A difference basis with respect to $n$ is a subset $A \subseteq \mathbb{Z}$ such that $A - A \supseteq \{1, \ldots, n\}$. Rédei and Rényi showed that the minimum size of a difference basis with respect to $n$ is $(c+o(1))\sqrt{n}$ for some positive constant $c$. The best previously known lower bound on $c$ is $c \geqslant 1.5602\ldots$, which was obtained by Leech using a version of an earlier argument due to Rédei and Rényi. In this note we use Fourier-analytic tools to show that the Leech--Rédei--Rényi lower bound is not sharp.