happen to have a new MacBook Pro, it is possible to draw a lot more power as it is specially equipped for providing power:

When you connect headphones with an impedance of less than 150 ohms, the headphone jack provides up to 1.25 volts RMS. For headphones with an impedance of 150 to 1k ohms, the headphone jack delivers 3 volts RMS

And yes, it is theoretically possible to play aloud the audio while drawing the power - but it is not practical. The audio signal basically is the power, so for practical purposes you would try to maximize power output by effectively playing non-music (i.e. basically loud "noise").

From https://support.apple.com/en-us/HT212856 (thanks to @jksoegaard for the reference in their answer!), the adaptive-impedance audio interface circuit on the new MBP operates in two modes:

1. When you connect headphones with an impedance of less than 150 ohms, the headphone jack provides up to 1.25 volts RMS.

2. For headphones with an impedance of 150 to 1k ohms, the headphone jack delivers 3 volts RMS.

From the basic physics of resistive circuits:

P = V^2 / R

This means that we can infer the following:

1. In low-impedance mode, the best-case voltage is 1.25 Vrms. Presuming a flat power curve (i.e., the voltage drops accordingly to maintain constant power through a reduced resistance), this means that at 150 Ω the power delivered into the load is P = V^2 / R = (1.25 Vrms)^2 / 150 Ω = 10.4 mWrms. If the power curve is not flat then the available power may be higher or lower for lower-impedance loads.
2. In high-impedance mode, the voltage is fixed at 3 Vrms. Maximum power develops through minimum resistance, so at the lower end of the 150-1kΩ impedance range we have P = (3 Vrms)^2 / 150 Ω = 60 mWrms.

In both cases these are optimal parameters derived from a combination of electrical characterizations on Apple's part and a simplified theoretical model on our part. Real-world conditions are not as optimal and will likely see a slight reduction to these maximums.