Unknown potentiometer in schematics

Question

In a circuit schematic of a stereo audio amplifier, I found the following component:

It is the potentiometer used for the balance. Upper line is one of the channels, lower line is the other one. Central, bold line is ground. The variable resistor seems to be "bridged" between the two lines.

1) Conventional potentiometers have a simple, unique output which is a fraction of the input voltage; but this one needs to have two complementary outputs. Does such a potentiometer have a specific name?

2) Removing this potentiometer (which is very similar to this image), the amplifier still works, with just one channel. How is it possible? Looking this part of the schematic, after removing the variable resistor, both the channels should be isolated with respect to ground: so, they should exhibit the same behaviour.

Answer 1

Figure 1. A four-terminal or centre-tapped potentiometer - not to be confused with a pot with a fourth terminal to connect an earth wire to the case.

Four-terminal potentiometers were common at one stage on hi-fi volume controls to give automatic "loudness" control - see What's a potentiometer with four terminals in this schematic?. These were logrithmic law or "audio taper".

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. Probable balance control circuit.

My guess is that it's a linear pot and that the centre-tap is used to provide a minimum load to each side.

• With pot in centre position the signal fed into 'L' is \$ LEFT \frac {R3}{R1+R3} \$ and the signal into 'R' is \$ RIGHT \frac {R4}{R2+R4} \$.
• If we move the pot wiper all the way to towards 'L' we end up with \$ L = LEFT \frac {R3}{R1+R3} = LEFT \frac {0}{R1+0} = 0\$. The 'L' channel will be fully muted but 'R' will remain unchanged.
• If there was no centre-tap the right channel would get loutder in this situation. This would normally be considered "a good thing" as the overall volume would remain the same and you are only changing the "balance" between left and right. In this application the designers may have reasons for not doing this.

Answer 2

The thing is called a center tap potentiometer and all that means is that the center of the resistive track has a pin available on the component. I don't know whether or not they are in mass production, but it's certainly something that is available as a custom part.

As for the second question, it's impossible to tell with out seeing the rest of the circuit why only one channel works. I suspect that it has to do with biasing being gone.