File:Navy carbon radio amplifier 1915.jpg

English: An early carbon amplifier for radio signals used by the US Navy around 1915. Made before vacuum tubes, it used carbon microphones for amplification. A carbon microphone consists of a diaphragm attached to a "cell" containing carbon granules between two electrodes. An incoming sound wave makes the diaphragm vibrate, putting varying pressure on the carbon, varying its resistance. When a current is passed through the microphone from a battery, the varying resistance causes a varying current to flow, creating an audio signal. Since the carbon microphone didn't generate its own output current but modulated an external current, it could produce more output audio signal energy than it received from the sound wave, thus achieving amplification. A carbon amplifier consists of an acoustic driver, a small speaker, coupled to a microphone. The weak incoming signal was applied to an electromagnet in the driver which vibrated an iron armature, and the vibrating armature was attached to a carbon microphone, creating the output signal.

The above Navy "Ampliphone" device was used to amplify the weak audio signal from a crystal radio. It contains several carbon microphone amplifiers in series, which gives a large gain. The carbon amplifier was a very primitive, unsatisfactory device. It's frequency response had a sharp peak due to the resonance of the driver armature, and the carbon produced electrical noise, which caused a "roaring" sound in the background. It was only used in a few applications such as telephone repeaters, and was replaced by the triode vacuum tube amplifier around 1916.