Fascinating stuff, this.
Definitely going to follow how it goes - if he delivers, it promises to be a hyper capable version of the moderately GASsy but quite limited Onde Magnetique.
But his commitment to open development is as interesting, with every stage of design (it’s still very early stages) being published for free on his Patreon https://www.patreon.com/TheSoundofMachines
Here’s the Mk1 to give a bit more of an idea of what it might sound like (I guess). Wonky as anything. Got me hooked!
I like that he is now breaking down the mechanicals on the cassette motor drive, so that you can buy those separate components rather than finding and recycling that part.
We’ll see how far he gets to making this a complete purchasable product. That is a big step along in the process.
Also fun is the setting the A & B channel mix on the keys and switching. My imaginary version would be setting speeds of A & B on the keys and then morphing between, with the motor speed change delay.
Since this is going to be relatively detailed construction plans, you could make modifications and make it even more mellotron like by putting in multiple cassette drives and heads.
This is very close to this :
I am loving that he has started with concept (basically “more” and form factor “bigger but the same”) and is then working backwards towards building the internals to deliver on his ideas. And am already thinking about how to use that twin cassette loop/mix/pitch shift thing to create some interesting new dronescapes.
Also note that this is entirely mechanical / analog. Nothing digital.
Man it’s really cool of that dude to be so open with the building process and plans for anyone who wants to DIY their own. Respect to those like him who work hard to bring joy to the people of the world.
Ive held on to three old recorders and a literal duffel bag of tapes for something like this. Hoping his second version is a big hit.
I also think of Jeremy Bell, who did and shared the ScrubBoard – also a cassette tape thing. He did a Kickstarter with it and it failed unfortunately. ( post )
Also in the realm of cassette tape audio generation devices is the HC-TT from Landscape.
Either of these could be blended into this sort of system, as they all take and amplify signals from magnetic tape heads.
So is the Cassettone in stereo ? How about using all four tracks on a cassette, with a four track head ?? Then mix that down, or allow quick cuts between heads with an LFO signal or two. This would all be easy using modular gear.
ADDED : Alternately there is a speed controllable four track modular cassette player device called Noodle Rider. ( post )
A youtube thumbnail that makes it look like it’s ‘just’ a VST and a video that doesn’t show it in action … :\
Its not a sales video, its a report on progress 
Basically it will sound like the Mk1 but with two tapes and the ability mix between them (assuming he manages to pull the design off)