Unamplified desktop speakers?

I want a pair of stereo speakers that I can plug in to a desktop (or iphone, android, etc…) headphone jack, without batteries or a power supply- and not have to wear (i.e. Not headphones). I know this can be done- I found a 16 ohm speaker cabinet that could put out decent volume with the (30mw?) output of one channel of an iPad headphone jack, with clear sine bass response due to the cabinet's amplification. I am looking for a pair of speakers designed for this purpose with a lower impedance (4-8) designed for this purpose, but I don't know where to look, since most sources I see say "not enough power" or "the headphone jack only puts out line level", etc.

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