User:Olli Niemitalo/Electro-mechanical amplifier

A mechanical amplifier is a device for increasing the power of a mechanical signal. The term may also refer to any amplifier that is partially mechanical.

Mechanical amplifiers were used in the pre-electronic era in specialized applications.

Purely mechanical amplifiers
Mechanical power is the product of a force on an object and the object's velocity, or the product of a torque on a shaft and the shaft's angular velocity. In purely mechanical amplifiers, the input and the output as well as the internal operation of the amplifier are mechanical.

Pneumatic amplifier
A pneumatic amplifier may be realised by using some kind of sensitive valve, which requires little force to operate, to modulate the flow of a stream of compressed air. They were used in compressed air gramophones before the days of electronic amplification, as in the Auxetophone, the Stentorphone, and similar devices. The basic principle of the valves used in these devices was to pass the stream of compressed air through two partially overlapping combs. The sound vibrations to be amplified were applied to one of the combs, causing it to move laterally in relation to the other comb, varying the degree of overlap and so altering the flow of compressed air in sympathy with the sound vibrations. The Auxetophone was capable of producing sufficient volume to broadcast public music performances from the top of the Blackpool Tower, and was said to be loud enough to cause people to vacate the front rows of seats in an auditorium.

Torque amplifier
Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.

Fluidic amplifier
Fluidics refers to construction and use of analog and digital elements that operate on fluid flow signals. A fluidic amplifier has no moving parts, except for the fluid itself. It operates by directing fluid flow by a control stream that can be weaker than the controlled stream.

Electro-mechanical amplifiers
Before electronic amplification using vacuum tubes was available, electro-mechanical amplifiers were devised for amplification of electric signals.

Johnsen–Rahbek effect amplifier
The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen–Rahbek effect was not understood until the semiconductor era.

Electrostatic drum amplifier
The electrostatic drum amplifier used a band wrapped partway around a rotating drum, and fixed at its anchored end to a spring. The other end connected to a speaker cone. The input signal was transformed up to high voltage, and added to a high voltage dc supply line. This voltage was connected between drum and belt. Thus the input signal varied the electric field between belt and drum, and thus the friction between them, and thus the amount of lateral movement of the belt and thus speaker cone. Other variations on the theme also existed.

Carbon microphone and carbon amplifier
One of the first devices used to amplify signals was the carbon microphone (effectively a sound-controlled variable resistor). By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications; analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were also used as amplifiers in telephone repeaters for long distance service.