Low Frequency Mains Noise

[ruffrecords]

On several tube mic 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?

Cheers

Ian

 

[PRR]

> The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +-20 to 30mV.

Sounds like 1/f noise.

Or rather, looks like 1/f noise; you can't hear <1Hz wobble directly.

I had a galloping case of subsonic noise in a commercial phono preamp. They put a subsonic filter in front of the amplifier. That meant the "first grid" saw an impedance which rose very high below the audio band. This "first grid" was really a BJT chip, with input current noise "negligible" in low-medium impedance, but large for large impedance. This in addition to the universal tendency for noise to rise as frequency falls ("1/f noise"). The result was ~~1Hz bobble large enough to kick-out speaker "DC" protection relays.

In most such cases, the wisest path is, I believe, to structure a low-cut filter to do minimal harm to audio yet substantially cut sub-audio signal.

There is a special case for a 2-stage R-C amp with overall NFB. The R-C cut bass, sure. The NFB flattens response to a point. But if that point covers enough phase-shifts, NFB becomes PFB and response bumps-up before finally dropping. The Dyna preamp is prone to this. 20Hz-20KHz it is as flat as anybody could ask (for the price). And in 1958 we did not have ready access to simulators or sub-Hz signal generators. The deep end of the Dyna was always loose, and when you explore 5Hz down to 0.1Hz you find a bump, and right around the warp rate of a 33RPM disk. The height of the bump is critical about tube and capacitor tolerances. (The 2-transistor Dyna has a similar thing, but it also depends on the source impedance.) And while you don't hear these bumps, they keep the amplifier operating point wandering up and down, device parameters always changing in random ways.

 

[Samuel Groner]

I think we're talking about a PSU design and not an amplifier?

If you want to reduce those effects (probably a result of slightly fluctuating mains voltage) you need a regulated PSU; just adding low-pass filters is not gone address things entirely--there's always a low-enough frequency which passes the filter unattenuated. A regulator works down to DC, so catches it all. It has 1/f noise itself, but this can be made much lower than the couple mV you see.

Samuel

 

[ruffrecords]

[quote author="Samuel Groner"]I think we're talking about a PSU design and not an amplifier?

If you want to reduce those effects (probably a result of slightly fluctuating mains voltage) you need a regulated PSU; just adding low-pass filters is not gone address things entirely--there's always a low-enough frequency which passes the filter unattenuated. A regulator works down to DC, so catches it all. It has 1/f noise itself, but this can be made much lower than the couple mV you see.

Samuel[/quote]

I am rapidly coming to the same conclusion. It just surprises me this is not mentioned in PSU design literature.

Cheers

Ian

 

[bcarso]

[quote author="ruffrecords"]It just surprises me this is not mentioned in PSU design literature.
Cheers

Ian[/quote]

Tubes have so much 1/f noise that the noise in the power supply is often small by comparison. As well, most tube audio circuits are many-times highpassed via R-C or transformer coupling between stages, and the magnitude of both PS and 1/f fluctuations reduced.

Also, for most of the classic tube circuits developed before most of us were born, making a regulated supply was a PITA. So you didn't see it done much.

I like regulated HV for tubes because it is fairly easy to do with sand, and presents one less thing to worry about.

The audiophiles will worry that they are hearing the regulator though, somehow. Well, you can place a final R-C or L-C filter before the circuit's HV connection, and try to please everyone. You won't of course.