Living with unsolvable hum - Any audio detectives out there?

Hum is frustrating  !!!    Nothing worse than hearing that during a quiet passage.   A while back I had a problem where it was not a constant hum.  It was intermittent.    Well after doing all the  usual tests and remedies ,it turned out to be one of those Glade plug in air fresheners.   Threw those in the trash and problem solved.   

As good as Jensens are, any transformer will introduce sonic artifacts which vary with the equipment either side of them.

Take a look at the below troubleshooting guide for this type of problem.

https://www.jensen-transformers.com/wp-content/uploads/2014/08/ts_guide.pdf

It's a no brainer that 60Hz hum is likely a ground loop.

A Loop Break in the amps from Earthing (Grounding) Your Hi-Fi - Tricks and Techniques (sound-au.com) may solve the issue and keep the system safe.

!!_DO NOT RUN THE SYSTEM WITH THE SAFEY EARTH DEFEATED_!!

Bingo.

I have a ground loop problem....at the very end of the system chain and NOT caused by other equipment (the other gear were very good shiny object distractions).

Performed systematic changes as previously described.

As soon as I got to the D100A amp with and without the 3 to 2 power cord convertor....60 cycle speaker buzz gone/appears.

* Using same existing wall outlets as before; no changes for scientific method approach.
* No donut / toroid in the mix
* Amp plugged straight into wall outlet with no conditioners/regenerators.

I cannot confirm (and I don't feel like sleuthing this deep), but my music room wall shares a wall with the garage. Even though all garage outlets use a separate and dedicated fuses, I am wondering if the wall mounted Electric Vehicle transformer on that shared wall, the garage refrigerator along that shared wall, or the tankless water heater along that shared wall are tapping into the same ground.  

Regardless....I have zero 60 cycle hum from my speakers when I lift the ground for the D100A.  

Montaldo, my hunting is done.  I have given you one process / approach.

Happy hunting, Mr Montaldo.

-stu

Test 1:

Pulled Audio Research D100A out of rack. Put amp into my garage workbench away from all RF devices.  Plugged amp right into wall outlet. No donut between amp and wall outlet.  Plugged speakers right into amp.  Zero 60 hertz cycle. Amp is fine as Scott mentioned.

Test 2:

Plugged my old iphone 6 into patch cable with 1/4 inch to RCAs.  Plugged RCAs right into amp.  Before turning on iphone, checked speakers with amp. Zero 60 hertz cycle.

Test 3:

Turned on iphone with no music at low volume level.  Checked speakers with amp. Zero 60 hertz cycle.  Turned iphone volume to max. Zero 60 hertz cycle.

Test 4:

Turned on iphone with music...listened specifically between music gaps = zero 60 hertz cycle.

Results

Amp is quiet when it is by itself and NOT near equipment.  Using garage wall outlet power and no line conditioner / regenerator. Using no donut toroid.  This is baseline for the amp in an isolated environment so I don't need to further test/discuss the amp power supply, caps, transformer, etc. in my situation. DONE.

Next Step For Ground Loop Tests

Perform lifted ground technique per Scott.  Lifted ground = all
equipment in stereo rack plugged into wall outlet WITHOUT ground
 = insert a small 3 prong to 2 prong (plus wire for ground)

https://www.homedepot.com/p/GE-Polarized-Grounding-2-Prong-Adapter-Orange-20-Pack-46851/312551290?NC...

 into ALL stereo / related devices in rack EXCEPT for the most sensitive pickup device. In this case, this would be my phono tube amp.  Phono tube amp is plugged directly into wall outlet WITH normal 3 prong ground   = only ONE source for ground across all equipment to baseline all equipment.  Listen for 60 hertz cycle. 

If 60 hertz cycle through speakers....then probably not a ground loop issue and a local proximity 60 hertz transmission being picked up by the phono power cord.  If this occurs, then I will purchase clamp on ferrite and place on the amp.

https://www.amazon.com/Roctee-Suppressor-Electronic-Ferrites-Diameter/dp/B07XCMY6LT/ref=sr_1_12?dchi...   

If no 60 hertz cycle through speakers with single ground source (phono amp), then systematically begin removing the 3 to 2 power connectors.  Remove one at a time. Remove from the most sensitive to least sensitive gear.  In my case I would then remove line amp 3 to 2 connector, since that is next in the chain after the phono amp, which plugs into line amp.  
Test for 60 cycle, and continue proceeding.  

If 60 cycle surfaces, then that device "may" be the one or one of several contributing noise generators.

Results to follow.

Does one connected power amp only hum? i.e. completely remove one amp from the system.
Try each amp. One amp may be quieter than the other.

If no hum with only one amp, try reversing power transformer input leads on the other power amp. 

I had a similar issue with factory buggered, err built, Tubes4HiFi / VTA M-125s
See  ieLogical VTA M-125

Ok before you go out and buy anything, do this first!

Take a good look behind your equipment. Look at all the power and signal cables. Make sure all of them cross each other at about 90 degrees. No parallel cables should be touching or near each other.

Anything running parallel should be at least 12" apart...better if it is 18".

To some degree, reorganize your components to accommodate better cable management.

Undo looping cable. Use shorter and longer power cables to tailor to the need.

If you need, use small blocks of wood or zip ties or velcro to create separation and organization.

Also get rid of wall warts and non audio equipment connected to the line.

Separate the analog stuff from the digital stuff. They should not share the same line.

This worked for me...and I have 2 dedicated audio lines. The noise appeared when I quickly and carelessly changed out my power conditioner for another. The noise is most likely coming from your own equipment.

Look inside any point to point wired preamp or amp...everything crosses at 90 for this same reason.

Might be gain sensitivity on the amp- must be too high and therefore picking up an otherwise inaudible hum. Ask Bering about padding it down perhaps.

This may sound crazy but have you lifted the ground on one channel at the power amp or, alternately, the pre-amp?  Kerry_

Lifting the signal ground is a fine idea, when there are multiple signal wires.

These are mono amps, so I would suggest the OP try cheater plugs on one or the other amp first, and if that works, get a hum buster (or whatever they are called).
Actually, the OP should unplug 1 mono amp from the power completely, and see if this doesn't fix it.

A common wiring arrangement in pro audio to stop ground loops is to lift the ground of interconnect cables at one end. We did it all the time building amp racks and outbound racks. We would lift the ground on the inputs to the power amps when linking them together and from the outputs of the crossover/processer. Easy to do with balanced XLR connectors. We always did it on the input end of the cable. We also never lifted power grounds, it was forbiden by my boss at the time.  I have never done it with unbalanced RCA cables but it is worth a try.

Alan.  

I have not checked/cleaned the preamp fuse. Will do that first, as well as disconnecting wifi and other simple tests suggested, such as trying a backup amp to see if the interaction with my amps is a problem. Then check pre tube pins seatings. Then will see if I can take it to a friend's house. if it hums at friend's, maybe replace power supply tubes. Then will embark on other ideas mentioned.

Can't thank all you guys enough for your time and ideas!

Cable tv was the culprit in my system. Put a $15 isolator between cable and box, quiet as a mouse!!

OP before I read through this, did you pull the valves tighten the pin pockets and swap the rect valves? Did you pull the fuse and inspect the fuse pocket, clean it and replace the fuse. Did you replace the PC? Just asking?

Regards

Amp no likey the preamp!  

After 40 yrs in the communication business, there are 2 main sources of 'hum'. The first obvious one is an extraneous ground. The 2nd and most difficult one is an unbalance line load. All of the suggestions here are very good ones indeed. I read most of them and don't recall a suggestion or simply running a temporary power supply from the initial termination (from the power company) directly to your equipment. Make it as short as possible. Shielded wiring just laying on the ground if necessary, as short as possible. That would eliminate beaucoup  possibilities. Doesn't take much resistance to duplicate your problem. Secondly, just a minute amount of carbon build-up on a selector switch can also duplicate your trouble. Don't recall how old your preamp is, but clean, clean, clean.  Again, I apologize if these are duplicate suggestions, I did not completely read all postings. Simplify the power path. If you have performed most of these suggestions and resolved nothing, plz see oldhvymec suggestion, take a toke and just hum along...good luck. AB

I had this same problem years ago with a BAT VK30. I tried everything in your list, Drove me crazy. I called the BATmeister who replied "my preamps don't hum", and hung up. I even bought a PS Audio Humbuster which changed nothing.So I left it with a very good audio tech. He called me back and said he had put it on his test bench and it was better than spec. I responded that the sound seemed to come mostly from the area of the right channel toroid transformer. He said, "ah so, I know about that kind of problem, let me call you back". He called back 10' later and said it was fixed and I could pick it up. He loosened the fixing bolt and rotated the transformer on axis for minimum hum and checked the left channel also. I picked it up and put it in the system... dead quiet.

I left a message with Chris at https://www.kf7p.com/KF7P/Welcome.html
and will be chatting with him to see if he has any other customers that experienced similar challenges.

I have as much interest as Montaldo to solve this similar issue.

----

For my grounding, I have two 6 foot copper rods driven into the ground and wired these only to the D100A.  This did not solve my issue.

And recall, I recently had the D100A power supply filters caps replaced (past year) and no 60 cycle noise on tech's bench similar to Montaldo. 

I have an email in to Scott Frankland to talk about ground loops.

backtothe80s. "The fix was to properly ground the cable splitter at the point of ingress to my home."

montaldo. "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"

Best so far:

Happy hunting, Mr. Wick. 

https://www.youtube.com/watch?v=PjFKALOxB0U
For one gold coin I will eliminate your problem. But it will have to be done under the table.

Montaldo, it is always your choice to live with the hum. To me it would be like having splinter. 

Since hum is not affected by volume check the umbilical chord also for any broken pin or bad connection.
G

I had a very similar issue years ago, and it drove me crazy for a while.  The fix was to properly  ground the cable splitter at the point of ingress to my home.  The inbound cable and electric  services entered my garage at the same place.  Grounding the splitter solved my hum completely and permanently.   I'm  not suggesting this is definitely  your problem,  but it's a simple fix to try.

Could be worth a try, but!
Only difference with your situation is your hum transfers to speakers.
My amp has ten power rectifiers for dealing with design inherent or dirty-power before signal output which shut the problem down.
That said the amplifier hum was a real problem until I inserted a holton dc blocker. 
Various mains power conditioners just didn't work!
I learnt that once you get up to certain sized toridal transformers mains power can become a significant problem. My amp has a 400va pair of transformers.
As per its' instructions the mains distributor is fed from the blocker unit, which is mains plugged to the wall.
This reduced the hum to negligible audibility. I also got an acoustic advantage across the dynamic range with particular reduction in the high frequency band.
Hope this offers solution, cheers

Literally the first line in the ops post:

For over a year I have put up with a hum in my system, coming through the speakers (not chassis hum).

wsrrsw "Did you try not using power conditioner?"

montaldo "I'm using a Shunyata T6000 distributor (the hum existed prior to this, and the Shunyata didn't solve it"

oldears "You’re looking under the wrong rock:It is not an electrical hum problem, it is mechanical and coming from your preamp power supply. The S:N test of your preamp found no problem. You hear it next to your rack."

montaldo "But I had my local tech hook up the preamp on his bench and it is quiet as a mouse."

Stu - Assumed montaldo gave his local tech both the pre amp and the pre amp power supply....but I could be completely wrong.

Maybe this was mentioned and I missed.

Did you try not using power conditioner? Did you plug pre directly into wall?

     I looked at the advice in the posts here, both good and bad. So far, there is no mention of trying a line level audio transformer inline with either the output of your pre amp or (one at a time with all other inputs-Disconnected) with one of the inputs. This is mostly to discover where the source of the problem lies, but might actually be a good enough fix sonically as to keep it in place. Sorry, I do not have a lead on who makes a fine quality unit for this. It will however isolate either the input or output of your unit.

You're looking under the wrong rock:It is not an electrical hum problem, it is mechanical and coming from your preamp power supply. The S:N test of your preamp found no problem. You hear it next to  your rack.
Use an old mechanic's (weight doesn't matter) trick to imitate a stethiscope. Take a screwdriver. Place the plastic handle at your ear and touch the chassis of your preamp power supply to see if you hear the hum. If so, then the problem is there; probably due to a poor orientation of a toroidal transformer, or possibly bad rectifier tubes.

BIF suggested:

"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."

No push back on this idea..and expanding on your approach....

Montaldo should just take his preamp to a friends house and test it. No shipping charges. Safer. No minimum bench cost to open up the preamp.  If preamp has noise at friend's house, then send preamp for sure.  No use spending money if you can do a quick and easy test at friend's house.

If preamp has no noise, then Montaldo has to hunt and kill the noise source....or suppress the noise.

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.

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.

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.

@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  

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.

BTW,  use the power extension cord technique (cheap and easy).. and use it with your Cardas...screw how much you paid for Cardas...go to home depot and get a 12 foot 12 guage extension and wind a donut.  If this partially solves the problem...do you really care how much you paid for the Cardas but got rid of the annoying noise?  Or do you want a rich warm sound with an annoying 60 cycle hum underlying everything?  

Not evil...I am just curious.  ;)

Couple more thoughts and experiments:

* I did a brief read on Shunyata and their power distributors. I might have missed it, but they did not call out specifically how their internal distributors actually work and corresponding components. It might be worth doing a YouTube search to see if someone has opened up a T6000 to actually see what components Shunyata is using in your distributor. The website refers to toroids, transformers and capacitors, but it is really unclear what actual parts are used in this device. If no one has opened up their T6000...be the first to do it and see if they have transformers, caps, toroids or if this is merely a "power strip"

* Cheap and easy....borrow an AC line conditioner, regenerator, etc. from a friend to see if it makes a difference, for your preamp. As usual, scientific method (plug this borrowed line conditioner into same outlet that your preamp is currently using and only plug the preamp into this device. If no difference, then one by one add more of your stereo components. You are trying to see if another device (close proximity) is creating the noise.

* Remember....as I found, multiple number of man made /house appliance and device RF noise sources could exist and no silver bullet. As audio2designs reminded me, even devices such as dimmer switches can be a source of noise transmission that is picked up and amplified by your preamp.

* The torroid I specifically used (and was recommended), mix 31, is designed to trap man made noise low frequency such as transformers, walwarts, etc. If you do a quick google search to see what other people have done and their results, you will see pretty good results. Not a 100%, but again like insurance, you have to have layers of filtering for pesky noise inducing situations. That is why I could easily hear reduction in noise in my music room. Will I have to explore other possible capacitors, transformers, toroid mixes? Yes. Will only one component solve all MY issues....nope. Hence, keep an open mind to the different possible noise trap solutions and the different sources of noises...which leads me to....

* A cheap and easy way to "hunt" for man made / house noise is using an old battery operated pocket AM radio. Go outside away from your home and away from household noise inducing devices. Set the AM radio to the top of the band, 1600 (1600 kilohertz), or something close to that range that does not have a broadcast signal. Listen and learn for the speaker noise and pitch on that non broadcast signal. Now go back into your stereo room and put the radio near each transformer, device, light switch, light, computer, etc.. Your going to use this device as a noise transmission detector.

Slowly turn the radio 90 degrees relative to the device and within 6 inches from each household device, switch, power strip, walwart, etc. If you hear a change in pitch/noise.....your on to something. If it is a light dimmer, for example, maybe it is an old dimmer that needs to be replaced. If it is a new device, then you will need to get creative on how to eliminate the noise transmission from that device. In my garage I have a battery tender for my car. The battery tender is relatively new but it is located near my ham gear. I A/B tested (before toroid / after toroid) with the toroid and noticed a small but perceivable reduction in noise. Again, you may have several different devices that contribute to the noise so you have to prepared to hunt for different noise sources and use different techniques to suppress / kill noise.

Happy hunting, Mr. Wick.

Wow Evil Stu thanks for all that! I don't have much slack in the power cord and it is heavy-ish Cardas Golden Ref. Hmm.
Good Stu

2 items I haven't seen mentioned:
1-Did you move your system to a new room?  I know it's not easy to say the least.  You could run a really long extension cord from another side of the house?  Not sure it will help.
2-Did you try different amps.  It might just be a compatibility issue.  Don't use that junk receiver but borrow a nice high output power amp.
Your system has great components but that doesn't mean the kids will all get along well.

The issue is the ferrite toroidals are not very effective at 60hz. More likely they take out the effects of 120Hz events, like dimmer turn on but do little about 60Hz fundamentals. Hence a small isolation transformer on the preamp as a better test.  Attenuation at 60hz with that ferrite toroid will be near 0.

Google photo link to some of my ferrite "donuts"

https://photos.app.goo.gl/GPdtYtB66gNJPtP49

The "donut" on the carpet with the black power cord picture is what I am experimenting with for the D100A amp.

You may need to start hunting and unplugging other household devices (walwarts, computers, lights with transformers, etc.) as you listen to your stereo system (scientific method, one by one approach)....experimenting is fun!

Hi Montaldo,

You received good advice to "process of eliminate" the possible 60 hz hum and possible narrow this to the preamp.

One of my other hobbies is amateur radio and after reading your trails, this reminded me of what I have been doing for my stereo system, which like you, I did all the same recommendations you did to no avail.

In amateur radio, we use common mode chokes (CMCs) to eliminate man made noise. These are VERY VERY easy and cheap to build (buy the right toroid donut, wind your coax (for antennas) or your power cords (as in your preamp power cord around the toroid donut....plug and play...you will either solve it or not.

I had a LOT of man made noise entering my ham radio receiver (noise signal 7 to 10)...it was ridiculous...but I have have solar panels over my garage (where my ham gear is; and also close to my music room with the stereo gear, btw), over 75 computing / wireless devices in my house (I am in AI neural research for Alzheimer’s and build a lot of stuff).

I first replaced 7 shops lights in my garage that had the old ballast and replaced with LED non ballast shop lights. Ballasts transmit a LOT of noise into all radio gear. Man made noise reduced to signal 5 to Signal 7, but still very unacceptable. I then installed mu metal covers over my solar convertor boxes and EV vehicle transformer boxes (located in the garage). Not much help.

I then asked for help on eham.net (my ham radio Internet club); the older guys told me to try building inexpensive toroids for my antenna coax (transmission line) and my power cords that power all my ham gear.....this made a big difference...i am now down to normal atmospheric noise with Signal 1 to 3....more than acceptable.

After going through these experiments, I decided try using my spare toroid donuts with my stereo gear (which happens to share an adjacent wall to my garage...go figure).

Stereo Equipment

SOTA turntable / ET2 tone arm / Shelter 901 mk3 ---> Wavestream Kinetics phono tube preamp --> Wavestream Kinetics line tube amp --> Nelson Pass passive analog crossover (bi amp output):

--> Nelson Pass full range / mid crossover outputs --> Moore Franklin Associates (MFA) 200Cs tube amps --> Linkwitz LX Mini full range speakers

--> Nelson Pass low/mid crossover outputs --> Audio Research D100Bs solid state amp ---> Linkwitz LX Mini mid/low driver.


Experienced the 60 cycle hum occurred as you have experienced.


Like you, I narrowed down the equipment issue and eventually all data pointed to the Audio Research D100a amp as the "possible cause". I eventually turned the D100A into Scott Frankland (of MFA fame and now Wavestream Kinetics fame...he still actively building and repairing gear in San Jose, Ca...not far from me). He changed some of the power supply filter caps and didn’t get any more 60 cycle hum noise on HIS bench...except when I put the D100A back into MY house/system...like you, I experienced the 60 cycle hum.

After performing my ham radio ferrite donut noise reduction experiments, I thought, what the heck, I have two spare ferrite donuts...I am going to wind the D100A power amp cord around one of the ferrite donuts.

Sure enough...I reduced about 60% of the 60 cycle hum = my power amp cord is acting like a receiving antenna and picking up noise either from all my research equipment, wireless devices, or computers in the house/garage or other stereo gear.

The ferrite donut is a common mode choke (CMC); these have been around for a LONG time and are used in many of the power supply conditioners.....

Below is the pdf link to common mode chokes (this is an amateur radio piece so you will read colloquialisms pertinent to two way (transmit /receive) equipment; don’t be scared or concerned). Document is on what, why, where, and how to build common mode chokes.

http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

Read page 7

Why Use Common-Mode Chokes? The most common reasons for using common-mode chokes are: (1) to reduce the fraction of the RF power that is fed to your antenna from your transmitter, but then is conducted back to your shack via common-mode current on your feedline, causing RFI trouble in the shack or elsewhere in your house; (2) to keep the transmitted RF power that 60-Hz power, telephone, TV, and other cables in the field of your antenna pick up, from bothering susceptible devices connected to these cables in your own and neighbors’ houses; and (3) to keep the RF noise that all the electronic devices in your house generate, from being conducted via 60-Hz power, telephone and other cables to the outer shield of your radio, and from there along your feedline(s) to your antenna(s), in common-mode.

(more info in the document...but the point is your preamp power cord "may" be picking up the 60 cycle transmitted noise from other household devices....in YOUR environment and not in the shops where you had your gear tested/serviced.

Read page 9 where to install common mode choke


Best part....you can build this for about $25 in parts and 10 minutes of your time. If it doesn’t work....well it is much cheaper then other next options like hiring an electrician or replacing your power conditioner, which you already know is not likely going to solve this issue.

The best price for ferrite toroid donuts is here:

https://www.kf7p.com/KF7P/Ferrite_chokes.html

Yes, an amateur radio site, but cheaper then the mouser / digikey.

I specifically purchased the "Monster toroid mix 31" for $25.


You basically wrap your preamp power cord around this ferrite donut...as shown in the pictures.....I recommend about 16 winds if you have enough preamp power cable length....YMMV and you many need to experiment...but it is cheap!

My Audio Research D100 did not have a long enough cable, so I purchased a good quality extension cable and wrapped this around the ferrite donut. One end of the power cable donut went into the wall outlet. The other end went into the D100A power cord = 60% reduction in 60 cycle hum.

My next experiments

a. increase the number of winds around my existing donut for the D100a
b. I have another monster ferrite donut mix 31...and I will probably wind the power supply cord driving my Sota turntable.
c. observe 60 noise...and continue adding donuts to other stereo gear...especially anything with a walwart like device... until I reach diminishing returns.

Good luck!

Stu
WB6YRW

I have found that most elusive hum is due to DC components that tide on AC.  Unfortunately it can come from just about anywhere in your local grid - basically anyone connected to your shared power utility transformer.  It can come from your next door neighbor that is using an electrical wall heater (half-wave rectifier), hot tub heater, microwave, clothes dryer, dimmers with an SCR... too many sources to list here.  Unbalanced AC lines just are evil.  You can also check your cable or satellite grounds.  They don't have to be in circuit to cause hum issues.  Installation techs just look for the easy way, not necessarily the proper way to find earth.  Then again, I once found a persistent system hum due to an interconnect pair that had telescoped grounds with one connected in opposition to its brother.

Hums suck.  Big time.

@montaldo 

So many knowledgeable members have offered great suggestions.  I doubt that I have much more to offer. In light of all the great suggestions relating to ground loops, critical components and I/Cs near house 120v/60Hz lines, and the like, ... I assume that grounding and ordinary 60 cycle hum issues are not the problem.  But here are few other simple ideas:

1. Are you sure the preamp is working properly.  You posted that:

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.

Ok, but have you sent the preamp back to the factory for a complete check-up.  I  own ARC gear and some of my pieces were getting long in the tooth.  Even though I do not like shipping my gear to anyone ... period ..., one of my ARC pieces had a problem so I sent everything back to the factory for a check-up.  What a nightmare.  To my total surprise, all the tubes (almost new) in one piece were toast.  ARC replaced all the tubesin the unit.  Go figure.

2.  Incompatibility?  Let assume that your pre is in proper working order, is there some oddball incompatibility with your amp and pre?  Have you tried swapping out your preamp with a temporary borrowed pre and then checked to see if the hum went away.  If you still have the hum problem, same with the amp.

3.  Upstream Issues?  I assume you already isolated upstream source gear so you are satisfied that the hum is sourced to only the pre or amp.

I apologize in advance if the above suggestions were already offered by others.  Just seems to me that a rig should not pass a hum unless (1) there is an external 60Hz signal that is inducing the hum in your rig, (2) there is a grounding issue,  or (3) there is a problem with your gear.

Good luck and keep us informed.  Happy New Year.

BIF

Did it ever work without hum?

There is no way in the world I would live with hum.

You have a temporary spare, I would ship that thing back to David, get him to give you a very good deal on a new one if you love his stuff so much.

http://davidberning.com/support