this hefty idle current demands to that of a series regulator with the same load: as little as 15 mA at idle and 4.005 amps at the other extreme. With savings such as these is it any wonder that series regulator predominates.
    However, when the load draws a constant current rather than a varying current, both the series and shunt regulator can use a lower wattage active device. In the case of the series regulator, a high wattage resistor can be placed in parallel with the pass device. This resistor would provide the larger portion of the current flowing into the load and the pass device would have the task of fine tuning the output. Or, in the case of the shunt regulator, the shunting device can be scaled back in terms of dissipation, as it would only have compensate for the small straying from the reference voltage due to ripple. For example, if the load is a Class A amplifier that draws a constant current of 4 amps, the modified shunt regulator must maintain a total current draw at idle of slightly greater than 4 amps, let's say, 4.01 amps, which means the losser device would handle only the 10 mA of the total. Another example, if the load is an SE Class A amplifier tube amplifier that draws an idle current of 150 mA, which climbs to 155 mA at full output due to rectification effects, the delta equals only 5 mA. This difference could easily be handled by a single section of a 5687. You see the active shunting device only needs to cover the delta (the difference) in current fluctuations. In this example, the 5687 placed in parallel with the output tube, need only draw an idle current of 10 mA, 5 mA of which it will give up when the output stage's current draw increases by 5 mA.

Regulator Analogies
   One way to look at the difference between the series and the shunt regulator is to view the series regulator as being analogous to a Class B amplifier and the shunt regulator as being analogous to a Class A amplifier. If the regulator is functioning as an amplifier, what is being amplified?

resistance, in parallel with a load that draws a constant or varying amount of current, creates a battery like effect of zero ohms termination for the load and the series resistor. In other words, while the load can be as wildly varying as a Class B amplifier, the power supply will not see a wildly varying current draw even as the amplifier draws a varying amount of current.

    This situation is vastly different from the that of the series regulator and, in many ways, preferred. It is as if the current variations produced by the regulator's load are trapped on the load's side of the series resistor, preventing those current variations from re-circulating into the power supply and from there into other circuits and stages.
    This benefit of increased isolation comes at the cost of a much greater quiescent idle current for the shunt regulator. For example, if the load is a Class B amplifier that has a current draw of 10 ma to 4 amps, then the shunt regulator's idle current would have to at least equal 4 amps! (And if we wish to cover a wide range of wall voltage variation, the idle current might have to be set to 6 amps. In this case the series resistor really should be replaced by a current source. And the argument can be made that in all cases, the shunt regulator should only use a current source, rather than the resistor; or in the case where our only concern is AC regulation, not DC, the resistor should be replaced by a choke.)
    When the shunt regulator faces an amplifier idling at only 10 mA, the active shunting tube or solid-state device will have to draw 3.99 amps; when the amplifier draws 4 amps, shunting device will have to draw zero current. Compare