Modular synthesizers are synthesizers composed of separate modules for different functions. The modules can be connected together by the user to create a patch. The outputs from the modules may include audio signals, analog control voltages, or digital signals for logic or timing conditions. Typical modules are voltage-controlled oscillators, voltage-controlled filters, voltage-controlled amplifiers and envelope generators.
Build the function generator circuit. The input is just the power supply, and the outputs are a sine, a triangle, and a square wave. If you’re not comfortable with soldering immediately, build it on a breadboard first. Test to see if the duty cycle and frequency knobs work as expected. If soldering, be very careful with potentiometers - make sure nothing gets inside and no solder attaches to the base of the pot. Try to keep the wires as short and as close as possible to the chip to avoid random pickup. Optionally solder banana receptacles to output wires for better connection. I used -12V and 12V supply, but anything up to 36V pp (+18V and -18V) is safe. Despite the suggestion of the datasheet, I found that the circuit only works if pin 11 is connected to ground and not the negative rail. The datasheet also suggests you use a 1N457 diode, but I found that a basic orange Si diode from the lab works better. If you’d like more information about how the circuit works, read my attached presentation or consult the datasheet for the ICL8038 chip https://www.mit.edu/~6.331/icl8038data.pdf
Build the audio amplifier circuit. Test the output before connecting the speaker. Do the gain and volume knobs produce the amplification you expect? Again, solder the pots very carefully. The gain knob has one of the ends not connected to anything and the 10R resistor (R1) is actually 10 Ohm, not 10kOhm. Try not to connect input and output ground to the same line as it gets noisy. You can read about what the parts do in my attached presentation or on https://www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/
Now you can build on this - for example, you can add high and low pass filters. You can use my schematics or design your own. I recommend using filter design tool by Analog Devices: https://tools.analog.com/en/filterwizard/
These normally utilize multi-stage filters, first stage being a simple buffered filter, connected in series with Sallen-Key filter stages. Sallen-Key filters are nice because they use a non-inverting amplifier, so voltage can stay at the same level as before the filter, and they are easily stackable for better results.
Here you go, now you have a nice base to build off of when adding new modules! Consider adding a mixer, a VCA, and an LFO for more fun sounds.