[PeterD]

On Fri, 31 Oct 2008 11:01:16 +0000, Ian Bell
wrote:

On several preamp PSUs I have built in that last couple of years I have
noticed the same thing. A very low frequency low level variation in the
output voltage. My latest PSU uses a series of five 100uF caps with 1K
resistors in between and it still exhibits this problem. The 'noise'
seems to be below 1Hz in frequency, fairly random and peaks typically
between +- 10mV with occasional excursions to +20 to 30mV.

Because the noise is so small, about the only place you can see it is on
the smoothed output with a scope set to ac input.

What kind of scope are you using that when set to AC input will show a
less than 1 Hz ripple?

I suspect this noise
is mains borne but I don't know how to see such small low frequency
signals on the mains itself. Any ideas what it is, how to look at it on
the mains and how to get rid of it?

Cheers

Ian