A split-Stirling cryorefrigerator including a cylinder containing a movable displacer containing regenerator material and having an attached movable rod with a free end located in a housing enclosing a gas-spring chamber containing a gas. The cylinder has an orifice near its warm end which is in fluid communication with a gas source having a cyclic (typically sinusoidal) pressure. The movable rod slidably and generally sealably engages the housing and the warm end of the cylinder. The cryorefrigerator also includes a mechanism for adjusting the void volume of the gas-spring chamber, such adjustment allowing for increased cooling capacity of the cryorefrigerator when the amplitude of the cyclic pressure of the gas source is increased.