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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
4,788 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.
.
sbrownnw OP
717 posts
 
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.
jea48
4,788 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
sbrownnw OP
717 posts
 
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
sbrownnw OP
717 posts
 
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
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