
The Magnetic Amplifier – A Lost Technology of the 1950s - segfaultbuserr
https://www.nutsvolts.com/magazine/article/the_magnetic_amplifier
======
jhallenworld
There are a lot of weird amplifiers:

Old welding transformers are controlled by a magnetic shunt- field lines are
directed through a movable shunt instead of the secondary.

[https://www.youtube.com/watch?v=k5684mQJQRU](https://www.youtube.com/watch?v=k5684mQJQRU)

There are magnetic core logic circuits, see:

[https://www.youtube.com/watch?v=p7SkE5pERtA](https://www.youtube.com/watch?v=p7SkE5pERtA)

You can make very loud record players using compressed air:

[http://www.douglas-
self.com/MUSEUM/COMMS/auxetophone/auxetop...](http://www.douglas-
self.com/MUSEUM/COMMS/auxetophone/auxetoph.htm)

You can make audio amplifiers using friction:

[http://www.douglas-
self.com/MUSEUM/COMMS/trumechamp/trumecha...](http://www.douglas-
self.com/MUSEUM/COMMS/trumechamp/trumechamp.htm)

Friction can be electrically controlled:

[https://www.gettyimages.com/detail/news-photo/this-wall-
tele...](https://www.gettyimages.com/detail/news-photo/this-wall-telephone-
uses-the-chalk-receiver-invented-by-news-photo/90773875)

You know that magnetic tape hiss? That makes a great radio detector:

[https://www.youtube.com/watch?v=4CAxWoKD-
fw](https://www.youtube.com/watch?v=4CAxWoKD-fw)

~~~
moron4hire
You can use a flame as a triode to build an amp
[https://www.youtube.com/watch?v=-aAwyUoawkc](https://www.youtube.com/watch?v=-aAwyUoawkc)

Tangentially, you can use a flame to emit sound directly
[http://onetuberadio.com/2018/07/02/flame-speaker-science-
fai...](http://onetuberadio.com/2018/07/02/flame-speaker-science-fair-
project/)

------
Animats
Power control for serious power was really hard until about 30 years ago, when
power MOSFETs and IGBTs got good. There were a long series of not very good
devices from 1900 on. Ward-Leonard drives, dynamotors, magnetic amplifiers,
motors with multiple switchable windings, thyatrons, ignitrons, silicon-
controlled rectifiers... People needed smooth control of high power, but it
wasn't easy to get.

Now you can get IGBT modules able to handle the power for a locomotive motor.
They're not that big - you can hold one in your hand. The next thing seems to
be going back to MOSFETs, but in silicon carbide. Tesla switched from IGBTs to
SIC MOSFETs. At last, high power control is a solved problem.

~~~
blueprint
yeah but what are the specific benefits of magnetic amplifiers that you cannot
get from SICMOSFETs

~~~
vibrolax
I guess there are fewer ways to destroy stacks of laminations and copper
windings. But that doesn't make up for the extra weight, space, cost, and
labor to manufacture them

------
mikewarot
I had the job of repairing some of these. They were made by Westinghouse in
1964 as part of a control system running an 6 stand rolling mill. The nice
thing about them is you can have several inputs, all electrically isolated
from each other (separate sense windings) and they worked all the way down to
DC, which surprised the heck out of me.

The driver was a 10khz square wave, so the frequency response was DC to
1khz... fast enough for control loops.

~~~
lpmay
Down to DC? That should be a surprise, because it's certainly not true!
Faraday's law of induction only applies to varying fields, which means AC or
physical movement. Remember, it's perfectly normal and common to have a square
wave with a DC component of zero!

~~~
cpcallen
I think he means the response frequency, not the frequency of the AC drive
(which he states was 10 kHz).

~~~
tonyarkles
Yeah, they’re almost definitely referring to the control signal. Since it’s
controlling the saturation of the core, it’ll happily go down to DC just fine.

------
dharma1
Looks like Lars Lundahl (famous for high quality audio transformers) wanted to
build an audio magnetic amplifier in the 90s, but the project got canned. Some
German company ended up making it a decade or so later

[http://www.auditorium-23.de/MagAmp/MagAmp.html](http://www.auditorium-23.de/MagAmp/MagAmp.html)

[http://www.acousticplan.de/html/magamp_english.html](http://www.acousticplan.de/html/magamp_english.html)

The technology has also been used in a completely passive multi-band
eq/saturator - [https://www.soundonsound.com/reviews/gyraf-audio-gyratec-
xxi...](https://www.soundonsound.com/reviews/gyraf-audio-gyratec-xxi-magneto-
dynamic-infundibulum)

------
dwheeler
Amazing, I had never heard of these things.

There is a 1960 Popular Electronics article about them:
[https://www.rfcafe.com/references/popular-
electronics/magnet...](https://www.rfcafe.com/references/popular-
electronics/magnetic-amplifiers-jul-1960-popular-electronics.htm)

~~~
fuzzfactor
It's really the 1850's when you think about it.

Transformers were all there was before the Edison Effect was discovered and
vacuum tubes were developed.

A lot can be accomplished with just copper and iron and its always been
expensive and heavy, sometimes like a baseline for cost.

By comparison alternative systems can be much more complex with way more
points of failure if they can just beat that cost to a certain extent.

There's more proven technology on the trailing edge than the leading edge can
be expected to provide.

In one person's lifetime there always exists more forgotten technologies of
the ages than truly novel technologies that can be developed over a single
career.

It could be worthwhile to try and balance the creation of new technologies
with the preservation of technologies at risk of loss.

~~~
blattimwind
Funnily enough, the "Edison" effect was originally discovered in the 1850s. It
got rediscovered approximately every ten years after that.

------
aj7
Saturable reactors are critical in semiconductor manufacturing. They are the
key components in the magnetic pulse compression circuits that drive the
excimer lasers used as light sources in photolithography and in the annealing
of thin film LED displays.

~~~
segfaultbuserr
Thanks for the comment, just learned something new today. I thought magnetic
pulse compressors are only for physics experiments and EMP weapons, never knew
they are used in photolithography.

~~~
aj7
Mass production. Thousands of units. When I learned this engineering, I
briefly looked around for other contemporary applications. I couldn’t find a
single one! Other than research and defense, pulse power is only applicable to
driving pulsed lasers with very short upper state lifetimes. And magnetic
pause compression, not being cheap, is only applicable in high repetition rate
systems. Hence excimer lasers in the semiconductor industry.

------
sleeved5
Mag amps are still used for post-regulation in switching power supplies, such
as the Mean Well QP-200 and some 80%+ efficient ATX power supplies.

~~~
segfaultbuserr
+1.

See also: _Magnetic Amplifier Control for Simple, Low-Cost, Secondary
Regulation_ by Bob Mammano, Unitrode Corporation (now Texas Instruments).

[https://www.ti.com/lit/ml/slup129/slup129.pdf](https://www.ti.com/lit/ml/slup129/slup129.pdf)

------
Aloha
Another lost amplification technology is the mechanical amplifier - basically
a transducer which is mechanically coupled to a carbon button mic diaphragm -
obviously only useful in the AF range, but it was how early telephone toll
circuits were amplified and extended beyond what heavy gauge copper and
loading coils could do on their own.

------
bilegeek
I've heard of these before; they are very cool. The only problem, as far as
I've seen, is a smaller frequency range and lower gain compared to tubes or
transistors. Not crazy smaller or lower, but don't expect a single magnetic
ham radio amplifier to get you from 160m to 10m at 1.5kW legal limit.

------
agumonkey
Oh I recently came across a similar topology in welding machines. The Amp
output was a large screw shifting a rod (shunt?) in the transformer. I had no
idea it was a larger principle at play.

------
jcims
So if you wound a 1:1 transformer on a coil that has additional windings to
’preload’ it, could you use this as a speed controller/voltage regulator?

~~~
segfaultbuserr
Yes, it's called a saturable reactor [0] and can be considered as a very
simple type of magnetic amplifier. You control the impedance of the reactor
using the extra winding, conceptually similar to how you control the
conductivity of the pass transistor in a solid-state voltage regulator. You
can find them in old Tektronix oscilloscopes, here's a long (70 min.) and
comprehensive video [1] on the design of the Tektronix 555 power supply,
including the operation of the saturable reactor voltage regulator.

[0]
[https://en.wikipedia.org/wiki/Saturable_reactor](https://en.wikipedia.org/wiki/Saturable_reactor)

[1]
[https://www.youtube.com/watch?v=CAbeWXcuYOE](https://www.youtube.com/watch?v=CAbeWXcuYOE)

~~~
jcims
Awesome thank you!!!

------
divbzero
> _EMP-proof_

This detail caught my eye. Very cool.

~~~
jhoechtl
But why? An EMP would still induce energy into the coils, largely disrupting
operation, no?

~~~
jacquesm
But afterwards they'd be fine unless their isolation was compromised somehow
through for instance arcing. And even in that case, there are ways to stop an
arc and leave the system functional (self repair, essentially the coil
material evaporates a bit further from the breach point than the isolation
material).

In silicon if such an energy pulse would reach the circuitry it would be
ruined forever.

------
peter_d_sherman
> _" The mag amp can modulate, switch, invert, convert, multivibrate, audio-
> amplify, radio-amplify, frequency-shift, phase-shift, and multiply. Stages
> can be cascaded. Simple feedback techniques enable gains in the millions.

The mag amp can even compute. Trouble-proof magnetic binaries replaced the
less reliable vacuum tubes used in some early digital computers."_

This is my new favorite HN article in the field of electrical engineering...

 _Basically, what a transistor is to DC -- A Magnetic Amplifier is to AC_...
plus there is additional functionality...

This device is highly worthy of future study...

------
b1c837696ba28b
I love a good used book store. Two years ago I found this:

Storm, Herbert F, Magnetic Amplifiers, Wiley, 1955. LOC# 55-6432

Covers applications and theory, contains All The Math.

------
yummypaint
Here are some more schematics for audio amplification and receiving radio:
[http://www.aes.org/aeshc/pdf/how.the.aes.began/vincent_magne...](http://www.aes.org/aeshc/pdf/how.the.aes.began/vincent_magnetic-
audio-fundamentals.pdf)

Im very curious what the tone of such amps is like. Anyone heard one in
person?

------
pdkl95
Interesting, this seems like a practical implementation of the a-field
(magnetic vector potential) "circuits" described here[1].

I wonder how hard it is to make an _electro-electret_...

[1] [http://amasci.com/elect/mcoils.html](http://amasci.com/elect/mcoils.html)

------
amigiac
Looks hard to manufacture on scale for cheaper than a transistor amp. It was
probably sidelined due to complexity of assembly.

------
bawana
Can this be used to control the massive magnets needed for nuclear fusion in
tokomaks?

------
SomeoneFromCA
I've heard that some modern PC PSUs use these devices inside.

------
TedDoesntTalk
why did they fall out of favor.m?

Are they slow? Expensive? Something else?

~~~
hcarvalhoalves
I guess mostly price - cooper is more expensive, and the economy of scale in
semi-conductor manufacturing should be much higher.

------
senectus1
huh, I always wondered what those little loops of what looked like a "life
saver" lolly was when I was pulling shit apart as a kid.

~~~
akovaski
Those would most likely be inductors or transformers. It is mentioned in the
article how the simplistic Figures 2 & 3 magnetic amps would act as a
transformer.
[https://en.wikipedia.org/wiki/Toroidal_inductors_and_transfo...](https://en.wikipedia.org/wiki/Toroidal_inductors_and_transformers)

------
mercurywells
How much EMI does it cause?

~~~
sleeved5
How long is a string? No more than any other magnetic component, such as the
switching transformer in your cellphone charger...

------
redis_mlc
One of the most interesting articles I've ever read.

I did a fair amount of reading about vaccum tubes back in the day, and my
family owned a tube-powered commercial Seeberg 45 RPM jukebox, but I never
made the mental leap to all-magnetic "circuits."

This jukebox was in our living room (size of a refrigerator):

[https://www.prosoundweb.com/put-another-nickel-
in/](https://www.prosoundweb.com/put-another-nickel-in/)

One disadvantage of magnetic amplifiers that I can think of is that any heavy-
current application would require specialized training to service. For
example, I work with 1-phase electricity as needed, but I won't even open a
3-phase panel. Can you imagine poking a screwdriver around in a magnetic
circuit? :)

Fun fact: after the first vacuum tube amplifier was patented, patent trolls
would solder random collections of vacuum tube logic (VTL) and patent those.
But when cross-examined in court while defending their patents, they could not
explain how they worked. Kind of like the first spaghetti code, but in
hardware.

It would be very cool if somebody made a graphical simulator for magnetic
circuits and amplifiers, like Kerbal Space Program (KSP.)

~~~
lixtra
> Can you imagine poking a screwdriver around in a magnetic circuit? :)

I wouldn’t bet my life on it but my expectations is that you don’t feel much
of the high frequency magnetic fields in your screwdriver.

------
peter_retief
This could be really useful.

