DC blocker?

[stevehertz]

I've seen something about 'DC Blockers'. Here's the words to accompany a commercially available product:

Nowadays the AC mains network is heavily polluted by various domestic appliances, industrial electrical consumers etc. This can lead to buzzy toroidal transformers (especially the power ones). Proposed DC trap (blocker, filter) will remove or at least lower the buzzing.
Schematic is similar to this one used in well known Vladimir Shushurin’s Lamm-1 amplifier. An article with in-dept explanation of how DC blockers work can be found on Rod Elliot’s site (http://sound.whsites.net/articles/xfmr-dc.htm).

I'm confused and bemused. Is this another snake oil/voodoo/smoke and mirrors product ?! I've read the article at the link - is it a 'true story' ? Is it only toroidal transformers that are affected ? Anyone (sensible) use one ?

[G6Tanuki]

In times-past, when there were loads of TVs and radios using AC/DC technique and half-wave rectification of the mains, there was sometimes a resulting (barely noticeable) DC component superimposed on domestic mains.

These days nothing worth bothering about half-wave-rectifies the mains. So no significant DC component to worry about!

I'd suspect that your referenced device is not going to make a significant improvement to anything. Certainly never needed such a device in the decades I've spent working on serious radio gear, some of which involved Kilowatt-rated toroids...

[Nuvistor]

I have only ever used toroidal transformers less 300 VA so no experience.

Is the question out of interest or do you have problems with transformers buzzing?

[barrymagrec]

Toroidal transformers as usually used in hi fi equipment are more sensitive to DC on the mains than other types and occasionally this can cause problems - not often though, depending on the quality of your mains.

I have never personally had this problem but I know of people who have.

A blocker may be worth trying if you have a buzzing transformer but there is no guarantee it will fix the problem.

[stevehertz]

Originally Posted by Nuvistor

I have only ever used toroidal transformers less 300 VA so no experience.

Is the question out of interest or do you have problems with transformers buzzing?

Just interested.

[kalee20]

A thumping big capacitor (1,000uF or more) in series will remove DC. This has a reactance of 3 ohms at 50Hz, so 250 VA load won't exactly cause much voltage drop.

If you want to try it, a chunky electrolytic from a low-voltage power supply will be OK for a few moments. You'd probably want to bin it afterwards, because it will have suffered a few volts in the reverse direction, but it will be OK to see if the buzzing stops.

Another option is just an isolating transformer. The secondary will be DC-free. Down side is that this might hum as much as the toroid you're trying to quieten, but you can at least site it remotely.

[Graham G3ZVT]

Originally Posted by barrymagrec

Toroidal transformers as usually used in hi fi equipment are more sensitive to DC on the mains than other types and occasionally this can cause problems - not often though, depending on the quality of your mains.

I have never personally had this problem but I know of people who have.

A blocker may be worth trying if you have a buzzing transformer but there is no guarantee it will fix the problem.

If I understand correctly, toroidal transformers tend to be operated closer to saturation than an equivalent conventional one.

Years ago there was an article in RSGB "Radcom" about an Amatuer whose PSU fuse would blow at a particular time in the evening, but only if he was transmitting at the time. This was repeatable on subsequent evenings, but each event was a precise number of minutes and seconds later than the last.

It turned out after much investigation, that the electricity board was using a signalling system that effectively superimposed DC on the mains to turn on and off street lamps, and this was briefly driving his transformer into saturation and blowing the fuse.

[G6Tanuki]

The circuit the OP refers to has a 1000uF electrolytic capacitor in series with the mains, with a parallel 4-diode network. From my reading of the circuit, at any one time the capacitor has in effect two forward-biased diodes shunting the capacitor for each half of the AC cycle, which will turn-on if there's more than a Volt or so of DC present - so nullifying the effect of the capacitor.

If I believed there was a need for ~~DC-blocking~~ I'd do away with the diodes altogether and build a 'big polarity-indifferent series capacitor' using motor-start capacitors which are available remarkably cheaply.

[Graham G3ZVT]

Originally Posted by kalee20

A thumping big capacitor (1,000uF or more) in series will remove DC. This has a reactance of 3 ohms at 50Hz, so 250 VA load won't exactly cause much voltage drop.

If you want to try it, a chunky electrolytic from a low-voltage power supply will be OK for a few moments. You'd probably want to bin it afterwards, because it will have suffered a few volts in the reverse direction, but it will be OK to see if the buzzing stops.

Another option is just an isolating transformer. The secondary will be DC-free. Down side is that this might hum as much as the toroid you're trying to quieten, but you can at least site it remotely.

I doubt if discussions about 12 bore firearms cartridges are allowed here, but I suspect your capacitor would become one.

[Craig Sawyers]

I built a small circuit to actually measure the DC on mains. Just fired it up, and read 10mV at this time.

I haven't seen much beyond 40mV, and 100mV maximum.

So too small to worry about where I live.

But yes - if there are substantial amounts of DC - usually caused by significant even order distortions as opposed to a real DC offset (which I cannot think of a mechanism for) - can cause transformer buzzing because of core saturation, usually with large transformers >500Va where the effective resistance is low. Simple DC blocks of diode protected back to back electrolytics of 10,000uF or so will sort that out perfectly.

The Rod Elliott design is good, or you can buy one already made from http://sjostromaudio.net/shop/mains-...uilt-unit.html .

Craig

[Boom]

Originally Posted by G6Tanuki

In times-past, when there were loads of TVs and radios using AC/DC technique and half-wave rectification of the mains, there was sometimes a resulting (barely noticeable) DC component superimposed on domestic mains.

Used to see it when working in a workshop with a benchful of Grundig thyristor sets running.

[nutteronthebus]

hi the art of sound forum also has this up as a topic http://theartofsound.net/forum/showt...hlight=blocker and the price of some is snake oil prices

Dave
mods please remove link if not allowed

[Radio Wrangler]

I'd be a little wary. I looked on there and spotted a statement that a certain amp didn't need a DC blocker unit adding because it already had DC blocks between the driver valve and the power output valve.....

DC on the mains seems to be a lot less than I remember, though wave shape distortion seems pretty bad.

Of course. I'd look a right idiot on any audiophile web group because I wouldn't know if any of the awfully esoteric names in the lists of their equipment they need to list at the ends of all posts, is real or made up for a joke. Shades of that Rowan Atkinson sketch?

David

[mhennessy]

A friend of mine has a Bryston 4B SST with a DC blocker. The capacitors are 33,000uF, but as they only ever see ~1.2V DC max, thanks to the bridge rectifier, they are only 6.3V. Still physically large though.

A reverse voltage of 1.2V won't harm a cap; something Bryston must be pretty confident about that given that this amp is guaranteed for 20 years

[Cobaltblue]

Oh dear so providing the DC component doesn't exceed the foward voltage drop of 2 silicon diodes all is well then?

I do admire the audio industry thinking up new ways to fleece the hard of thinking.

I did all that Audio Phoolery in the 70's and 80's ( when to be frank I had more money than sense)

All went out of the window on the birth of my first child, haven't visited that world ever since.

Now is content over delivery for me.

Doesn't mean I dont like well made quality audio equipment but its appeal is limited and I no longer feel the need to compete.

Cheers

Mike T

[mhennessy]

In fairness, Bryston are one of the better manufacturers out there, preferring engineering to audiofoolery.

I have used noisy large torroidal transformers which were improved by a DC block (though I prefer to specify custom-wound units that are run at a lower flux density so aren't saturating, but this adds to the cost and size-per-VA).

Right now, I have about half a volt of DC on my mains. Given that a blocker can only remove around 1.2V, yet can make a worthwhile difference in transformer saturation (observed with a current transformer as well as ears ), that shows just how "on the edge" some of these big torroids can be.

[Craig Sawyers]

Originally Posted by mhennessy

A friend of mine has a Bryston 4B SST with a DC blocker. The capacitors are 33,000uF, but as they only ever see ~1.2V DC max, thanks to the bridge rectifier, they are only 6.3V. Still physically large though.

A reverse voltage of 1.2V won't harm a cap; something Bryston must be pretty confident about that given that this amp is guaranteed for 20 years

It is typical of the careful engineering of Bryston that they have put the DC block in the neutral line, so it operates near ground potential, and not (as is more usual) in the live line. Rod Elliott's and the Sjöström designs all operate in the live line, and so float at 240V.

I too tend to use toroids that operate a low flux levels (wound by an old friend of mine, Paul Houlden of TEC transformers). That means that they are physically larger, but they don't buzz.

Craig

[Craig Sawyers]

Originally Posted by nutteronthebus

hi the art of sound forum also has this up as a topic http://theartofsound.net/forum/showt...hlight=blocker and the price of some is snake oil prices

Well, the price of the Sjöström unit pictured in the link above is indeed about £50 shipped (I bought one). But what you get is a populated board that has been designed with mains voltage in mind, with creepage and clearance taken into account, 4oz copper, double sided board. And sprung connectors for wiring up. That is not a bad price for a small production volume cottage industry.

Or you can buy the bare board and populate it yourself - for £8.60 plus shipping.

Once I measured that the DC on my mains is actually very low, I've never used it.

[G8HQP Dave]

People seem to doubt whether up to 1.2V of mains DC can cause problems. After all, it is only 0.5% of the AC voltage; how can that have any effect?

Let us do some sums. Big toroidal transformers can be quite efficient with low DC resistance. Let us assume that the regulation of our transformer is 5% - a big one could be even better than that. This means that at rated current it drops just 5% of voltage. Now assume that half of this drop comes from the primary. So at rated current the primary drops 2.5% of voltage. Turning this around, we conclude that 0.5% of DC voltage will impose a DC current equal to 20% of rated AC current. Many transformers would not cope with what is in effect a 20% overvoltage on the primary, and go into saturation.

My conclusion is that DC on the mains as small as 1.2V could cause problems for a transformer with low DC primary resistance which is running near saturation under normal circumstances. Having said that, it is quite likely that some audiophiles think they have mains DC when they have not or believe that a DC blocker will do some good even when their DC is too low to do any harm. However, just because something has become a popular fad does not mean that some people are not genuinely suffering from it.