A turbocharger, comprising: a turbine mounted on a rotatable shaft for rotation about an axis of the shaft; a radial compressor comprising a compressor wheel mounted on the shaft and disposed within a compressor housing, the compressor wheel comprising a first impeller and a second impeller each having a hub and a plurality of compressor blades extending from the hub, a back side of the first impeller being adjacent a back side of the second impeller such that the impellers are in a back-to-back arrangement and air enters the impellers in oppositely directed axial directions, the compressor housing defining an inlet to the first impeller and defining a common volute that receives pressurized fluid from the first and second impellers via a common diffuser leading from the respective impellers into the volute, the compressor housing defining a first shroud that is adjacent tips of the blades of the first impeller, and wherein a second shroud is adjacent tips of the blades of the second impeller; a center housing mounted between the compressor housing and the turbine housing and containing bearings for the shaft, the center housing defining an inlet flow path for the second impeller; a shroud ring mounted between the center housing and the compressor housing, wherein the shroud ring defines part of the second shroud and the center housing defines the remainder of the second shroud, the shroud ring further defining one wall of the common diffuser; wherein the second shroud defines at least one bleed port therethrough and the center housing at least partially defines a bleed flow passage leading from said at least one bleed port into the inlet flow path for the second impeller at a position upstream of said at least one bleed port such that fluid can pass in either direction between the inlet flow path and the second impeller via said at least one bleed port and said bleed flow passage, wherein said at least one bleed port is formed by a generally annular space defined between the shroud ring and the center housing and wherein the center housing is a one-piece integrally formed structure. a turbine mounted on a rotatable shaft for rotation about an axis of the shaft; a radial compressor comprising a compressor wheel mounted on the shaft and disposed within a compressor housing, the compressor wheel comprising a first impeller and a second impeller each having a hub and a plurality of compressor blades extending from the hub, a back side of the first impeller being adjacent a back side of the second impeller such that the impellers are in a back-to-back arrangement and air enters the impellers in oppositely directed axial directions, the compressor housing defining an inlet to the first impeller and defining a common volute that receives pressurized fluid from the first and second impellers via a common diffuser leading from the respective impellers into the volute, the compressor housing defining a first shroud that is adjacent tips of the blades of the first impeller, and wherein a second shroud is adjacent tips of the blades of the second impeller; a center housing mounted between the compressor housing and the turbine housing and containing bearings for the shaft, the center housing defining an inlet flow path for the second impeller; a shroud ring mounted between the center housing and the compressor housing, wherein the shroud ring defines part of the second shroud and the center housing defines the remainder of the second shroud, the shroud ring further defining one wall of the common diffuser; wherein the second shroud defines at least one bleed port therethrough and the center housing at least partially defines a bleed flow passage leading from said at least one bleed port into the inlet flow path for the second impeller at a position upstream of said at least one bleed port such that fluid can pass in either direction between the inlet flow path and the second impeller via said at least one bleed port and said bleed flow passage, wherein said at least one bleed port is formed by a generally annular space defined between the shroud ring and the center housing and wherein the center housing is a one-piece integrally formed structure.