[Ian Bell[_2_]]

flipper wrote:
On Sun, 02 Nov 2008 11:30:36 +0000, Ian Bell
wrote:

flipper wrote:
On Sat, 01 Nov 2008 10:21:18 +0000, Ian Bell
wrote:

flipper wrote:
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. 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?
From what you describe it sounds like mains noise, as you suspected,
and with the thousands upon thousands of things running, coming on and
off, or whatever, throughout the grid lord only knows what causes each
'blip'. AC mains are neither 'clean' nor stable, at least not to the
degree you're talking about.

Indeed. However, I would have expected the transformer to attenuate
noise in the 1Hz region
You have a couple of dubious assumptions here with the first being
it's a 'separate' signal. It's more likely riding on the AC with the
AC acting as, essentially, a carrier.

Yes, of course, it's amplitude modulation caused by the varying overall
load - no wonder it gets through the transformer.

The second is that the AC 'noise' has the same frequency component as
your filtered measurement.

I don't think I was assuming that, only that it contained a component at
the frequency I meausured.

That goes without saying but the point I was getting at was your
mystification on how such a 'low frequency' could get through the
transformer. It riding on the AC carrier is one way but the noise
doesn't have to be 'low frequency' to give you a 'low frequency'
looking result after filtering. Like, for example, a transient surge
or a (relatively speaking) HF noise burst can cause a 'blip' that,
after filtering, will look like a 'LF' blip because that's the filter
response.

Agreed. It should have been obvious but for some reason it just did not
occur to me.


Of course, I'm just speculating but it doesn't really matter what the
cause is because you have no control over it. I mean, even if you
could track it down to surges from a local water pumping station it's
not like they're going to redesign the things, or rearrange the power
grid, for your preamp.


Indeed. However, at first I was not certain if it was mains borne. It
might have been noise generated by the electrolytics themselves for all
I knew.

plus the five stage RC filter I am using is over
120dB down at 50Hz so even a decade or so lower I would have expected
its attenuation to be significant.
I'm sure it is 'significant', which gives you an idea of just how bad
it is on the AC mains itself. It isn't a 10mV blip out there.

Which means I could probably see it on the mains side - where's that
scope probe?

Maybe. But you don't really know what to look for nor, since you say
it's random, 'when' to look.


Indeed. I have now built a new smoothing section with a 470uF reservoir
followed by five RC stages of 1K and 470uf each and the LF noise is now
below the broadband noise.

Cheers

Ian

With it out of band and low I'm not sure what you're trying to fix but
the common choices are to filter till it's below whatever tolerance
you decide upon or regulate.

I wreaks havoc with distortion measurements at 100Hz when you are
expecting a result below 0.1%.
Sounds like maybe a measurement problem because out of band signals
shouldn't be in the measurement.

Cheers

Ian