3.5 mm had been around for a long time, maybe first made for transistor radios? Personally I like RCA jacks as an output. I've never hooked up extra speakers but in the late 70's a lot of the quality boomboxes pushed having those external speakers.
I found this: “The miniature 3.5mm jack version was introduced in the 1950s for the headphones of transistor radios,popularized by the famous SONY EFM-117J radio released in 1964, then by the first Walkman, again from Japanese brand Sony, in 1979.” Source: https://www.audiophonics.fr/en/blog...ature 3.5mm jack,Japanese brand Sony, in 1979.
The mono 3.5mm plug was very standard indeed. You could often recognize it by the indicator words "EAR" or a small drawing of an ear symbol. I'll keep my eyes open to see if I can find the stereo version somewhere on a device that is older than the TPS-L2. Maybe a computer or something. Because I suspect it played a mayor role in its miniaturization that makes it so special and contributed to its popularity. I've also read that Sony is one of the last to stick with the stereo 3.5mm plug on their mobile phones too. RCA is great. Here in Europe we had the DIN plug. I didn't like that at all. In practice, I used to connect my boombox to a stationary hifi via the 6.35mm headphone plug for playback. Just like one would connect a modernish portable MP3 music players or walkman via the 3.5mm headphone plug I guess. In my early 80's situation the cassette playback quality of my boombox was much more stable and much better quality than the quality of my first walkman, a Grundig Beatboy 100.
There's other versions of the 3.5mm with seperate zones, I think one zone is for the inline remote? I'm paging through old Funkshau Magazines, there might be something in here but I can't read it, great photos and ads though.....
