Snap together the main circuit.
We will start by assembling power, keyboard, oscillator, two filters, envelope, and a speaker bit to construct the main circuit.
Start with a power bit. Snap your power bit to the input of a keyboard bit. Then snap the output of the keyboard bit to the input of an oscillator bit. Then snap the output of the oscillator bit to the input of the first filter bit. Then snap the output of the first filter bit to the input of the second filter bit. Then snap the output of the second filter bit to the input of the envelope bit. Finally, snap the output of the envelope bit to the input of the speaker bit.
We now have our main circuit, but we need to build a circuit to control the cutoff frequency of both filter bits at the same time.
Snap together the control circuit.
We will make a control circuit by assembling wire, dimmer, and split bits. We will then snap this circuit to our main circuit.
First, snap a wire bit to the input of a dimmer bit. Then snap the other end of the wire bit to the trigger output on the keyboard bit. This will supply power to our control circuit. Snap a split bit to the output of the dimmer bit. Finally, snap the two ends of the split bit to the cutoff frequency inputs on both of the filter bits.
Moving the dimmer bit will allow us to control the cutoff of both filters at the same time.
Calibrating the cutoff of the filter bits.
To get a standard 4 pole filter response, both filter bits must have the same cutoff value on both knobs. I have found that the best value for these knobs is to position the notch around five to six o-clock. This will allow the widest frequency sweep from the dimmer bit. Make sure both knobs are at about the same value.
Calibrate the peak on the first filter bit.
Turn the peak on the first filter bit entirely counter-clock-wise. Since the filter bit will self oscillate when the peak knob is near its maximum value, we will only need to adjust the peak knob on the second filter bit.
Optional: attach a bendy straw to the peak knob of the second filter bit to easily tell which one should be adjusted.
Play the oscillator thru the 4 pole filter.
You should notice that the sound of the oscillator is darker in timber than it usually is. Be sure to sweep the dimmer bit to change the cutoff of the filter. An easy way to hear the difference between a 4 pole filter and a 2 pole filter is to simply un-snap the first filter bit from the main circuit. After snapping the second filter bit back into the circuit, you should hear that the oscillator is now brighter in timber.
Learning: Why does running two filters in series create a different timber?
In a filter, the main circuit that controls the timber of the final sound is called a pole. The easiest way to change the timber of a filter is to add or subtract the number of poles the input signal runs through. The number of poles in a circuit will determine how "dark" the timber of the filter is. You may even notice that when sweeping the cutoff of your four pole filter that the output sounds smoother than the regular two pole setup.
Learning: How does a filter pole work?
A filter pole will remove parts of the input signal. To get technical, all filter poles will remove parts of the input signal at a rate of 6dB per octave. The standard filter bit is a two pole filter, that means it also has a slope of 12bB per octave. A 2 pole filter and a 12dB per octave filter refer to the same number of poles!
Learning: Why do most filters contain an even amount of poles?
In a filter pole, subtraction must take place. This is why using a filter to sculpt a sound is referred to as subtractive synthesis. It also just happens that subtracting the signal over one pole will invert the input signal. The easiest way to get a non inverted signal out of the filter is to simply run it though a second filter pole! It is like multiplying by -1 two times, you end up with the same value you had to begin with.
Learning: Why is a 4 pole filter useful?
Many vintage synthesizers use 4 pole filters instead of 2 pole filters to shape their oscillators. If you have been having trouble getting the exact sound you hear on an old song, you may be able to get closer using a 4 pole filter!
Now that you have this set up, you can try adjusting the cutoff and peak on both filter bits. You will want to document what settings you like so that you can use them later. You can even replace the dimmer bit with an envelope bit to control the filter cutoff automatically!