If the objective is to learn how an analogue synth works from the basics, there are a couple of free VST modular synths that I heartily recommend.
- Kamiooka
- Sonigen modular
These have separate modules for every function - oscillators, filters, envelopes etc. which you connect together to create your own synth (this all works in real time, so experimenting is easy). There are plenty of built in patches too, so you can see how experts go about sculpting a sound and have some instant sounds for noodling with.
This way you can experiment with each component of a synth in isolation, so that you develop a really good feel for what each section does, and what role it plays in creating a sound. As you learn each new feature, your patches can 'grow' with you - and it's often surprising how few modules you need in order to create a satisfying sound.
A bit of experimenting on a modular system will also help you discover which features of a synth mean most to you. which will help you with selecting other instruments in the future. There are some pretty good on-line resources too, that can guide you through the process of constructing a patch. The Sound On Sound "Synth Secrets" series is an excellent, in depth, guide written in nice plain English.
My main recommendation would be to find a couple of synths with interfaces that you're comfortable with, and get to know them really well, before you start filling folders full of free VST downloads or splashing out on commercial plugins. It's really easy to end up not learning so much when you are constantly switching between different interfaces - something that gets much easier when you learn a bit more about what happens "under the hood". Plus, you can then spend money more wisely when you know exactly what synth features you really need for your music.
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