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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
sbrownnw OP
693 posts
 
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.
magnumpi205
504 posts
 
Dimmer switches in the house? Removed mine and removed the hum.
bigshutterbug
383 posts
 
Scott..Are you referring to an audible hum that you hear through the speakers, or is it a mechanical hum from the transformers themselves?
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
4,425 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.
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