Synth: Coisa Linda (Square Wave Section)

This project started out with a schematic I found in the LM386 datasheet (1) for a square wave oscillator. I was a little surprised since I was more familiar with the LM386 as a power amp in circuits like the Earthquaker Acapulco Gold (2) but once I got a working breadboard it was interesting. Eventually I found out you could make an electronic piano with an LM386 (3) and worked the tuning pot/switch combination the the design, which gives you about 1 octave of range that you can change with a B10K potentiometer. The square wave section is then followed by a gain recovery stage using an LM741-based op-amp that is very similar to the MXR Distortion + (4). There are five controls in this basic design:

1. VOLUME - A100K, controls master volume out

2. GAIN - B1K, connects pins 1 & 8 of the LM386 and affects the gain/grit of the square wave

3. BASS - B100K, usually fixed at 1K, but by adding resistance with a potentiometer you can vary the pitch dramatically and get sub-bass tones

4/5. TUNE 1/2 - B10K, adjust pitch of the square wave

This module by itself produces a tunable sound but also has the option of blending in another effect (at the two pins flanking the 20K R2 in the schematic below). The idea was to make a useful synth that people can customize however they like. For my first try I used the PT2399 section of Mad Professor's Deep Blue Delay (5) from a PCB and patching in/out at the right spots on the board. You can see some results in the video below. Since the square wave is already pretty fuzzy, you might have best success with modulation effects like phaser, chorus or other delays. Additionally, one could swap out R2 with an LDR and  incorporate an LFO-driven LED to make a tremolo-like effect.

Fig 1. schematic  

 

PERFBOARD LAYOUT

 

Fig 2. Perfboard layout with hardware and BOM

 

PERFBOARD BUILD 

Fig 3. Perfboard build

DEMO VID

This video was made with the full build but I have the delay switched off (using an SPDT) and blended out so that the "clean" square wave signal can be heard at the beginning. The delay mod is turned on at 1:35 and you can see what it adds to the signal.

Fig 4. Coisa Linda bass line rinse out sesh