  We introduce a large N version of the spin quantum Hall transition problem.
It is formulated as a problem of Dirac fermions coupled to disorder, whose
Hamiltonian belong to the symmetry class C. The fermions carry spin degrees of
freedom valued in the algebra sp(2N), the spin quantum Hall effect
corresponding to N=1. Arguments based on renormalization group transformations
as well as on a sigma model formulation, valid in the large N limit, indicate
the existence of a crossover as N varies. Contrary to the N=1 case, the large N
models are shown to lead to localized states at zero energy. We also present a
sigma model analysis for the system of Dirac fermions coupled to only sp(2N)
random gauge potentials, which reproduces known exact results.
