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
One way to be sure (if you can get your hands on one) is to try an isolation transformer as this will stop any DC cold as they cannot pass DC. Your problem might be poor quality AC however, such as low voltage, lagging current/voltage issues, etc. These could be more difficult to remove. Cheapest solution is to see if the power corp can check it first.
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.
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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one other thing you can try (which costs money, of course!) is to use a AC power regenerator (like one of thos PS Audio units P300, P600, P1200 or Power Plant Premier) or from PurePower.
The AC power from your wall will be used to power the electronics inside the AC regenerator but the AC power regenerated should be clean.

Looks like you might have a mechanical hum in the xformers. Does putting a brick or some heavy weight on top of the xformer stop the humming? Of course, put the heavy weight on the xformer when your preamp is off to avoid any personal harm.
Dimmer switches in the house? Removed mine and removed the hum.
Scott..Are you referring to an audible hum that you hear through the speakers, or is it a mechanical hum from the transformers themselves?
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.
So, I took the case off the preamp today and tightened all power supply screws (none really budged). Powered it up with every breaker turned off except my dedicated 20 amp AV line and only the preamp plugged into the dedicated outlet. Still hums - grrrr.

It looks like I have very few options, some which I cannot afford and 1 I really am not sure I want to do:

1. Send back to Audio Plus Services (Seattle to NY). Anyone had any experience with this shop and their service dept? I can see a $250 diagnosis bill plus shipping both ways and them coming back saying there is either nothing wrong with it or they are unable to fix it. Really not sure about this route. Probably a No.

2. A custom or 3rd party power supply is probably out of the question after looking at various wires coming off the factory power supply. No.

3. Trying a PS Audio or similar power regenerator. Too expesive. No.

4. Going to an integrated amp if I can find an Audio Refinement Alpha Complete in black that does not hum in 8/10 shape I just might do it. I've heard it is a good unit but not as good as my Audio Refinement separates, especially the Pre 5. A Maybe.

5. Trying my best to get the power company come out and test the line. Hopefully something that is easily addressable and I can actually get the power company to show up and test the line and do it for free (I do pay them for a resource with hopefully some kind of service level agreement backing it.

6. Going to try a Environmental Potentials DigiPlug Stationary in series before my Porter Port on my dedicated 20 amp line. They will do a 30 day refund trial and I can do the wiring myself. A Yes and hope it fixes things.

What I really don't get is that all 3 preamps I've tried do hum and got better with using the PS Audio Humbuster, but again, I lost the high frequencies so the PS Audio Humbuster is out. If it is that common of a problem, why is there not more of an industrial standard that avoids this problem. On top of that, the Linn Classik K music (integrated amp with tuner and CD player) just cranks away with no hum but does not sound as good as my current Audio Refinement gear and probably loses out to the Audio Refinement Complete Alpha integrated also.

Things that annoy you with their hum...
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.
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 and everyone else, I appreciate the answers and ideas. Yes, it is a toroidal transformer. Usually YBA, which made the Audio Refinement line does not use toroidal ones but they did for this line. And I think the Linn Classik does use an EI.

I had all the grounds checked when I bought the house in 2010. House was built around 2004 and, yes it is quite a new and large housing development with a business park or two down the road and a retail strip mall village at the entrance of the development.

Jea48, do you know of any plug and play 120v in to 120v out EI transformers that would have decent wattage, good to excellent quality and not break the bank? Or maybe a high quality EI transformer that I could add a plug on the in wires and a outlet for the out wires plus a nice box?
....
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, thanks again for the info. Here are the preamp's specs:

Output imp: < 20
Power consumption: 6.5 watts
Max output voltage: 3.8 v

With the above specs, is there a cheaper EI power supply I can try? Or maybe I can put my amp on the same EI supply. The power amp consumes 850 watts. Would the one you found be powerful enough for both the preamp and amp?

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.
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
So I am going to try an isolation transformer. I found one of the two new (returnable if it does not solve my problem):

- Tripp Lite IS1000 in black (1000W) for preamp only
- Tripp Lite IS1800HG (1800W hospital grade in off-white) looks like it is just under the 2000W rating Jea48 recommends for preamp and amp

I am going to check around Seattle today to see if someone has the IS1000 in stock. Hopefully this works and gives me some more evidence to use if I can ever get the power company to come out and test my line.

If the IS1000 does not work for eliminating the hum in my preamp, it looks like it will have to go in for repair. Does anyone have any good repair shops besides Audio Plus Services?

Again, TIA everyone,

Scott
Found the following locally:

Tenma 450 VA Isolation Transformer, Isolated Outlet (2): 450 VA Continuous, 550 VA Intermittent, Direct Outlets (2): 900 VA, Isolation Leakage: <0.1 mA, Dimensions: (W x H x D) 6 x 5 x 9 , Weight: 16 lbs, Power Requirements: 120 VAC, 60 Hz

Going to give it a try for just the preamp. Fingers crossed.

Scott
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.
I still see two ways DC offset can still infiltrate your system ... even with all the breakers off except the dedicated line ...

First you can be picking it up from your neighbor who is producing it and sending to the transformer you share ... his flat screen .. dvr/cable box .. computer .. hair dryer and anything else in his house that has a switching power supply with a half wave bridge rectifier will produce DC offset by drawing current in un-even pulses

I also not sure if a Isolation transformer or regenerator will help ... an isolation transformer will block common mode noise riding on the line ... DC offset is not like noise riding on the line ... it is a distorting of the sine wave .. so I would think that the Iso Tranfo will not remove or straight the distorted sine wave but just passes it through

My proof is your neighbors are creating plenty of DC offset and it is ending up in your home after it passes through the transformer on the pole on the street ...

The way to dissipate the DC offset is to pass it through a capacitor bank ... capacitors can not pass DC and the action of the capacitor merely straightens the sine way as the current passes through it

DC offset vs DC

Just for clarity (I hope) .. DC offset is when the sine wave doesn’t cross correctly at the zero crossing point were the wave turns from positive to negative phase ...

Here the half wave bridge rectifier in the switching power supply lops off half the wave and creates some DC from the AC wave form ... this created DC rides on the current and can only be dissipated by a capacitor bank which will not pass DC and this is what straightens the wave back out to its symmetrical wave form

Regenerator won’t help either ... I’ve been using a Exact Power EP15 since 02 and it’s no benefit .. here is why

The humming of the transformer you are hearing is the “Symptom” and not the “Real Disease” ... the Real Disease and silent killer as you can’t hear it is ... the reduction of the transformer’s efficiency

By the Offset shifting the Zero crossing point ... it reduces the transformer’s efficiency and this silently reduces dynamics and head room ... unlike the presence of RF that you clearly can hear ... the reduction in dynamics and headroom are not realized until the Offset is eliminated and dynamics are restored

Simply put DC Offset created by switch power supplies has a profound effect on dynamics and headroom

So now you add a regenerator and I don’t care what brand ... the DC Offset will hit the regenerators transformer first reducing it’s efficiency long before it hits the regenerating circuit ... so the generating circuit is now running down on power provided from it’s compromised transformer

Again IMO it is the reduced efficiency that is the perpetrator and not the sound of the transformer humming which I view as the symptom

When I unplugged my 500w Phase Linear amp and Arye D1xe player for the EP15 and put them into a PS HumbusterIII ... the improvement in slam and authority was immediate and profound ... wanna buy an EP15 cheap

I also didn’t experience the roll off in the highs as you did ... but that’s not the issue at this point or of this thread

Second point of entry ... with all the breakers off and only using your dedicated AV line ... I realize that you have shut off your video equipment also and you would think that you were safe but ...

All that video equipment .. Flat Screen .. cable box and or DVR have switching power supplies ... you would think by turning them off you’ve eliminated the effects of the switching power supplies ... but you haven’t

All these components work by remote and are in a constant stand by “ON” mode state waiting for a signal to turn them on ... in the stand by mode guess what ... the switching power supplies are still working to keep component in an ready state .. while waiting for a signal to turn on and thus creating the DC offset that continues to hammer your preamp even with those components turned off

Try UNPLUGGING your flat screen and cable box/dvr with all the breakers off and see if this stops the humming ... if it does and you still have the HBIII try re-plugging the flat screen and cable boxes into the HBIII ... this should eliminate the DC Offset from the video equipment and not roll the highs off to your stereo as the stereo is not plugged into the HBIII
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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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
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
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
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!
Scott, I sent you e-mail.
Ask your question over on Audio Asylum. Many more techies over there that can help you.

blue 84 carrera
Gbart, I sent a reply with some questions. Thanks,

Scott
Hi Guys - I am having a DC hum on my cinepro 3K6mk2 amp. I cannot afford a $300 humbuster right now. I'm not an electrician but I do know my way with a soldering gun fairly well. I appreciate any and all advice - thanks.
mark
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, I've been contacted ny Airkitty. He has confirmed that he's checked for all possible sources of hum, including a loose transformer mounting bolt. I'm shipping out a DC blocker to him tomorrow. What do I charge ? Not enough I'm afraid. Its hard to justify charging much for a handful of parts in a box. BTW, my e-mail address is sae2300@optimum.net

http://i994.photobucket.com/albums/af67/mr_classe/DC%20blocker/DC_BLOCKER_02.jpg

http://i994.photobucket.com/albums/af67/mr_classe/DC%20blocker/DC_blocker_002.jpg

http://i994.photobucket.com/albums/af67/mr_classe/DC%20blocker/DC_blocker_004.jpg
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
You might try communicating with PS Audio about their Humbuster in case you had a faulty unit. I've used one before and it didn't roll off the high frequencies whatsoever and I suspect they wouldn't introduce a product that would, if it was functioning by design. Did you measure the reduction in high frequencies and if so, can you be more specific (ie. how many dB and at what frequencies)?
Its easy enough to eliminate DC on the line. If you have DC, it can cause toroidal transformers to become noisy as the core of the transformer will saturate with very low DC levels.

Such DC filters should not adversely affect the bandwidth of the associated gear.
Gbart just built one of these DC blockers for me and I'm quite pleased with it. Transformer hum is gone, and there aren't any ill sonic side effects. There is still a little noise at the speakers if I stick my ear right up against them, but no one ever accused a Cary V12 of having "deep black backgrounds." On the whole, with this thing installed it's the quietest my system has ever been. Well done and very reasonably priced.
Van Alstine also makes a DC blocker.
I finally replaced the power supply in my preamp with another Pre 5 that was in rough shape but had a quiet transformer. The swap worked great and I now have no DC hum. I sold the custom DC blocker but have a PS Audio Humbuster III for sale if anyone is interested.