  The Cosmology Large Angular Scale Surveyor consists of four instruments
performing a CMB polarization survey. Currently, the 40 GHz and first 90 GHz
instruments are deployed and observing, with the second 90 GHz and a
multichroic 150/220 GHz instrument to follow. The receiver is a central
component of each instrument's design and functionality. This paper describes
the CLASS receiver design, using the first 90 GHz receiver as a primary
reference. Cryogenic cooling and filters maintain a cold, low-noise environment
for the detectors. We have achieved receiver detector temperatures below 50 mK
in the 40 GHz instrument for 85% of the initial 1.5 years of operation, and
observed in-band efficiency that is consistent with pre-deployment estimates.
At 90 GHz, less than 26% of in-band power is lost to the filters and lenses in
the receiver, allowing for high optical efficiency. We discuss the mounting
scheme for the filters and lenses, the alignment of the cold optics and
detectors, stray light control, and magnetic shielding.
