lørdag den 12. juli 2014

Aristoteles Logic Synth Update #2: Oscillators

The next module on the path of Aristoteles, the 'lunetta-style' synth, is finished! It's the central part of the synthesis: Oscillators. 8 of them. So essentially one could patch all 8 oscs into the 8-channel mixer and have 8 voice polyphony. They will also serve as the unique pseudo-CV clocks for the other modules, which is the main concept of CMOS-based synthesizers. The brain in all this is the classic 40106 chip, which is actually a logic NOT-gate flipping on and off. Below is a quick demo.

The oscillators have the following functions, as seen on the image to the right:
The first switch lets you choose if the osc should be constantly on or momentary, determined by the red button. This allows one to use the synth as a simple keyboard with a little willpower.

Next switch allows you to add extra capacitance to the oscillator, thereby setting the frequency range. The middle position of the switch is actually an off-position so the oscillator runs on the "offset cap" which is too small at the moment, so the frequency is very high. I wanted to have this range-switch so I could go with a potentiometer value around 100K for the frequency knob. This makes it easier to tune it without too big steps, as one gets if running on 1M or something in that range.

I chose to have the whole synth only producing square waves, to have it ultra lo-fi and simple. But I  thought it could be nice to at least have PWM for each osc though, so this is what we have below the freq pot.
 It's a simple diode technique for adjusting pulse width of the wave, but I discovered that it was actually an inverse PWM, for what I wanted. It was setting the lenght of the off-time, not the on-time, so the LED's would be constantly on, only turning off for a short time, so I used another 40106 to inverse the HIGH/LOW order. This is the reason for 3 of the chips in the design - 2 IC's for getting 8 oscs and another one plus the leftover gates from IC 2 to have the pulses inversed.

The LED's are switched on and off with the help of transistors, to minimize voltage drop on the outputs. All the outputs are grounded with 100K resistors as well to prevent frying the chip if you touch the banana jacks.

Here's my quick and dirty schematic drawn in MS paint :)
Feel free to use it for inspiration in your own designs.