DC Offset and DC blockers question

[Snoopy]

So simple question really.
Power outlets here have 230 volt 50 Hz and it's normal to have 10% +- fluctuations.

Looking at DC blockers (the stuff from IFI etc) they filter out 600 mv - 1200 mv and there is one from the Ukraine that filters around 3000mv.

So what exactly is a DC Offset, shouldn't a well designed amplifier / DAC filter that stuff out anyway.

And is 1200 mv even enough to make a difference ?

And would DC Offset me if the people in the apartments over me have stuff like washing machine, dishwasher running?

Edit:

Figured already out that SMPS are not affected by this anyway.
So I quess it only matters for a class A/B amp or DACs with toroidal transformers.

 

[sergeauckland]

DC offset really only affects amps with toroidal transformers. A large toroid has a very low DC resistance on the primary, so even a small amount of DC will saturate the core, which substantially reduces the transformer's inductance. This causes the transformer to overheat and causes mechanical hum, so a DC blocker is a Good Idea.

As to why there's DC on the mains, it can be caused by many things, but generally, half-wave rectification is a main cause. This can be hair-dryers on 'half' power, usually just a diode in series with the heater, ditto with laundry dryers. My understanding is that, at least in Europe, regulations have largely stopped this practice, although there are a lot of old appliances still in service.

By the way, LED lighting still mostly uses half-wave rectification inside the bulb, but at least here, with any random arrangement of lamps, the plusses and minuses should more or less cancel out, but of course, might not.

S.

 

[Snoopy]

DC offset really only affects amps with toroidal transformers. A large toroid has a very low DC resistance on the primary, so even a small amount of DC will saturate the core, which substantially reduces the transformer's inductance. This causes the transformer to overheat and causes mechanical hum, so a DC blocker is a Good Idea.

As to why there's DC on the mains, it can be caused by many things, but generally, half-wave rectification is a main cause. This can be hair-dryers on 'half' power, usually just a diode in series with the heater, ditto with laundry dryers. My understanding is that, at least in Europe, regulations have largely stopped this practice, although there are a lot of old appliances still in service.

By the way, LED lighting still mostly uses half-wave rectification inside the bulb, but at least here, with any random arrangement of lamps, the plusses and minuses should more or less cancel out, but of course, might not.

S.

So it wouldn't hurt to use one of these on a amplifier with toroidal transformer.
But is 1200 mV alot or is it a joke?

 

[DSJR]

My UK take is - and confirming (I hope) Serge's comments on some toroids...

Some toroidal transformers buzz and snarl at the slightest DC or nasties on the mains (I'm looking at you Naim Audio, at least in the past) and it was promoted in UK audiophool circles as a beneficial thing as it was showing how bad the mains was... Other toroids, even large ones, may 'booooiiing' a bit on turn on, but can be silent at almost all other times and it's been shown here on ASR that mains filters and so on are pretty much unnecessary these days.

So, if your gear (power amp mainly) has a snarly transformer only at certain times of day, a DC blocker may be just the ticket (if a pre-made one, make sure it can be returned if it doesn't work for you) as long as it's specified correctly. Otherwise, get yerself an amp with properly designed and specified mains transformer if the supply's a linear type

 

[Snoopy]

My UK take is - and confirming (I hope) Serge's comments on some toroids...

Some toroidal transformers buzz and snarl at the slightest DC or nasties on the mains (I'm looking at you Naim Audio, at least in the past) and it was promoted in UK audiophool circles as a beneficial thing as it was showing how bad the mains was... Other toroids, even large ones, may 'booooiiing' a bit on turn on, but can be silent at almost all other times and it's been shown here on ASR that mains filters and so on are pretty much unnecessary these days.

So, if your gear (power amp mainly) has a snarly transformer only at certain times of day, a DC blocker may be just the ticket (if a pre-made one, make sure it can be returned if it doesn't work for you) as long as it's specified correctly. Otherwise, get yerself an amp with properly designed and specified mains transformer if the supply's a linear type

I just returned the new Hifiman ef400 headphone amp (issues with the usb port) but the amp has a audible humm when being near the Amplifier. (Sound through headphones had no humm).

That's how I got interested in DC blockers.
I have no humm From my other amps.

 

[sergeauckland]

So it wouldn't hurt to use one of these on a amplifier with toroidal transformer.
But is 1200 mV alot or is it a joke?

In my view, it will do no harm at all, it's a sensible precaution. From what I can tell, 1200mV (two diodes' worth) should be enough in all but the most extreme cases, unlikely in developed countries with stable mains and strong regulations.

S.

 

[Snoopy]

In my view, it will do no harm at all, it's a sensible precaution. From what I can tell, 1200mV (two diodes' worth) should be enough in all but the most extreme cases, unlikely in developed countries with stable mains and strong regulations.

S.

I didn't even consider that this could be actually useful on vacation. I've spend months before in SE Asia
and I was always worried about plugging in a ps4 there.

 

[DSJR]

I just returned the new Hifiman ef400 headphone amp (issues with the usb port) but the amp has a audible humm when being near the Amplifier. (Sound through headphones had no humm).

That's how I got interested in DC blockers.
I have no humm From my other amps.

DC sensitivity as far as I can tell, has nothing to do with hum on the output if the affected unit is placed too close to an amplifier. That's magnetically? induced hum which is different. I'm talking physical buzzes and snarls (Naim amps at their worst) which come and go at different times of the day.

 

[DanielT]

DC sensitivity as far as I can tell, has nothing to do with hum on the output if the affected unit is placed too close to an amplifier. That's magnetically? induced hum which is different. I'm talking physical buzzes and snarls (Naim amps at their worst) which come and go at different times of the day.

"I'm talking physical buzzes and snarls"

In fact, it's the most annoying thing I can think of when it comes to amplifiers, and it can definitely be audible.

When buying a used amplifier, you should make sure how much buzzes, snarls, humming it can have. I made that mistake with a vintage NAD3020 amp. Bought it unheard of that is. Most thought that you should have such a vintage classic at some point in your life if you are interested in HiFi. The amplifier as such, with a reservation for low power answer, was quite ok but that damn buzz!

It is apparently possible to immerse old transformers in some kind of emollient liquid and in this way POSSIBLY reduce the buzz sound. That maybe works ..?
Please do not do this treatment of the transformer before talking to a knowledgeable person. Apparently, after the treatment, the transformer will also smell bad.

Edit:
I'm talking about a mechanical humming , buzz sound that comes from the transformer itself and not sound you hear in the speakers/ headphones.

 

[DSJR]

When new, the 3020 wasn't a mechanically noisy amp I remember (I had a 3020a on my bench running all day for seventeen years or so - don't know what happened to it after I left) BUT, the 7020 receiver (3020 amp and matching tuner in one box) used to get through mains transformers on occasion, so maybe a weakness there which is showing forty years on in the amps?

 

[DanielT]

When new, the 3020 wasn't a mechanically noisy amp I remember (I had a 3020a on my bench running all day for seventeen years or so - don't know what happened to it after I left) BUT, the 7020 receiver (3020 amp and matching tuner in one box) used to get through mains transformers on occasion, so maybe a weakness there which is showing forty years on in the amps?

If there would be any distinctive, general problem, for NAD3020 I do not know.This you wrote probably explains the thing: "forty years on in the amps?"
It's old stuff.The tooth of time that wears down.