Thanks for that great explanation. I also still believed it had an 18dB/oct filter, even though I usually got closest results simply using some 24dB filter. You’re making me curious about the transformed sawtooth now too. I do remember seeing wavetables with 303 waves that looked like malformed squares/saws, maybe analyzing clean original 303 samples is the best way to go at emulating first. It’s also good to realize that the inability to emulate the 303 is no real indication of a synth’s quality, as at some point I did believe.

In my view, it’s not that it’s hard, it’s that for years nobody did the work. Or used bad knowledge.
A long rumour was that the filter was 3-pole (18 dB/oct) but that’s a bad interpretation of the circuitry.
Another ‘problem’ is that the square oscillator is in fact the sawtooth being transformed by magic (i forgot how exactly, so I’ll pretend it’s magic… ^_^).
The glide/slide is another issue, because being a sequencer-based instrument the way it behaves cannot be accurately transposed into a pure MIDI instrument. That’s the best emulations (hard- or soft-ware) take care into reproducing the sequancer (sometimes to a fault, the insane way to enter the notes is optional…)
The accent being another quirk, because it affects the VCA in a very particular manner.
The TB-303 is a very bad, cheap design, and that’s why it’s harder than a clean analogue synth using quality components and good design. That’s both what makes it wonderful, but a pain to re-create.
Imperfections, chaos, are indeed harder to simulate simple predictable things. This goes for everything, audio, 3D CGI, AI, etc…

Unlike Audio Realism ABL 3 ($95) and d16 Phoscyon 2 ($119) the Midilab JC-303 costs $0. If you’re unsatisfied with it, why not just delete the synth from your VST folder and move along?

Life’s too short to get hung up on whether or not a free plugin sounds as good as hundred dollar commercial software.

Have you tried the new Arturia 303?

A common Mac issue with anything freeware/opensource stuff from tiny devs who refuse to pay the fee to Apple to sign the binaries. The solution is to open a Terminal and:

sudo xattr -rd com.apple.quarantine

You’ll then be asked for your admin password. Should work. Should. What do I know, The only MacIntosh I ever had was a Spectre GCR emulator for the Atari ST… :D

DITTO

That is interesting, I thought Apple Mac platform was immune to viruses and malicious software ?

I get the same message on Windows 10, but I just chose to ignore it, and plugs install fine.