Tonetta Blue by Flandersh Tech is a completely free semi-modular, eight voice polyphonic synth.
The front of the plugin sports a smart, well laid out user interface allowing for easy intuitive patch editing. The heart of the synth consists of two types of oscillators each with a choice of nineteen different waveforms to choose from: a 'Fat' supersaw style layered oscillator which gives epic sounding trance style leads and pads, as well as a secondary dual oscillator which is more suited to keys and bass sounds.
The front panel also contains a pair of analogue style filters, ADSR envelope adjustment, LFO and 'Curve' modulation, as well as a great sounding convolution reverb with six unique impulse responses. The 'Curve' modulation is particularly useful as it allows you to set an LFO with a completely customisable waveform, which allows for some great modulation effects.
The back panel of the synth (activated by the back button) is where things really get interesting, as this contains the modulation routings section where you can really start to play with all that fun modular stuff. There are many signal source and destination options to experiment with, a signal splitter and mixer, a modular delay effect, and even a step sequencer.
The synth comes complete with a number of great presets to get you started and features a built-in preset manager allowing you to save and load your own presets and organise them into banks. It's also worth noting that the synth comes complete with a comprehensive user manual which is a really nice touch for a free synth, and one which I definitely needed when navigating the modulation routing options for the first time.
In summary, Tonetta Blue by Flandersh Tech is a quality free synth which combines ease of use with an impressive set of features, allowing for many creative sound design possibilities.
Note that although the software developed by Flandersh Tech is free to use, their website does allow you to donate if you find their products useful in your productions. I would totally recommend supporting them in this way to help facilitate future development.
Tonetta Blue is available as a free download from the Flandersh Tech website as a 64 bit VST3 for Windows 8 and higher.
For more free VSTs check out this article