3 preamps with transformer hum, DC on the line?

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
.

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