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
