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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
jea48
4,534 posts
 
An isolation transformer will not pass DC voltage.

If that were the case the output transformer of a tube amp would not prevent the high B+ DC voltage, that is fed through the primary winding of the xfmr to the plate of the power tube, from passing to the secondary.

An EI isolation transformer will block the DC offset from reaching the secondary.

Will your DC blocker do the job? YES.... But the OP may not have the skills to build one.

He is looking for a plug and play iso EI xfmr.

Maybe the two of you could get together by email and he would pay you to build a DC blocker for him...

I still say the OP would be smart to take the preamp to a couple of friends houses and check out the mechanical hum.

It could be just a noisy Torrid xfmr.

http://en.wikipedia.org/wiki/DC_bias
.
almarg's avatar
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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
sbrownnw OP
710 posts
 
The isolation transformer (a Tripp Lite IS350) did not change anything. I think I will just live the minor hum. I cannot hear it from my listening position and only hear it when changing records. Would be nice to have it be quiet but we cannot have everything perfect, especially when it is not a necessity.

Thanks everyone
jea48
4,534 posts
 
The isolation transformer (a Tripp Lite IS350) did not change anything.

Sbrownnw,

If you can't send it back, you might try feeding your
oppo BDP-93 player from it.

Back to your preamp..... What do you have that thing setting on? Would that be working like a sound board?
Amplifying the mechanical noise from the torrid xfmr?

Reading the owners manual it says the xfmr is suspended to reduce transmission of its vibration to the rest of the circuitry. Did you notice that when you looked inside?

In the RECOMMENDATIONS,

"Do not place the unit on a glass shelf. We highly recommend placement on wood or granite."

And then there is this.....
"For best sonic results, correct orientation of the AC plug is important.
The prong with the red dot should be connected to the Hot connection of you wall outlet."

Is the plug on the end of the power cord just a two wire, 2 blades? Non polarized plug? Plug it in either way?
If that is the case make sure the blade with red dot plugs into the Hot of the receptacle.

Just throwing some things out there.....

Owners manual
.
Jim
sbrownnw OP
710 posts
 
I returned the isolation transformer within the 30 day return window, which was easy after buying it locally.

I might try ordering the parts my own humbuster / dc filter and have one of the two local audio service shops do the assembly. Can someone give me a schematic of the simplest proven design (side by side caps on either the live XOR neutral feed with diodes on each side?) and a high quality parts list that I would need? Should I go with the Bryston, the Crown or a simpler design? It would be great if that parts list included a nice black outer metal case / box if possible plus high quality IEC female connector. I would hope to be able to use one of Porter's power ports also.

Jea48, have you done something like this before? I saw a similar thread post by Gbart that he / she has made some of these for Audiogon members, but I am unable to email that user.

Please drop me an email if possible: white 78 911 at hotmail dot com

TIA again!
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