A question of balance

 Below is a picture of an old-fashioned amplifier that I received as an 18th birthday present! This article is about the balance control on the right hand side.

An exam question we did recently related to the balance control on a stereo. In the days before ipods, there used to be chunky bits of equipment that sat in the corner of the room connected to two different loudspeakers. It was called stereo because they record different tracks, one for each speaker. This continues, of course, but you'll notice it as different tracks to left and right headphones. The problem was that if you sat in one part of the room, you'd be closer to the right speaker than the left and so its track would be louder. The balance control allows you to turn up the volume of one whilst turning down the volume of the other. The exam question concerned the wiring. Look at my top diagram. Both speakers are controlled by individual variable resistors. Hence you can turn each speaker up or down individually. But you'd need two control knobs. The second circuit with a single potentiometer shows how it is actually wired. I've written on the potential differences assuming it is a linear potentiometer. You only need one control knob.