DC blockers.

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pre65
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#1 DC blockers.

Post by pre65 »

I do read other hi-fi forums.

On one the owner sells his own hi-fi products, one of which is a BMU (balanced mains unit). The transformers on these can hum, and it was suggested by one member that perhaps a DC blocker would help. That (to me) seemed a reasonable thing to do.

But it seems the reply he got was that DC blockers adversely affect the sound quality and that a big toroid (1000va) would cope with a bit of DC.

So, what is the opinion on here regarding DC blockers prior to a 1000va toroid wired for balanced mains.

The DC blocker I use on each 833a was made using advice from Nick G and I was happy with them. My intention was to transfer them to the BMU I lashed up the other day.
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Paul Barker
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#2

Post by Paul Barker »

I don't think that any torroid unless purpose made with an air gap will sound good for audio if it is exposed to DC current. It will immediately saturate and affect all audio considerably as it does so.

An ungapped interleaved EI transformer has a very small amount of gap, but it too will sound terrible when dc is applied. I don't expect that would sound good enough either, but folklaw says it is better. I have not proved that. A very nice mains transformer could be made with a gap but it would be huge relative to the equivalent AC only exposed transformer.

So in those scenarios a DC blocker should improve matters overall.

The concept that a larger transformer doesn't suffer from DC is nonsense, because the lower resistance means that more current will flow so the net result is the same. Sure if you only exposed the large transformer to the current which would flow through a small transformer it would not degrade sound. But the dc current will be inversely proportional to the resistance of the windings.

Only a complete nob with no practical experience of winding transformers and observing their behaviour when saturated and no experience of hearing the catastrophic noises which ensue saturation would say an ungapped torroid would not benefit from a dc blocker. You should walk away from where the ignorant talk.
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Nick
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#3

Post by Nick »

Its also (to add what Paul said) a bit of a mistake calling it a DC blocker. The problem is actually unequal AC waveform +ve and -ve, this has the effect of shifting 0v, so there is in effect a DC offset, but its the imbalance that causes the transformer to saturate. I have no real dog in this race, but I would have to say I have sold those blockers to several people to put in their balanced mains supply to cure hum. And I have sold them to a winder of such transformers who said they kill the noise on some 300VA transformers. Cant see why 1kva would be different.

Of course there are other reasons why a transformer is noisy that the blocker cant fix. Depends if he is talking about a buzz or hum.
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pre65
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#4

Post by pre65 »

Thanks Paul / Nick.

I'll try one of my "dc blockers" and see how it affects things.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
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