[Voxophone]

Reactance can be a strange concept but one you grasp it you’ll find it much easier to understand why certain components are used in radio and audio circuits. Since reactance depends on frequency, inductors and capacitors can be used to make a circuit behave differently at various frequencies, or to make currents at certain frequencies go where the designer needs them to go.

For example, a capacitor combined with a variable resistor can be used to make a circuit path more or less favourable to currents at certain frequencies. This is essentially what the ‘tone’ control does in some radios. It changes the frequency makeup of the audio output by changing how a circuit responds to bass, treble etc.

I’ve often found it useful when trying to understand circuits, to think of a fixed frequency (such as a radio’s i.f. frequency, or the upper/lower limits of audio) and redraw parts of the diagram with the capacitors shown as reactances. You can quickly see how the circuit operates by putting yourself in the current’s ‘shoes’ so to speak. A circuit might provide a path forward for some frequencies, while shorting others to ground and removing them.

Hope that makes sense!

Liam