# VCF.MTH.3

Lesson Summary

Compare various resistor values in the filter, contrast the simpler RC filter to the Moog transistor ladder, recognize differences.

Vocabulary
Electricity - The physical scientific processes related to the flow of electric charge.   Charge can be either positive or negative. The SI unit of electric charge is the coulomb C.

Conductor - A material that allows electricity to flow freely though it. Metals are great conductors, with copper being a commonly used conductor in electronics.

Insulator - A material that restricts the flow of electricity.  No material is a total insulator, common examples include glass and paper.

Voltage  - The electrical potential difference between two points, denoted by a V. A single AA battery holds 1.5V

Resistor - An electronic component that restricts the flow of electrical current and voltage. Resistance of the material is measured in Ohm's signified by Ω.

R1 = Resistor, R2 = Variable Resistor, R3 = Potentiometer.

Capacitor  - An electronic component that stores and discharges the flow of electrical current and voltage.  Capacitors use two layers of conductors, usually thin films of metal, aluminum foil or disks, separated by a polarized insulator made of glass, ceramic, plastic film, air, paper.  Capacitance is measured in farads, commonly displayed as micro farad or μF.

C1 = Capacitor, C2 = Polarized Capacitor, C3 = Variable Capacitor.

Low Pass Filter - A filter that passes low-frequency signals and attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.

High Pass Filter - A filter that passes high-frequency signals and attenuates (reduces the amplitude of) signals with frequencies lower than the cutoff frequency.

Cutoff Frequency - The boundary in a filter at which signal begins to be reduced.

RC Filter - A simple filter circuit using a single resistor and capacitor wired in series.

Exercise

## Experiment with RC filters in the Werkstatt audio line.

Materials
 1 x 10kΩ Resistor 1 x 2.2uF Capacitor Jumper Cables
Hardware

By using a 2.2µF capacitor and a 150Ω resistor we can create a simple low pass filter with a cutoff frequency around A440.  Make sure the Werkstatt settings match Figure 1.

Figure 1. Square wave

Now lets look at our RC filter circuit.  We need to run our signal into one lead of the resistor. We then connect the positive terminal of our capacitor to the opposite resistor lead.  This is the node that we will be pulling our output signal from.  Finally, we will need to pull the capacitor's negative terminal to ground to ensure a proper RC filter configuration.  The schematic is shown in Figure 2.

Our jumper cables should be connected as is shown in Figure 3.

Figure 3. RC Filter R = 150Ω C = 2.2µF, cuttoff frequency ≈ 440Hz.

Using the RC formula we can exactly derive the cutoff frequency of our filter is shown in Figure 4.

Figure 4.

Now lets swap our resistor for a variable resistor in the form of a 10k potentiometer.  Observe Figure 5 for jumper connection directions.

Figure 5. RC Filter with variable resistor.

Observe as we turn our 10k potentiometer how the cutoff frequency changes.  We are now getting closer to emulating the VCF CUTOFF knob located on the Werkstatt itself.  However, a very important feature that is missing from this circuit is resonance.

Disconnect all jumper cables and copy the settings in Figure 6.

Figure 6. VCF RES settings

Now when we sweep our CUTOFF frequency up and down we will hear a completely different sound.  This RES knob changes the resonance values in our filter and allowing us to create wild new sounds.  Experiment with both the CUTOFF and RES knobs to hear how they interact with each other.  A major feature of the "Moog sound" rely's on the transistor ladder filter and its control over resonance.

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