[sdiy] lm317t diferential input output voltage

[dragons]

Hi,
I just wondered if anyone knew what the lowest difference between the 
input and output voltage should be on a Lm317T ?
I'm working on a piece of equipment that I haven't got the correct mains 
transformer for, and it seems that the temporary transformer I'm using 
is slightly too low in voltage, causing the dc input too be slightly too 
low, which causes ripple on the regulated output, therefore causing hum 
and noise on the audio.
regards Peter B

( P.S. I did look at the data sheet but couldn't find any figures, maybe 
I wasn't looking carefully enough )