Well, crystal radios are electrically passive but usually the power was low and thus you couldn't produce enough "volume". So people interestingly built pneumat

Well, crystal radios are electrically passive but usually the power was low and thus you couldn't produce enough "volume". So people interestingly built pneumatic amplifiers (the principles are equivalent to electric amplifiers, I have studied some pneumatic devices and sometimes they are modeled with electrical circuits for convenience) using gas lamps, which were more abundant than batteries or mains at the time. This source of the article claims they will produce milliwatt power and the power of crystal radios circuits is measured in nanowatts. This is 1000 or 1 000 000 times more. They apparently do this by using multiple frequency ranges, while a cristal radio includes a adjustable electronic filter to frequencies, cutting the ones off above a particular, well cutoff frequency, you also don't want other signals of the same range in radios because you will create noise in your headphones. This is the basic principle, I'm sure there are some technical errors somewhere, but I'm still a student :-).