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 davehrab

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