2. amps-preamps
  3. 1333477504

3 preamps with transformer hum, DC on the line?

Hi everyone,

Searched the first couple of pages around transformer hum but did not find any specific answer around this. I have had the 3 following preamps in my main system with transformer hum, most of them on a dedicated 20 amp line:

- NAD T163 A/V preamp
- Audio Refinement Pre-DSP 2
- Audio Refinement Pre5 (current preamp)

Like I said, all had transformer hum. The problem reproduces with all other main breakers off except the dedicated 20 amp line and the preamp plugged into that dedicated line. I called my power company today requesting that diagnostics on the line into my house be done (check for DC and possibly adjust the AC frequency)? This would be great if this fixed my issue. I have also tried a PS Audio Humbuster III which did remove most of the hum but also killed the high frequencies so I removed it. If the power line diagnostics all check out, would you:

- Try to have the Audio Refinement Pre5 transformer repaired at Audio Plus Services which is USA YBA distributor and service minimum of $250 plus shipping both ways from Seattle to NY. I also have a good local solid state repair shop and I plan on calling them today to see if they can repair this locally. Is repair just a short term band-aid on what could be a tired power supply?
- Try to find a high quality replacement for the Audio Refinement Pre5 transformer. I have no idea what this would cost or how I could find a high quality drop in transformer.
- Have an external high quality power supply made like one from Welborne Labs made.

What does everyone think? The last two options look to be the cheapest route with new parts and warranties vs repairing used parts.

TIA very much,

Scott
sbrownnw
dhl93449
455 posts
 
Jea48

From your reference:

"It's also worth noting that DC is usually not a problem with toroidal transformers of 300VA or less. Their primary resistance is usually high enough that any DC will have little effect."

"Most of the time, the DC offset is transient - it appears for a short while, then goes away again."

And it can be easily measured on a good Digital Volt Meter, like a Fluke or Tektronix. But you have disable autoranging.

Was surprised to see a slowly ocsillating offest of about +/-300 mV peak (at a frequency of 0.1 Hz) on my AC line, but none of my torroidal transformer powered equipment (2x Parasound JC1, a JC2, and a P3) hum at all, even with this amount of DC.
jea48
4,534 posts
 
When you pulled the top cover off the preamp did you notice the power transformer? I assume it is a toroidal. Correct?

Have you noticed the mechanical hum being louder or quieter during different times of the day?

Try the preamp late at night.... After midnight.
Any house sharing the same utility transformer as your house can put DC back on the mains.

I take it you live in a residential neighborhood, houses all around you? No businesses?

Have you thought of taking the preamp to someone else's house, not on your street, and plugging it in one of their outlets?

BTW, the Linn Classik is part of my bedroom system with some Tukans and is not on a dedicated line and I have quite a few dimmers in the house.
The Linn might use an EI transformer instead of a torrid.

Cheap dimmers will cause DC on the mains.... But you said you turned everything off but the dedicated circuit.

If your problem is DC on the mains and it is not coming from your house you could buy a small isolation EI transformer. 120V in 120V out and plug the preamp into it.
.
sbrownnw OP
710 posts
 
Jea48 and everyone else, I appreciate the answers and ideas. Yes, it is a toroidal transformer. Usually YBA, which made the Audio Refinement line does not use toroidal ones but they did for this line. And I think the Linn Classik does use an EI.

I had all the grounds checked when I bought the house in 2010. House was built around 2004 and, yes it is quite a new and large housing development with a business park or two down the road and a retail strip mall village at the entrance of the development.

Jea48, do you know of any plug and play 120v in to 120v out EI transformers that would have decent wattage, good to excellent quality and not break the bank? Or maybe a high quality EI transformer that I could add a plug on the in wires and a outlet for the out wires plus a nice box?
jea48
4,534 posts
 
....
a business park or two down the road and a retail strip mall village at the entrance of the development.
04-05-12: Sbrownnw

I would think the businesses would have to be fed from the same utility transformer as your house. Down the road they are not.

Found this on ebay. 500Va. What is the power consumption, (watts, Va), of the preamp?
400Va or less would be fine. That would be 80% of the full load rating of the transformer.

It dawned on me last night, after I left a post, there is one test I do not believe you tried that may be causing the torrid to excessively vibrate... Some other piece of audio equipment connected to your system. Some piece of a equipment with a cheap switching power supply that would be putting DC back out on the line.

For a test unplug everything from the 120V dedicated circuit wall duplex receptacle except the preamp. Check for the mechanical hum.
.
sbrownnw OP
710 posts
 
Jea48, thanks again for the info. Here are the preamp's specs:

Output imp: < 20
Power consumption: 6.5 watts
Max output voltage: 3.8 v

With the above specs, is there a cheaper EI power supply I can try? Or maybe I can put my amp on the same EI supply. The power amp consumes 850 watts. Would the one you found be powerful enough for both the preamp and amp?

Yesterday I tried exactly what you stated by ONLY having the preamp plugged in to the dedicated 20 amp AV line with no other AV equipment powered up on the same line and all the other breakers off in the main fuse box. So, power was cut off to every breaker except the dedicated AV line and the only thing powered up in my entire house was the preamp. Still hums.
More to discover