Thaddeus Cahill's Teleharmonium | Hacker News Reader

Thaddeus Cahill's Teleharmonium

synthmuseum.com

35 pointsthrowup2389 comments

You can't throw down a claim like "Before the 1920's there was no way to amplify electrical signals" here without some pushback.

The triode theoretically dates from 1906, but took a while to be usable. There were various other techniques, using magnetics, that I think were more popular. Can someone give a better account of how, in 1919, people would have amplified an electrical audio signal?

Wikipedia: "The first practical prominent device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912."

It takes quite a bit of time to develop an electronic instrument, and the realization it could be done must have come some time after amplifiers became more widely used. So 1920 isn't unrealistic, IMO.

Magnetic amplifiers existed before triodes, dating back to at least 1885:

https://en.wikipedia.org/wiki/Magnetic_amplifier

However, these have the major drawback that they can only control the amplitude of an AC signal, and that with a lot of distortion. They could only be used to amplify audio with reasonable quality by amplitude modulation of a sufficiently high-frequency carrier, which itself was difficult to generate before vacuum tube amplifiers. As far as I know, the only two techniques that could do it were excitation of an LC resonant circuit using a carbon arc lamp, or using a very fast alternator that worked a lot like the Teleharmonium itself. See:

https://en.wikipedia.org/wiki/Arc_converter

https://en.wikipedia.org/wiki/Alexanderson_alternator

The AM signal could then be demodulated using various techniques that existed before the triode, e.g. a vacuum tube diode (invented before the triode in 1904), or if you don't want to use vacuum tubes at all, a crystal detector (principle discovered in 1874) or electrolytic detector (invented 1903), although I'm not sure those have the power handling capability to be useful for what the Teleharmonium was doing.

Magnetic audio amplifiers can be built. Here's a small hobbyist one.[1] You need a 35Khz power supply, which is no big deal today, but a problem in 1885. Alexanderson alternators could do it. Those things were huge because they wanted tens or hundreds of kilowatts out. The last one still working was rated for 200KW.[2] If you only need 50W or so, it could be much smaller.

This would be a cool hobbyist project today. Run a motor at 30,000 RPM (Dremel tools can do this), and have an 80-pole generator, which is just a slotted disk surrounded by windings, to get 40KHz. That's an Alexanderson alternator in mini size.

Once there's power, the magnetic amplifier stage can work.[1] Low-power audio comes in, and higher power audio comes out. Need a low-pass filter at the end. Getting the distortion down is probably going to be the hard part.

The keyboard end of things would use tone wheels, like a Hammond organ. Not as big as the ones in the Teleharmonium, though, because you're not generating high power audio that way. A magnetic preamplifier could be used to get those signals up to a useful level.

The Teleharmonium was built when insulated wire was hard to get. Inventors had to build their own parts. There were no instruments such as oscilloscopes to look at high-frequency waveforms. Early electronics involved a lot of wondering what was going on in there. Today, you can look at the waveforms and see what's going wrong.

So, not too bad to build today. Not useful, but might be fun. I miss the TechShop days.

[1] http://sparkbangbuzz.com/mag-audio-amp/mag-audio-amp.htm

[2] https://alexander.n.se/en/the-radio-station-saq-grimeton/the...

Another approach was basically a speaker connected to a carbon microphone, which compressed loosely packed carbon granules to change its resistance. They were used as repeaters in long distance telephone lines as early as 1904:

https://en.wikipedia.org/wiki/Carbon_microphone#Use_as_ampli...

I was so fascinated by the Telharmonium when I first discovered it years ago, I bought an obscure book - Magic Music from the Telharmonium by Reynold Weidenaar - which is based on the authors PhD. Cahill reads like a 19th century tech entrepreneur working on music synthesis and distribution before the age of electricity and electronics. Truly a man before his time. Cool to see it on HN.

A bit more modern take on similar idea https://gamechangeraudio.com/motor-synth-mkii/

And a digital version was made into a Eurorack modular synth module as well. http://www.makenoisemusic.com/modules/telharmonic

This is cool. We once programmed our drone to run the propellers and play notes based on a midi input. We used the accelerometer/gyro for frequency feedback.

https://youtu.be/oXBWA_A8kFU?si=iwomhS7nDFCIhhbb

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