"Pooh Bear"

** Hello Pooh !

Personally I've never found the couple of hundred microvolts output noise
(
audio band ) to be a problem. If you're getting more than that, your
problem
is something else.


** Supply rail noise from IC regulators easily finds its way into a mic
pre-amp or other audio circuits - whether they have inherently good SVRRs
or not.

The very low output impedance of IC regulators and the existence of
bypass capacitors of various values strewn around the PCB pattern are enough
to guarantee a noisy ground. Only a microvolt or so of such noise bleeding
into the input or feedback grounds is enough to destroy the noise figure of
a mic-pre.


If to make it really quiet, considered an emitter follower output buffer
after the 317/337 with an R to the base and C to ground ? Really cheap
although you'll lose about a volt.


** A much better solution is a simple RC filter - a 10 to 22 ohm
resistor in series with each rail followed by a 1000 uf cap grounded to the
REGULATOR ground reference point will produce a virtually silent supply
rail.


BTW The published data for the LM317/337 regs suggests they are noisier
than the LM78xx / 79xx series while experience shows very much the opposite.



.............. Phil