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
I think checking the main a second time is a good idea. Neutral and grounds. Do I think there is a problem in the main? NO. If your confident with you’re electrician.

Electricians and mechanics are NOT in short supply in my family, my brother retired as the local BA in Martinez CA. 12-15 last count. 4 HD local 3 mechanics.. No shortage of skilled labor, or muscle :-)

ALL of them learned what "tight" is from Uncle Scotty..(ME)
ALL were taught how to torque properly..

I use a Variac when I run into things like this.. Most electricians DON’T have them. Most old audiophilers DO.. You have a voltage difference between a "piece" of equipment and usually ALL the rest. or You have an old piece that need to percolate for 10-40 hours to tighten up the old caps.. So they just don’t leak all over..

Tube rectified in a preamp? MATCHED and checked for noise. NO excuse, on that one... You have to make sure it's not noisy at YOUR house... Not the Tec....Your house.

audio2design, mentioned an isolation transformer. A variac is, with the added feature of being able to VARY the voltage.

I suggest (only for a test) to very the voltage from 110 all the way to 122 VAC, SLOWLEY, and see if the noise goes away.

1. I would recheck MY main. I don’t trust anyone more than me.. NO
ONE. Check for 120 (A) 120 (B), 240 between. To ground less than 1% of the total being checked..

2. I would make sure I had a good PC going to my Pre amp. I’d change the PC no matter what. Just to test... I would pull the fuse clean the pocket with Isopropyl, LET IT DRY and inspect the HECK out of the fuse holder look for carbon trails.. LOOK close.. Reinstall a new fuse, a busman will do. NO fancy fuses for now...

3. The valves for the PS you know where the noise is coming from.. those valves would be changed (just for a test) MATCHED and checked for noise. NO EXCUSE on that one for sure. The pins and pockets on ALL the valves need to be cleaned and reseated, and pin pockets need to be checked for tightness and brushed and cleaned.

All this takes about 1-2 hours, from the mains to cleaning everything..

I would test NOW, did you get lucky, no luck go to # 4

4. I’d smoke a little weed about now.. :-)

Mind you, after everything you did THINK.

Clear your ears and

5. Hook up the Variac a DMM and turn it to 100 volts and go from there up to 122 or so.. You have to have a DMM, (no analog). LISTEN.

If it goes away, because of a voltage difference, LOL don’t be surprised.
If it doesn’t go away don’t be depressed.. Closer than you think..

Regards..
@montaldo,
Just connect the cables at preamp output without connecting to poweramp and with the multimeter check for continuity at exposed plug end between the signal pin and its earthing. If multimeter shows continuity then you have it. If not you eliminate that possibility.
G
Wow thanks for all the ideas. If the pre power supply tubes could cause hum, I should check them and sockets. That is one thing I have not done. I could just replace them. So are you saying that unmatched power supply tubes could create hum like this?
You have already tried many things and gotten a lot of good advice so I will offer a few short items.
First, did David Berning offer you another pre amp?  For starters, it would be interesting to see if the problem continues with an identical model.  If it doesn’t, then you could make some sort of deal with him about swapping your current one for the replacement.
  Second, I don’t know what a cheater plug is, but I have had a few hum problems many years ago.  One was solved by a PSAudio device that turned hum into a light (nightlight).  It was the size of a match box, attached to the power strip, and the first hum problem that I had disappeared completely.  A few years later with changing components a nastier hum wasn’t defeated by this some I bought something called a “Humbuster” from Sweetwater.  This required plugging the problem component in to the humbuster and then plugging that into the power strip and was entirely successful.  I couldn’t tell if the SQ from the component in question was affected