  An extensive investigation of low frequency noise in single electron
transistors as a function of gain is presented. Comparing the output noise with
gain for a large number of bias points, it is found that the noise is dominated
by external charge noise. For low gains we find an additional noise
contribution which is compared to a model including resistance fluctuations. We
conclude that this excess noise is not only due to resistance fluctuations. For
one sample, we find a record low minimum charge noise of qn = 9*10^-6
e/sqrt(Hz) in the superconducting state and qn = 9*10^-6 e/sqrt(Hz) in the
normal state at a frequency of 4.4 kHz.
