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What Is a Magnetic Amplifier?

By Benjamin Arie
Updated: May 16, 2024

A magnetic amplifier, or mag amp, is a type of electronic converter. The mag amp uses the interaction of electric currents, called electromagnetism, to enhance electrical signals without requiring any moving parts. This variety of power amplifier was a popular replacement for vacuum tubes, and is noted for its high reliability in harsh conditions. Basic magnetic amplifiers were built as early as the 1800s, but it was not until the 20th century that these devices saw widespread use.

Both the United States and Germany developed dependable mag amps for military purposes during World War Two. During the 1940s and 1950s, magnetic amplifiers replaced many vacuum tube amps, which performed the same general function but were more fragile. While modern transistors are now the primary method of power amplification, magnetic amps are still used in devices that must be resistant to severe conditions.

A magnetic amplifier produces a significant radio frequency (RF) signal when it is activated. This feature can be used for sending rudimentary radio messages. By toggling the amp on and off, the RF signal can be manipulated and received from a distance. For this reason, early radio pioneers experimented with using large magnetic amplifiers to transmit Morse code messages across oceans.

Magnetic amps use two duplicate coils of wire, which are each wrapped around a magnetic core. To operate the amplifier, one of the coils is energized with alternating current (AC) electricity. The circuit that is receiving amplification, called the “load,” is connected to the second coil. An electromagnetic field is generated when the first coil is powered, which also induces a charge in the load.

This setup allows the electrical load to be controlled without needing an actual physical connection or mechanical switch. Magnetic amplifiers typically have a long usable life, since there are no moving parts to wear out. For the same reason, mag amps are fairly immune to vibration or shock, and can be used in rough environments. Equipment such as arc welders and industrial power supplies sometimes use a magnetic amplifier due to these advantages.

A magnetic amplifier does have some disadvantages, however. Size is one problem. Modern transistors can perform the same function as mag amps while taking up significantly less room. Magnetic amps are also not as efficient as modern solid-state transistors, which are able to control circuits with very little power loss. The amplification ability or gain of a mag amp is relatively low compared to newer solid-state amplifiers.

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Discussion Comments
By anon963283 — On Jul 28, 2014

One huge advantage is cost/dependability. For switching large currents, or regulating them, there is still nothing available that can out-perform a simple Mag-Amp. I'm working on a unit right now that controls a motor over 150HP, with a small pot for full control and a small control unit with no cooling required.

It has a small problem, the first in over 50 years of hard use, and is still the desired control method for this type of setup.

By Charred — On Mar 01, 2012

@allenJo - I doubt you’ll be able to send signals halfway around the world. The magnetic amplifier is not a dedicated transmission device – that’s just one of its “side effects” if you will. I don’t have hard and fast numbers on how far the RF signal will travel, but my bet is that it all has to do with much power you feed into the system.

By allenJo — On Feb 29, 2012

@SkyWhisperer - I wonder how far the RF signal of a magnetic amplifier travels? I sometimes watch war movies where I see these guys tapping out their Morse code and the signals travel almost around the world.

I doubt you can get that kind of reception with a magnetic amplifier but it would have to be a decent distance if they used it for Morse code.

By SkyWhisperer — On Feb 29, 2012

@Mammmood - It’s easy, yes, but I wouldn’t recommend that beginners play with building their own magnetic amplifiers. You use AC electricity to get these things powered up and AC delivers quite a jolt which could kill you.

I’d recommend that only experienced technicians try to build their own amps. You can scrounge around from your existing audio and electronic scrap heap pile and find things that you can use to piece together a simple amplifier.

But again, I’d only recommend this if you really know what you’re doing.

By Mammmood — On Feb 28, 2012

While I do agree that the transistor has pretty much rendered the magnetic amplifier almost obsolete, there are still some reasons for its appeal.

The main reason is its simplicity of design. It uses a basic magnet, which can easily be created with coil wire and metal. That’s why they built these things back in the 1800s, but I think even today you can find some aficionados on audio forums who like to build their own magnetic amplifiers just for fun.

Also, because it does use magnetism, a magnetic amplifier can be retrofitted easily as another application which uses magnetism in my opinion.

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