It is often difficult for people to grasp how balanced audio cables are 6dB louder than single-ended cables or how two channels of a stereo power amplifier can have double the output voltage and 4 times the power output when in bridged mode.
It is difficult to understand because, in each case, only one output level is available. Put another way, if we have only 1 volt of signal available to us, how do we create a 2-volt signal?
The answer is easiest understood with an analogy using cars or trains. It’s somewhat the same thought process Albert Einstein used as he was figuring out the theory of relativity.
Motion and distance are relative to each other. You and a friend traveling in a car together at 60 mph are moving at the same speed so it appears as if you’re stationary. To an outside observer, you both whiz past at 60 mph.
In the case of a single-ended cable, the hot wire with the signal is moving away from ground (zero). For this thought experiment let us specify the maximum “distance” the signal travels as 1 volt.
How to double that voltage if we are restricted to only 1-volt.
Now, imagine two hot wires each with a maximum 1-volt signal. If we invert the direction of one signal we can double the voltage between the two signals. Signal 1 is moving away from ground in a positive direction while, at the same time, signal 2 is moving away from ground in the opposite direction.
Each is only moving a maximum of 1-volt. But measure between them and we get 2-volts (6dB louder or twice the voltage).
It’s very much like a train moving away from the station. If the train is moving at 60 mph, then we can say that in 1 hour that train will have traveled 60 miles. Now, add a second train leaving the station in the opposite direction and at the same speed.
Measure the distance between the two trains and we discover a 120-mile difference. One train would have had to travel at twice the speed to get that far, but two trains traveling the same speed in the opposite direction give us twice the distance.
The same is true for a bridged amplifier. One amp channel goes positive while the second amp channel goes negative. If you place the speaker terminals between the two outputs you get double the voltage resulting in up to (depending on the amp’s available power supply current) a 4X increase in output power.
Hope that helps.