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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

Showing 1 response by almarg

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almarg
9,670 posts
 
On the question of what an isolation transformer will and will not pass, I've got to agree with Jim here. A properly functioning transformer cannot pass the DC component of a waveform, regardless of whether that DC component is the result of a distorted and asymmetrical waveform, or a symmetrical waveform that is symmetrical about a non-zero level.

As to whether or not a distorted and asymmetrical waveform that has had its DC component removed can still have adverse effects on the sonic performance of a system, I would expect that it can. Removal of the DC means that the distortion will have no spectral component at 0 Hz, but there will be plenty of other distortion components remaining at frequencies higher than 60 Hz.

While those remaining distortion components could very conceivably affect system sonics, I'm not aware of any mechanism by which they could cause a toroidal transformer to hum.

Regards,
-- Al
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