The synth designer is the primary synthesizer engine for web synth. It works much like a traditional synthesizer would: accepting MIDI, having various configurable parameters, and outputting audio. However, internally it consists of three different kinds of synthesizer which can be chosen between at will and stacked together in any way.
The three existing synthesizer types that exist within the synth designer are:
You can read more about each of them by clicking the links above.
To switch the type of synthesizer you're using within the synth designer, use the "waveform" dropdown menu on the control panel for the slot you want to change. The first four are waveforms that are used in the [traditional-oscillator-based-synthesizer], and the last two are for the [wavetable-synthesizer] and [fm-synth] respectively.