More common in high-power pro amplifiers, the sonic characteristics of multiple rail amplifiers are varied due to the wide array of possible implementations. (They’re said to be similar to Class-AB when good design practices are used.)
However, multiple rail amplifiers suffer from the same problems as single rail amplifiers when it comes to driving reactive loads. The added dissipation usually occurring in one output transistor simply migrates another.
What does moving rail design really mean? Simply, the voltage rail moves with respect to the signal being reproduced – “tracking” it, in a sense. There are many particulars to this type of design, but the idea is to minimize the voltage across the output transistors and approach 100 percent efficiency.
Keep in mind that this is the efficiency of the output stage only. The power supply driving the output stage has its own efficiency, and the compound efficiency of the amplifier as a total package depends largely on the power supply (particularly in this type of amplifier.)
There are other caveats to contend with, such as the unpredictability of the input signal and the complexity of the power supply. The power supply rails in a moving rail amplifier can also cross zero in some implementations, allowing the output stage to act as a low-voltage amplifier at all times, regardless of output amplitude. Facilitating this type of operation is not trivial and can be quite expensive.
The power supply of a moving rail amplifier can be of the linear or switching variety as well, adding to the confusion. In the audio world, moving rail amplifiers are either modified Class-A or modified Class-AB, and suffer from the same problems when driving reactive loads. It should be noted, however, that the lowered voltage across the transistors minimizes these problems.
The hidden problem is that the power supply itself may be affected by load reactance since it has to drive the output stage. This is where the choice of power supply implementation has a profound effect on real world efficiency, and switching supplies are often the clear choice.
Tommy O’Brien is a power amplifier designer.