a variable displacement pump disposed in a main line and configured to receive a low pressure fluid and to output a high pressure fluid;

a pressure port disposed in the main line downstream of the variable displacement pump configured to port a portion of the high pressure fluid from the main line to a bias pressure line;

a controllable valve disposed in the bias pressure line configured to meter a pressure of the bias pressure line;

a pressure actuated main piston disposed in the bias pressure line upstream of the controllable valve relative to the output of the variable displacement pump, and wherein the pressure actuated main piston is connected directly to the variable displacement pump by a mechanical linkage and configured to control displacement of the variable displacement pump at a predetermined pressure set point;

a biasing member operatively connected to the pressure actuated main piston and configured to bias the main piston to a position to achieve a pressure output for the predetermined pressure set point; and

a pressure actuated biasing piston operatively connected to the biasing member and configured to bias the pressure actuated main piston, wherein the biasing piston is in the bias pressure line downstream of the controllable valve, wherein the output from the controllable valve is a bias pressure that moves the pressure actuating biasing piston to a desired position to change a preloaded spring force of the biasing member acting on the main piston.