This article shows you how to make an audio filter from recycled components.

This is the device that I made:

You can see the circuit working in those videos:

High Pass Filtering:

Low Pass Filtering:

Inserting a Headphone Plug:

Step 1: Design the Filter

I have drawn the circuit in the old PSpice simulation software student edition version 9.1:

A typical speaker is 4, 8 or 16 ohms. Headphones have higher impedance, and resistance values.

It is not likely that the old recycled circuit will have too many bipolar capacitors. You can try using electrolytic capacitors.

I also considered connecting a switch that would connect the Rf resistor directly to the input. However, this will also short-circuit Ch1, Ch2 or Ch3. Shorting any capacitor with a button/switch will cause permanent damage to the capacitor. This is why I avoided an additional Swh switch. An alternative is to connect a 10 ohm resistor in series with the button/switch. However, this would be an extra component that you will need to look for in an old electronic recycled circuit and an additional resistor will reduce the output signal amplitude.

The impedance/reactance (measures in ohms) of the capacitor is equal to:

Xc = 1/(2*pi*f*C)

Where: f is the frequency of the sinusoidal signal. Any periodic signal can be modelled with sinusoidal of specific amplitudes and phases (time delays). This is known as the Fourier series.

The bandpass frequencies of this filter could be complicated because it depends on load (speaker) impedance/reactance/resistance. However, if we ignore the speaker impedance we can calculate the output amplitude and phase delay with the following transfer function formula (Output Voltage/Input Voltage = Vl/Vin):

Vl / Vin = 1/(j*2*pi*f*(Cl1 + Cl2 + Cl3)) / [1/(j*2*pi*f*(Ch1 + Ch2 + Ch3)) + Rf + 1/(j*2*pi*f*(Cl1 + Cl2 + Cl3)]

The amplitude magnitude will equal to:

|Vl / Vin| = 

= |1/(j*2*pi*f*(Cl1 + Cl2 + Cl3)) / [1/(j*2*pi*f*(Ch1 + Ch2 + Ch3)) + Rf + 1/(j*2*pi*f*(Cl1 + Cl2 + Cl3))]|

= 1/(2*pi*f*(Cl1 + Cl2 + Cl3)) / sqrt[Rf^2 + (1/(j*2*pi*f*(Cl1 + Cl2 + Cl3)) + 1/(j*2*pi*f*(Ch1 + Ch2 + Ch3))^2]

The amplitude magnitude will equal to:

angle[Vl / Vin] = 

= angle[1/(j*2*pi*f*(Cl1 + Cl2 + Cl3)) / [1/(j*2*pi*f*(Ch1 + Ch2 + Ch3)) + Rf + 1/(j*2*pi*f*(Cl1 + Cl2 + Cl3))]]

Step 2: Simulations

The simulations show a bandpass filter output response:

Step 3: Make the Circuit

I made the circuit by twisting the wires together. I did not use a soldering iron.

The left blue and green wires are the input, the bottom yellow wire is the high pass filtering control, the top orange wire is the low pass filtering control, and the yellow and brown wire is the output. The orange wire is the headphone output.

Conclusion

You can see from the video that some capacitors are almost blocking the audio signal while other capacitors are creating an output sound that is exactly the same as the input. A good idea would be to use 22 uF, 33 uF, and 47 uF bipolar capacitors to implement a smoother transition from loud to filtered/quiet signal.