Simple synth
Possibly the simplest synthesizer I can think of is as setup with just 1 VCO, 1 ADSR and 1 VCA.
When you use a square as wave from the VCO, the pulse width can be modulated by the ADSR to change the sound.
That same ADSR is used to send the envelope to the VCA so the volume is controlled by the key-press.
So:
PITCH -> VCO EXT in
GATE -> ADSR GATE in
ADSR OUT -> VCO PWM in
ADSR OUT -> VCA ENV (middle) in
VCO OUT -> VCA ES in
VCA OUT -> MAIN OUT L
Ofcourse you could improve the sound by adding a filter before going to the VCA.
The ENV IN from the filter can then be fed by the same or a second ADSR.
And feeding the VCA out into a REVERB before going to MAIN OUT will surely improve the sound as well.
Basic synth
Here a basic synthesizer is build using a combination of VCO, VCF, VCA, ADSR and LFO.
The VCO receives a signal from the PITCH output to its EXT input to respond to the keyboard.
The VCO’s output, with square-wave switched on, is connected to the ES input of the filter with the ES knob turned up.
The ADSR’s output is connected to the filters’ ENV input and the ENV knob set to respond to a pressed key on the keyboard.
The ADSR receives a GATE signal in its input to make that happen.
The VCF’s output goed to the VCA’s ES input with a set level.
The LED switched ON and cable connected above from the ADSR’s output makes the VCA respond to the same ADSR that responds to a keyboard press.
To behave differently, use multiple ADSRs.
The LFO is connected to the VCA’s AM input to create a tremolo effect on top the envelope from the ADSR.
The VCA’s output is connected to the L output.