Amplification without transistors or tubes?

Question

Are there any means or technologies that can be used to linearly (or close to linear) amplify an audio or LF RF electrical signal that do not require transistors or vacuum tube technologies? (Or equivalent tech such as ICs, photo-transistors, klystrons, etc.)

Specifically linear (or "close enough" as are the usual slightly non-linear simple tube or transistor amp circuits), so disqualifying a two state relay as a possible solution.

Were any electromechanical gadgets tried before Fleming and De Forest?

Answer 1

An audio transducer (e.g., speaker) can be coupled to a microphone to provide gain in the audio frequency spectrum. This technique was used in early long distance telephony.

Answer 2

Negative-resistance devices such as tunnel diodes can be used as amplifiers/oscillators.

There are also magnetic amplifiers, most often used for electromechanical control.

A relay has gain, but it's highly nonlinear.

Are these the sorts of things you're looking for?

Answer 3

A generator without permanent magnets can be used as a amplifier. The extra power comes from the power provided by mechanically rotating the shaft. The magnitude of the output signal is then proportional to the shaft rotation speed and the field winding current. Since extra power is being added to the system via the shaft, you can get more power out than you put into the field windings. If you keep the shaft rotating at a fixed speed, the generator can be thought of as a amplifier from the field current to the output.

One problem with this is that these generators usually put out AC. That can be rectified, but will still have some ripple. If you wind the generator with enough poles and spin it fast enough, the AC output frequency can be higher than your highest frequency of interest. In that case, it can be AM demodulated to yield the final amplified signal.

Rigging up something like this to work at audio frequencies will be tricky. However, you can demonstrate the concept without too much trouble at lower frequencies. Connect a LEB (light emitting bulb) to the output of the generator and spin it to get typical line frequency out, like 60 Hz. Now you can control the lightbulb, or a bank of lightbulbs, with much less power than it takes to drive them directly.

The alternator in your car actually works on this principle to keep the rectified output of the alternator at the right voltage. The alternator spins at whatever speed it spins at. The control system modulates the field current so that the alternator output produces the right voltage to keep the battery charged but to not overcharge it. In this case the "amplifier" is being used as a voltage regulator, but the overall concept is the same. Less power is required to run the alternator field windings, which comes from the 12 V DC, than the alternator puts back onto the 12 V DC line. The extra power comes from adding a mechanical load on the engine.

Answer 4

A pure mechanical audio amplifier called an auxetophone uses an air compressor and a gate that is mechanically modulated. This video shows a recreation of an auxetophone where a solenoid is used to move the gate, but it can be mechanically moved.

In a similar manner, the gate could be replaced with an adjustable power resistor, so you can build a mechanical amplifier that converts the small signal to mechanical movement, which then modulates a much larger power source.

Answer 5

A transformer can provide gain i.e. a "step-up" transformer can magnify the input voltage by the turns ratio but it isn't a power amplifier. They can be fairly linear.

A tuned circuit (L & C) can also amplify a voltage signal by many times but it is limited to a small band of frequencies close to the resonant point. Beyond that frequency signal output levels fall off.

Answer 6

Parametric amplifiers can be made with nonlinear capacitors.

In the early days of radio, a device called the coherer was used.. It could be described as a detector with gain. Not linear at all.

There were various devices in the early history of instrumentation that provided something like gain without actual power gain. For example, the optical lever, the galvanometer, the thermopile.

Answer 7

Here's a schematic of an amp purely only using capacitors:

From https://www.eevblog.com/forum/beginners/tested-capacitor-amplifier/