Alternatives to the vacuum tube and transistor?

As I build my class A simple amplifier, and delve into the current-controllable-variable-resistor concept which is the transistor, I keep wondering if a similar device could have been invented earlier? This is for audio amplification applications.

Has anyone come up with a variable-resistance relay that will do a similar job? Of course reliability will not be the same as a transistor, and performance may degenerate. Maybe a motor controlled by a small current moving a mechanical potentiometer?

The circuit I envisage is a simple amplifier circuit. (Single ended etc)

Alternatively, could a simple oscillator circuit using a capacitor be set up, with the input audio somehow made to vary the pulse width of the signal, this in turn used to create an amplified signal at the output sort of a class D without the transistors?

The first idea is within the realm of possibility, and I intend to try it out sometime. The second one...

There is also the magnetic amplifier but sound quality is said to be not so great, aka distortion.
They made very powerful speakers using an air compressor, a modulated valve exiting into a giant horn.

Maybe you could get the grooves of a record to modulate a micro-machine valve with some air pressure behind it - and get that into a horn (or a pair...) in a way similar to the 78 players of old. Have two conduits running down the tone-arm into separate horns. No electronic at all - just a little background hiss.

I once owned a portable Victrola wind-up. Replaced the mica diaphram with a plastic one having more compliance. Changed the sound! Became just a bit less squeaky.


2019-12-28 9:16 pm
You could use magnetics and use opposing currents to control other currents - similar to controlling arc furnaces.

Do missed the latter point.

The option then is digital but then you have the fun of modulation and filtering.

You could use light but the cost of a lab setup and the reality of usability would be an issue. An open optics bench also has safety requirements whereas a solid state “on a chip” is both immature with only a few universities being able to creat test designs..
For example you could use parametric downconcerstion to create a quantum entangled photo pair the. Sent one downs fiber optic to the read head and the other to the sensor. Then you count the photons and you know what happened in the read head.

That replicates the Chinese quantum camera/radar.. so cutting edge:)
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Has anyone come up with a variable-resistance relay that will do a similar job?
Relays are variable resistance, either infinite or almost zero, so conceptually they can be used to build a Class D amplifier.
Now given switching rate limitations, only sub-Hertz signals can be amplified.

Maybe a motor controlled by a small current moving a mechanical potentiometer?
Also conceptually possible, again only practical for sub-frequencies.

Pneumatic/hydraulic amplifiers have been used for decades.


Paid Member
2003-06-12 7:04 pm
Maine USA
> the ONLY place I've read about them:

Hunt has a couple mentions on a couple of pages.
HUNT, F V, Electroacoustics, Harvard University Press, Cambridge, MA (1954)
The 1954 printing can be found on used-book sites. There is a 1980s reprint by the ASA. I found a bad PDF which is surely bootleg infringement. Harvard arranged for on-demand reprints/PDFs, not cheap, but may be worth it: it's got more action than a Shell Scott novel.
Electroacoustics — Frederick Vinton Hunt | Harvard University Press


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I first read of those carbon mic amplifiers here. Actually this is the ONLY place I've read about them:
Electro-Mechanical amplifiers

This is it! This is the concept I had in mind. From the aforementioned article. I would think that a simple plunger sliding against a resistance coil would do the trick. Might last longer as well.

"Mere lack of resource has never blocked human ingenuity, and there was a handy fact to exploit. Carbon microphones, as used universally in telephones until the mid-1960's, are not mere transducers that turn sound power into electrical power, but actually give a power gain of about a hundred times. The microphone is a variable resistance, made up of fine carbon granules, that controls the flow of current from a DC power supply; it does not merely convert acoustic energy into electricity. This power-amplification technology was the essential basis of the first practical telephone systems."

Great link, thanks.
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