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

Showing 2 responses by dhl93449

dhl93449
455 posts
 
DC on an AC power line is highly unlikely unless you live in the third world. You can easily check this with a DVM. Just set it to DC volts and put it across the line.

The AC frequency is set at the generating power station and cannot be "adjusted". It's usually very accurate and variation will not cause hum problems.

DC is also unlikely to cause "hum", if you are speaking about electrical noise. Are you speaking of an audible noise coming directly from the equipment (and not through speakers/headphones)? Are the transformers overheating?

Have you checked the grounds in your house wiring? Bad grounds could account for hum problems.
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.
More to discover