All components from the vacuum of space to the great pyramid have properties that can be affected due to changes in the space around them so nothing is totally passive. A mag amp/carbon mic repeater are amp 'circuits'. We are changing the magnetic field transfer coupling ratio in the case of the mag amp with a decoupled signal controlling the hysteresis ('materials effect') or changing microphone resistance (compressing carbon granules 'materials effect') in response to a decoupled moving plunger powered by a voice coil ('electromechanical effect') in the repeater. The carbon mike just by it's self has an effect that can be simulated by a large switch quickly changing the electrical values of a single electrical component across the input port -> output port in a circuit. Thermodynamically there is no electrical device gain as the input/output are still directly coupled electromagnetically and is still passive as we are just controlling how much of the inputs total possible energy is split (with a acoustic to electrical resistance transducer) between components so no matter how much energy is input the output will be equal or less. With an active circuit the input can (in theory) control a infinite amount of output power because the input control port can be (largely) electromagnetically decoupled from the output.
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