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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 8 responses by jea48

jea48
4,733 posts
 
DC on an AC power line is highly unlikely unless you live in the third world.
04-04-12: Dhl93449
Not true.

You can easily check this with a DVM. Just set it to DC volts and put it across the line.
04-04-12: Dhl93449
Not true.

Are the transformers overheating?
04-04-12: Dhl93449
DC offset on the mains will cause an xfmr to run hotter than normal.

Have you checked the grounds in your house wiring? Bad grounds could account for hum problems.
04-04-12: Dhl93449
Not xfmr mechanical vibrating noise.

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1209767274&openflup&6&4

http://www.altavistaaudio.com/hum.html#trafo

http://sound.westhost.com/articles/xfmr-dc.htm

http://sound.westhost.com/articles/xfmr-dc.htm#dc1
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jea48
4,733 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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jea48
4,733 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.
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jea48
4,733 posts
 
The power amp consumes 850 watts. Would the one you found be powerful enough for both the preamp and amp?
04-05-12: Sbrownnw
No...... For a power amp that consumes 850 watts I would not want to use an xfmr rated any smaller than 2.5KVA, 2500Va / watts....



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
04-05-12: Sbrownnw

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)

If not for your original post I would say the Audio Refinement Pre5 is the problem.....

Take the preamp to someone else's house, not in your neighborhood, and check for the mechanical hum...

If the hum is the same, try one more persons house. Hum still the same it is the preamp......

http://www.audiorefinement.com/handbook/Pre-5-Manual.pdf
jea48
4,733 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
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jea48
4,733 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
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Jim
jea48
4,733 posts
 
Airkitty,

If you posted an email address Gbart might contact you..... Not sure what he charges to build a DC blocker.
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jea48
4,733 posts
 
Gbart,

Thanks for the info. I printed out your post, with email address and links, for future reference.

I suggest others do the same in case the post disappears..... Sure do miss the old Agon email member feature.....
Jim
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