What might cause a 20hz noise?

[Bowie]

  Tried searching this but I get a lot of unrelated subjects.  I was checking out the noise floor on my most recent LA-2A build and the floor is low (-70dB on my converters, -85 in the DAW's metering) but, when looking closer, the little bit of noise that exists is mostly around 20 to 30hz.  It jumps around by about 7dB but is pretty consistent whether it's racked, unracked, lid open, etc.  The 60 cycle noise is actually really low, lower than any LA-2A I've used and about 10dB underneath this 20hz noise.  I'd like to figure out why there is anything going on at 20hz, even if it is really low in the noise floor.  Build is P2P, Nicholson's in the power section, with values as stated in the Cayocosta layout. 
Thanks.

 

[emrr]

Definite tone, not just tube shot noise?

 

[JohnRoberts]

A steady noise might be a beat frequency.. I recall seeing a 1Hz noise from two computer monitors in adjacent offices beating against each other.

A jumpy noise might be I/F (shot?) noise.

JR

 

[iomegaman]

For me its called tinnitus but it doesn't show up on a screen because in truth its not really there...but like  John said this sounds like monitors or perhaps something in the test circuit itself...although I've had Nicholsons that passed but were borderline...(decoupling caps) just enough to add some noise.

 

[ruffrecords]

You say it jumps about a bit and its at 20Hz. First  I expect 20Hz is the limit of your measuring system so anything below this will be recorded as 20Hz. Second, the jumping around is typical of the very low frequency noise you get from tubes and simple RC HT supplies. I have in the past AC coupled a scope to my very well smoothed HT line and it jumps about a little bit, quite possibly due to fluctuations in the mains voltage.

Cheers

Ian

 

[Bowie]

Definite tone, not just tube shot noise?

A steady noise might be a beat frequency.. I recall seeing a 1Hz noise from two computer monitors in adjacent offices beating against each other.

A jumpy noise might be I/F (shot?) noise.

JR

For me its called tinnitus but it doesn't show up on a screen because in truth its not really there...but like  John said this sounds like monitors or perhaps something in the test circuit itself...although I've had Nicholsons that passed but were borderline...(decoupling caps) just enough to add some noise.

I don't know enough about shot noise to say, though it looked stable enough to be a tone.  I don't see any pulsing, though the meters and software might not be sampling fast enough to show that.  Just a little wavering of about 7dB on an already low-level noise.

You say it jumps about a bit and its at 20Hz. First  I expect 20Hz is the limit of your measuring system so anything below this will be recorded as 20Hz. Second, the jumping around is typical of the very low frequency noise you get from tubes and simple RC HT supplies. I have in the past AC coupled a scope to my very well smoothed HT line and it jumps about a little bit, quite possibly due to fluctuations in the mains voltage.

Cheers

Ian

Yes, the DAW's analyzer I used starts at 20 Hz and the peak starts at 20 so it could possibly be coming from an even lower, louder frequency.  I have my audio gear all powered by a balanced isolation transformer and the first check I did was powering up the Sta-Level that sits right below this unit and the 20hz wasn't there.  There are two PC's at the end of my rack, literally 2' from these units.  Moving it around didn't alter the noise though. 

Thanks for all the replies so far!  This isn't really causing any problems but the perfectionist in me wants to chase it down and my insatiable curiosity wants to know what's causing it.  If I should forget about it, let me know or I'm going to spend hours trying to figure it out.

 

[gyraf]

How is your frequency response below 20Hz? The L/C coupled LA2 output stage may very well have a relatively large frequency response bump at low-cutoff point, which would translate to more noise gain in that area..

Jakob E.

 

[ruffrecords]

If you have an oscilloscope I would recommend you use it to look at the output. You should easily see if there is a tone there or low frequency noise.

Cheers

Ian

 

[Markus_Krippner]

Could it have something to do with DC-Offset ?
Afaik, using an audio analyzer, a noise floor around 20-30Hz might be a sign for a DC-Offset problem.

...markus