[quote author="toby"] i understood this preamp had a low input impedance?[/quote]
Load impedance, what the microphone "see's" and source impedance, what the first stage of the amplfier see's are two different things.
In the case of a mic pre with a transformer on the input side, the turns ratio squared is the impedance ratio. So, for 1:10 transfomer, the impedance ratio is 1:100.
So, let's say the source impedance (the mic) is nominally 200 ohms, this gets transfomed to 200 x 100 = 20k. This is the source impedance that the first stage of the amplifier see's. It is the first stage of the amplifier that determines the noise.
From the mic's perspective, the load impedance that it see's is transformed at an equal ratio. If the amplifier has an input impedance of, say 100k, this get transformed down to 100k / 100 = 1k. So the mic is loaded by 1k.
And like NewYorkDave said, the "free" voltage gain is a bonus (or curse in case of hot signals) rather than a reason for the transformer to be used.