  The Variable S-matrix approach offers a unique way to extract low energy
threshold parameters for a given NN potential. We extract those parameters for
the np system from the NijmII and Reid93 potentials, to all partial waves with
total angular momentum $j \le m5 $, using the generalized effective range
expansion % $$ ({\bf f}^{sj})_{l,l'} k^l k^{l'} = -({{\bf a}^{sj}}^{-1})_{l,l'}
+ \frac12 ({\bf r}^{sj})_{l,l'} k^2 + ({\bf v_2}^{sj})_{l,l'} k^4 + + ({\bf
v_3}^{sj})_{l,l'} k^6 + + ({\bf v_4}^{sj})_{l,l'} k^8 + ... - {\rm i}
k^{l+l'+1} $$ % where $ {\bf f}^{sj} = ({\bf S}^{sj} - {\bf 1}) / (2 {\rm i} k)
$ is the scattering amplitude and ${\bf S}^{sj}$ is the unitary S-matrix in
coupled channel space with total spin $s$ and total angular momentum $j$. Our
calculation includes all the relevant contributions of the full amplitude to
order ${\cal O} (k^8) $ in the CM momentum. We also discuss the validity of the
generalized effective range expansion in the region of analyticity $ k \le
m_\pi /2$.
