I normally tell people interested in analog computing to just buy some modular synthesizer gear (or download one of the many free software implementations) but this is a surprisingly affordable alternative. The documentation is especially good, since modular synths and their documentation are aimed at musicians rather than engineers, and so tend to begin with oscillators rather than scalars.

The analog thing uses 3.3v signals which is a common standard, but 5v is nowadays much more popular on modular synths due to the predominance of the Eurorack format. If you know for sure that you enjoy making bleep bloop noises as well as doing Science! then you might want to go with modular gear form the start, since you'll also have access to a vast universe of filters, function generators, and so on.

If you'd like to know more about this, please enjoy this fantastic tutorial by my late friend Rob Hordijk: https://rhordijk.home.xs4all.nl/G2Pages/index.htm

Cool. Just realized they were also featured on this episode of Veritasium: https://youtu.be/GVsUOuSjvcg

Telefunken slapped their name on the pairing of the legendary Neumann U47 microphone with their amplification for the USA market:

https://wikiless.org/wiki/Neumann_U47

So Frank was correct to use the Telefunken U47 term, but audiophiles and engineers found it a bit misleading.

I'm not sure where you were, but what you saw was almost certainly a service van with the Telefunken name and logo because Telefunken was one of the major brands of hifi radio consoles and table radio sets at the time. As the availability of FM radio increased in North America, so did the popularity of the living room hifi console. FM Stereo broadcasting furthered the boom. When community cable tv companies began to include FM Stereo stations in their services, often with stations normally out of range of home antennas, there was a further sales boom. As with all major brands, Telefunken hifis came with a record turntable and AM/FM (later FM Stereo) radio band receiver, but Telefunken also included Short and Long Wave bands as standard features. Major North American brands integrated TV sets into their high end consoles while Telefunken remained radio-turntable only. Eventually the market demand for large hifi consoles died off and now they are collectible relics, boat anchors, or landfill.