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Living with unsolvable hum - Any audio detectives out there?

For over a year I have put up with a hum in my system, coming through the speakers (not chassis hum). I cannot make it go away. It seems to be related to the preamp because it stops when I replace the preamp. But I had my local tech hook up the preamp on his bench and it is quiet as a mouse. I've also corresponded with its designer, David Berning, who has been very responsive and helpful. But no luck solving it. I thought it may be related to the separate power supply's umbilical but David Berning said likely not. Earlier this year I even bought a star grounding component from Granite Audio and connected everything to it. Didn't work. After trying everything the engineer at Granite could think of (he was great), he was stumped too. These people have forgotten more than I'll ever know about the subject, so I gave up at that point and just lived with it. I had also tried everything they and a few knowledgeable friends have suggested (see below). But now I would like to take another swing at solving it. Any ideas? What kills me is that now I can't recall when it started, which would be very helpful to diagnose. The system sounds as good as I've ever had it now, and I LOVE the Berning preamp. So replacing it or other major components is not an attractive proposition for me.

For any intrepid detectives, here are the facts:

- Hum is typical 60 cycle sound- both channels equal volume of hum- loud enough to hear at the listening position, but just barely. Quite noticeable when standing at the rack.
- Hums with any source, not volume dependent, still hums with no source components attached (I even tried unplugged them from the wall too). But the hum stops if preamp is disconnected from amps.
- System plugs into a dedicated 20 amp line with eight plugs. Nothing else is on this circuit except my audio system. I had an electrician verify and tighten all the ground connections. The service is a relatively new 200 amp service. The electrician tested and found no ground issues or noise in the dedicated line.
- Tried shutting down all breakers in the house except my dedicated audio line. No effect, surprisingly. I had high hopes for that one!
- Tried cheater plug on everything including the preamp. No effect.
- Tried different interconnects between pre and power amps... No effect.
- Replaced all linestage tubes. No effect.
- Moved components around, moved the power supply, even used long interconnects to move the preamp three feet in front of the rack. No effect.
- Tried an extension cord to plug the preamp into a different AC circuit. No effect.
-The only thing I know of that could try, but have not tried, is replacing the power supply tubes, but I didn't bother because on the bench it made no noise for my tech.

My system:
- Power: Temporarily I'm using a Shunyata T6000 distributor (the hum existed prior to this, and the Shunyata didn't solve it). All Cardas Golden Ref or Golden power cords, except T6000 is plugged into the wall with Shunyata Sigma HC cord.
Analog: Koetsu Rosewood Signature Platinum, Jelco TK-850, Cardas Golden Cross phono cable
Digital: CEC transport and Audio Logic DAC, Golden Cross interconnect.
Preamp: Custom Berning Octal tube preamp with separate tube rectified switching power supply, built-in Jensen transformer MC stage at 24x gain (on the high side, I know, but it sounds amazing compared to other winding options)
-Power amps: Quicksilver v4 monos with KT150 tubes
-Two REL G2 subs (hum existed before them, and persists when they are disconnected and unplugged)
Somehow the interaction between the preamp and other components seems to be creating the problem. Source components don't seem to matter, but amps are Quicksilver v4 monos. Speakers are Verity Audio Parsifals. Interconnects, speaker cables and power cables are Cardas Golden Cross.
Speakers: Verity Audio Parsifal Encores. No surround sound or home theater.

montaldo
audio2design
1,351 posts
 
Stuogawa,

By "Mix 31" they mean Fair-rite 31 ferrite material. It really does not work that great at low frequencies.


Here is the material data sheet: https://www.fair-rite.com/31-material-data-sheet/

Here is an example ferrite with it: https://www.mouser.com/datasheet/2/150/2631803802-1666141.pdf

How well it works is a factor of cross-section, so the big one you listed is better, than the one I linked above, but not much better. It only has 2x the cross-sectional area. As you can see the attenuation at 100KHz is quite low. At 60Hz, there will be virtually no attenuation. The hum improvement and hence why you only got partial attenuation in your application is it attenuates the bursts of noise that typically happen every 120Hz with most electronics. It would not have attenuated 60Hz.

An isolation transformer will provide huge common mode attenuation at 60Hz. There are very high permeability common mode cores, but the listed type 31 ferrite is not one of them.
bifwynne
2,206 posts
 
@montaldo 

Picking up on the last several comments, take a look at the Google search I did on Faraday cages:  

https://forum DOT allaboutcircuits.com/threads/holes-in-a-faraday-cage.69512/

If a simple metal screen mesh can attenuate ambient 60Hz noise, that might be a simple way to solve your problem.  It seems that the posters to the thread thought that an aluminum screen might not be conductive enough.  Perhaps winding copper wire around the cage might augment its effectiveness.  Think Occam's razor!  

Techies, please weigh in.

I reiterate my suggestions that you should send your preamp back to the factory for a thorough check-up.  For all you know, a cap used to filter out low frequencies might be leaking.

I would also swap out your preamp and your amp to check for compatibility issues.

BIF  
millercarbon's avatar
millercarbon
12,531 posts
 
In less time than it takes to read this thread, let alone try out all the many ideas, you could stick a few pieces of fO.q tape on your tone arm and have a sonic improvement you can actually hear. 

Just sayin.
stuogawa
45 posts
 

No arguments with the specs and not operating at low frequencies (60 to 120 specifically); but at the same time THESE reduced man made noise in my stereo amp music environment. YMMV regardless of the Fair rite specs. More investigation on why, in my environment, noise reduction with mix 31....TBD..and curious to find out why.

I have 6 Nest powered cameras, whole house alarm with some walwarts, 6 Orbi Wifi mesh satellites, 15 SunPower panels (that each have its own micro inverter per panel, an EV transformer (adjacent the the music room and mounted in the garage, a big screen TV in the music room, a dimmer powering 6 recessed lights (house was built in 2015), 2 mac workstations in the music room. Fast and easy calculus... I wanted to test and hear if the toroid would solve my noise issue to get a North Star solution. I am not including the other 50 plus workstations, servers, ham gear, etc. that I would have to test in my environment.

We can say with high certainty: I have a 60 cycle transmission emitting in my house and that is received and amplified through the D100A and NOT the MFA amps or the WaveStream preamps.

Bottom line...my donut reduced 60 cycle noise for the D100A when no source music played, similar to Montaldo’s situation. Did it go away completely. No. Do I need to experiment more? Yes. I had these Fair rites on hand, so I used what I had. Was it cheap and easy? Yes.

If I just used the Fair-rite specs, I would agree with you....but because I had these readily available and tested it...the donuts are staying put until I can hunt and kill the other noise sources in my house. Maybe my house is the exception with all the gear I am running despite the house being newly built in 2015.

Jim Brown did a very extensive writeup on RF and discusses Fair-rite’s Mix 31 for 1500 cycles to 30 megahertz.

http://k9yc.com/RFI-Ham.pdf

Pages 21 on provide very insights and how you could modify the chokes.

He also provides cook book recipes for transformer, cap, and toroid filtering solutions in this doc.


audio2design
1,351 posts
 

stuogawa22 posts01-03-2021 3:39pm
No arguments with the specs and not operating at low frequencies (60 to 120 specifically); but at the same time THESE reduced man made noise in my stereo amp music environment. YMMV regardless of the Fair rite specs. More investigation on why, in my environment, noise reduction with mix 31....TBD..and curious to find out why.


This one is easy @stuogawa.  If you look at things like dimmers, linear amplifiers, and linear power supplies, most USB chargers, etc. they draw power in bursts at 2x the line frequency, i.e. 120Hz or 100Hz. When they make that current draw, it is a burst of high frequencies. The noise spectra ends up being 120Hz mixed with high frequencies, and harmonics of both. When those high frequencies that are harmonics of the high frequency mixed with 120Hz enter your equipment, it modulates back down to 120Hz in your audio equipment.

The ferrite will take out the high frequency harmonics so that they cannot modulate back down to 120Hz. It cannot remove common mode noise at 120/60Hz that is transferring from the pre-amp to the power amp if that is what is the cause.



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