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Sep 23 2013

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AC/DC … or a Highway to Hell

Your wall socket outlet, what’s in there ?
Is it AC or DC ?

Did I hear you say: “AC” ?
Sure ?
Quite sure ?
Are you REALLY sure ?   ( I mean: really, really, really SURE ? )

But what if you are wrong ? What if this is just a case of mainstream wishful thinking ?
What if I were to tell you that your wall socket delivers AC/DC ?
That it delivers BOTH ?

That your wall socket, the one that you use to power your audio equipment, provides you with lots of AC, and just a small, little, tiny tiny touch of DC ?

Let’s say that you live in Europe, and your wall socket provides you with 230 Volts of AC …..   and on top of that, it provides you with 0.275 Volts of DC ?
Peanuts ?
Is that what I heard you say ?
That I am splitting hairs ?
That such 0,275 Volts of DC is negligible ?

And what if I were to tell you that such a minuscule amount of DC, when entering into your audio mains transformer, embarks your equipment on a Highway to Hell ?

At this point, I wish to express a great amount of thanks, to a great mind – to Mr. Rod Elliot, of Elliot Sound Products. His site contains an ocean full of DIY projects, but … let’s just concentrate on the AC/DC issue at hand. For a full, somewhat technical version, please check out the details under this source link:

http://sound.westhost.com/articles/xfmr-dc.htm

For the sake of DIY efficiency, let’s just say that I shall be skimming only the tip of the iceberg on this.
I suppose that I can give you a “short” layman’s version of the problem, of the remedy, and of the implications that this imposes on the sound of your system.

The bad news is:  That 0,275 Volts of DC, that may pop up in your wall socket can potentially raise havoc in the efficiency and performance of your mains power, audio grade  transformer.

Within that source link as above, there is a Table no. 1, where it shows the conditions and the performance characteristics of ONE AND THE SAME  toroidal transformer, that is powering a hiend-audio device.
The left column – shows typical operating conditions, when the toroid is working of an idealistic (i.e. not existing) AC mains input voltage.
The right column – shows the operating conditions of the same toroidal transformer, when it is fed from a contaminated AC mains input voltage. Contaminated by a small DC voltage offset, of  “ONLY”  264 millivolts DC.  But mind you, this small “little” DC offset voltage – it wrecks havoc in your hiend-audio mains transformer that powers your equipment.

The transformer magnetizing current (RMS – root mean square, as measured by any cheap multimeter) increases, in the presented example, from 15.7 mA up to 218 mA.
The Magnetizing current value, “peak-to-peak”, increases from 50 mA  to 1 full Ampere.
You can see, that the secondary Voltage starts sagging, and falls from 31.8 V RMS  to 31 V RMS, and this with no load connected.
The no load amount of VA lost on this transformer rises from 3.77 VA up to 52.32 VA.
The primary Impedance falls down on its face – from a healthy 15.3 kilo-ohms to only 1.1 kilo-ohms.
The effective Inductance of the unit falls down from 48.7 Henry down to  3.5 Henry.

The data, as reported by Mr. Rod Elliot, is as measured on a typical 500VA toroid.

As you can see, is seemingly minuscule amount of DC voltage within your mains socket, a DC voltage that is a nuisance that is “superimposed”, or riding “on top” of your AC voltage, causes fairly dramatic complications.

Anyway, I’ll say it in another way:  I have my own bad experiences with this phenomenon.   As I mentioned elsewhere, I have built an Output Transformer-Less vacuum tube amplifier. For it’s construction, I used a 1200VA toroidal ( …yes, that is ~1,2 kilowatts, give or take).

Now, I am seriously considering building this DC blocking circuit, as presented by Mr. Elliot of http://www.westhost.com, and this is because that each time my woman switches on that damn hair dryer in the bathroom, and switches it on at half-power (as she always does), the toroidal mains transformer within my OTL amplifier, situated in the living room, starts groaning in agony.
In fact, it moans so terribly, as if it were short circuited – and believe me, I have seen many a short-circuit in my life, with the fumes of melting and/or burning toroids that accompany that. For the sake of clarity – let’s just say that I am “allergic” to the “sounds” of toroidal transformers, those that are moaning in agony.

The DC-Blocker schematic is TRIVIAL – it is comprised of only THREE elements: a high power rectifier bridge, and two big electrolytic capacitors.
If you decide to do a “full symmetrical” version of the circuit, the count of elements shall increase to SIX.  That is all.

DC Blocker

DC Blocker

ATTENTION:  This circuit is TRIVIAL….  But at the same time – it is very dangerous,
If you decide to build it, please kindly refer to the link as signaled at the beginning of the article,
and read the material in full – especially the SAFETY considerations. You shall be working here with live wires directly connected to the mains socket, so I advise special precautions and safety measures.

Believe me – it is a very easy add-on circuit to you system, to your power supply distribution box, or whatever you have there. But you have to be fully aware of the dangers.  And of the inherent risk of a resonance circuit, that you could build by accident. Whatever you do … DO NOT even THINK of trying to reduce the capacity of those electrolytic capacitors. They are supposed to be big.  2x 4700 uF / 63V at the least.  DON’T reduce that.  (unless you want to potentially see a highly efficient resonance circuit powered directly off the mains, a frying toroidal transformer and exploding high voltage electrolytic caps.  I have seen it – and I do not want to see it again).

The capacitors … They need to be capable of handling very high ripple current. That means, that they need to be capable of dissipating big amounts of heat. Such a feature is typical of capacitors that have HUGE dimensions.  Now, in order to achieve HUGE dimensions, we take a pair of fairly HIGH VOLTAGE electrolytic capacitors here. This is done WITH A PURPOSE. You could say: “Hey, that shorted bridge silicon rectifier, with the plus and minus pads shorted – this is essentially a grouping of two diodes in series in one direction, and parallel to that – a series connection of two diodes in the other direction”.  That is true. You could also say: “Hey, the voltage on the pins of those capacitors will NEVER achieve a value bigger that two conducting P-N diodes in series, that means that the voltage at that spot will NEVER be higher than say a few volts. So why on earth do I insist on using 63 Volts rated electrolytic capacitors ? ”

Well, as I said before - these need to be capacitors with “RADIATORS” – so to say, ones capable of disseminating HEAT.  They need to handle huge ripple current, so they need to be BIG in SIZE, in order to radiate and disseminated heat. The only capacitors that come to mind that fulfill such a request are the high voltage electrolytic’s – which obviously have a bigger volume. They are simply BIGGER. And therefore, SAFER.  Please do NOT go below 63V voltage rated capacitors.

The reason we connect them “face to face” with their pluses together, is that in end effect we wish to achieve a “unipolar” capacitor of 2350uF, capable of accepting a charge in any of the two possible directions.  This is because we cannot a priori determine, in which direction shall the DC offset position itself.  The “plus” of a DC offset can come from “either direction”.

To be on the safe side, I would suggest no shortcuts related to the rectifier bridge. Take a KBP 35/1000 (35 Amperes, 1000 Volts).  Just to be on the safe side. These diodes within the bridge are simply not allowed to fail, under any circumstances. They simply CAN NOT Fail. Just imagine what would happen if they …. did. 

But, … where was I ?   Ah yes.  When you are listening to the quieter sections of the music that is playing, this “moaning” of the toroid – it is simply irritating. OK, you say. So I got a “moaning” toroidal. Big deal.

But does this have an impact upon the quality of the “sound” of my system ?  Actually, it does.   If you use the DC-Blocking circuit, you shall observe a “better” performance of your system.

But what is the correlation between something so abstract as 0,275 volts of DC in the mains socket and the sound of my system?   I think I can “try” to explain that.

It seems to me that any audio device that has a solid, strong, stiff power supply with low internal output impedance, and hence with a stable and ‘non-sagging’ voltage delivered to all the interested circuits, one that delivers a constant voltage, irrespective of the dramatically changing variety of possible load currents – such a system is bound to sound “good”.

However, consider the same device as before, with the same mains power transformer, but suddenly, … it ‘magically’ turns out to be a flimsy, frail, weak power supply, one with a high output impedance and hence, one with a voltage that “dives”, or sags, on each and every account when the circuit that constitutes the load wishes to pull some above average, substantial current peak from such a power supply. Such a system, driven by an inefficient and sagging power supply – it simply must sound worse.

If you look at the article of Mr. Rod Elliot at the beginning of this text – especially look upon that first table with the results of his measurements, it is enlightening to note how one and the same transformer can have completely different performance characteristics and working conditions.

How terribly those parameters change (to worse), as soon as the smallest amount of minuscule DC offset appears in the wall socket. This seemingly irrelevant amount of DC current, flowing through the primary winding – this results in such gruesome effects.

But if you think about it  -  it is very logical.

Please concentrate on, or calculate, the value of  ”Ampere-Turns” – which is a result of simply multiplying the current flowing through the primary, by the amount of turns that constitute the primary winding. This physical quantity, the “Ampere-Turns”, when further combined with the magnetic permeability of the iron core within that transformer, actually gives you an idea of the total magnetic flux that is flowing through the iron core.  A magnetic flux that might be just a tad too high.

Take a small DC current, say 1 Ampere (peak-to-peak) and multiply this by the number of turns in the primary winding.

But how much is that actually ? Well, from my experience when fiddling with extra windings added to an existing toroid transformer, I observed that each additional “turn” around the core of a toroidal transformer gives me roughly about 0.1 ~ 0.15 extra AC Volts per turn. This means, that a primary, which is supposed to work off from 230VAC, such a primary will probably be constituted of something like 230VAC / (0.115 VAC/turn) = 2000 turns, give or take.

So, now, returning to our calculations:

the “minuscule” 1 Ampere of DC offset current, peak to peak, flowing through the primary winding, multiplied by 2000 turns, gives a whopping result of 2000 Ampere-Turns. Now, throw into this the fairly high magnetic permeability of the iron core, and you embark upon an AC/DC Highway to Hell, as the magnetic flux in your iron core reaches significant values, expressed in Gauss / Tesla, and may be dangerously approaching core saturation levels  ( a pessimistic scenario), or at least an area of high non-linearity (slightly less pessimistic).

I suppose that “close to saturation” operation conditions of the iron core are by no means optimal for such a toroidal transformer. Especially from the viewpoint of the secondary winding, which is trying to generate your secondary voltage and currents that are needed to feed the rectifier, the filter capacitors, and ultimately – your whole hiend audio device.

Thus, in magical terms, the one and the same power supply mains transformer may have times, where it is “Strong” and “Powerful”,  capable of hefty current capability, but then, at other times, it suddenly becomes a flimsy, clumsy weakling, that falls down on it’s face every time a meaningful current is demanded by the load. Every such meaningful demand essentially translates to a sagging voltage.

But these above – these are only my wild assumptions.

However, let me provide you with an insight as to my practical observations:

As soon as my woman turns on that damn hair dryer in the bathroom, with the 1/2 power switch activated (essentially constituting a silly rectifier diode, that unsymmetrically loads the mains sine wave),  my 1400 VA transformer in the OTL begins to complain. This coincides with the voltage readings on the output power tubes slightly sagging in value.
Not much, but it is a correlation that I observe time and again, so it is not a coincidence. The sound originating from my poor feeble 1400 Volt-Ampere toroidal trafo is sometimes very much similar to that of an overloaded welding machine. When I first heard it, it was a shock and I was worried to such an extent, that I was about to pull the plug out of the socket. At that time I did not have the slightest idea of the reasons that are causing this phenomenon, and what was more irritating, why is this phenomenon “coming and going” – it suddenly appears, and then, for seemingly no obvious reasons, just simply disappears again, just as suddenly as it appeared.

As mentioned, I am “allergic” to the the sound of agony and the moaning of a toroid. Any toroid, because it then seems that there is some fault in the secondary circuit, some short-circuit, and that the fumes shall shortly appear again. The more so, because it happened to me once before – I actually fried a toroid once, it melted the plastic insulations and the stinking fumes were abundant.  The breakfast tray under it – that one melted also.

One more observation.  When that cheap hairdryer is shedding havoc, the music from my system is still playing.  Providing that the process of hair drying lasts just a bit too long, I can actually observe, that the Power supply block of my OTL, … … it actually becomes much hotter, than in normal operating conditions (i.e. = with no hairdryer being used in the bathroom).

Happy soldering !  … and PLEASE, do not get yourself killed, while you are at it.
Death – is permanent.   And most of the time – it is also painful.  Common sense – applies.

Best Regards,
zjj_wwa

P.S.  Last but not least, I would just like to mention, that I have some aliases to my hiend-audio.com site.  Any of the following links as below shall bring you to these pages: http://capacitorless.com 
http://currentdriven.com  http://diy-hi-fi.com http://diy-hi-fi.com.pl  http://diy-hi-fi.eu  http://diy-hi-fi.pl  http://hi-fi-diy.com  http://hi-fi-diy.com.pl  http://hi-fi-diy.eu  http://hi-fi-diy.pl  http://hiend-audio.com.pl  http://hiend-audio.eu  http://hiend-audio.pl  http://hiend-system.com  http://hiend-systems.com  http://hiendsystem.com  http://hiendsystems.com  http://highend-system.com
http://highend-systems.com  http://otlaudio.com  http://straightwiregain.com  http://transformerless.com  http://zjjaudio.com 

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