The Quilcom Spectre is a free VST synthesizer instrument and it was designed to be a relatively easy-to-program additive synth.
There are 20 sine wave partials provided. The stack of 10 on the left go to the left channel and the 10 on the right go to the right channel. The bottom two partials can be sent to the FM input bus on the partials above them, and each partial can then have their FM modulation levels set individually.
There are 2 AM LFOs at the bottom of the panel and each partial can have its level adjusted from each LFO. Each partial can have a simple delay time set and this can be used, for example, to stagger the attack phase or produce a beating effect if set low. Any value for the pitch ratio for each partial can be entered. The velocity response is variable for each partial; clockwise is maximum response and anticlockwise is no response.
Below the MIDI options area is a pitch envelope. If you want no pitch change just cntrl-click the knob to set the default of zero. Velocity response is adjustable for the pitch envelope. The whole synth is affected by this pitch envelope.
At the bottom centre is a pitched noise generator synth which sends to both left and right channels.
To the right of the noise synth is the button for showing the effects chain, then the Mono to stereo width knob and finally the output volume.