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Burn-in

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    Posted: 21 Apr 2008 at 10:31pm
Burn-in

Here is the first part of my long awaited article on burn-in. It is a lengthy explanation, so as usual I’m doing it in stages.

Introduction

Most people realize that it takes two wires to make a light bulb work.

Few realize that it takes two or three wires to make an electronic circuit work.

Let’s just concentrate on the two wires – three just makes it harder to explain.

Across these two wires will be a voltage of 3 to 36 volts, depending on the design, for a preamp circuit, and in the region of 80 to 100 volts for most solid state audio power amps. For tube amps the voltage can be in the region 200 to 500 volts.

The type of voltage is DC (direct current). In most cases it is derived from the AC (alternating current) mains by means of a transformer and rectifier. This produces a waveform of one polarity operating at a frequency twice that of the mains supply. What we need is a waveform-less voltage, otherwise we’d hear the twice mains frequency waveform (and a lot of distortion). This frequency is reduced to give an acceptably low ripple voltage on the DC by the property of the capacitor. A very large value, often 1,000 micro farads or many times more, is needed to “smooth” the rectified waveform to “flat” DC.

Often in the precision circuits of preamplifiers the DC has to be far smoother, so a voltage regulator is placed after the large value capacitor.

Sometimes batteries are used instead.

Whether battery or voltage regulator, each exhibit some output resistance which would steal power from the load. Enter the capacitor again - placed across the supply after the regulator, the charge on it makes the resistance tiny at audio signal frequencies.

Now, the signal is a two wire thing too - the wire that carries the signal and the wire that carries its return. Remember, the minimum number of wires to make a circuit is two.

The return wire in an audio circuit is called ground. Most people see ground as being the negative (usually) wire in a two wire supply (the middle wire in a three wire supply), but in actual fact the ground is both wires! (or all three in a three wire supply).

The AC resistance (impedance) between the wires of a DC supply must be zero or as near as damn it. A short circuit would achieve this but then there’d be no power, just a blown fuse or a burnt offering…

How do you short the power rails together at signal frequencies but not at DC? Yes, a capacitor again!

To get the high value of capacitance required and still have room in the case for the rest of the parts means using electrolytic capacitors. And because they connect the signal between the power rails, you have a case of electrolytic capacitors in the signal path – try doing it without them.

A lot of people who’ve been fed misinformation may detest this fact and argue against it, but all currents flow in circles – to make a light bulb work requires two wires to make the circuit, doesn’t it?

The output of an amp is delivered to the load from the power rail(s) and then to ground. It then has to return to the power rail(s) to complete the other leg of the circuit. What through? The electrolytic capacitor!

OK, you’ll have heard of bypassing, but that’s a smaller (film usually) capacitor which can’t do the bass end, and quite often doesn’t do the midrange either (depending on the impedance of the load). So the bass and some of the midrange of the signal flows in an electrolytic capacitor, period.

Yes, even if the input, output and negative feedback network are DC coupled. Sorry to upset the “all capacitors are sh*t” brigade, but they ought to try getting a proper education. Try doing it without these electrolytic capacitors and the signal will be far worse than sh*t.

Why so long though

Just why does it take such a long time for a high performance (low signature – high musicality) piece of audio electronics to burn-in?

This is why my introduction has concentrated on the requirement for electrolytic capacitors.

Although every component within a circuit has some burn-in time associated with it, by far the longest is that of the electrolytic capacitor. All electronic components comprise a junction or number of junctions between one substance and another. Before power is applied for the first time, these junctions that have been thrust together in the manufacturing process are in tight intimate contact with one another.

When power is applied current flows. Current flow exists because electrons flow (in the opposite direction). Electronic components are so designed that electrons can flow – the physics of each substance are such that an electron can escape the atom of one and join the atom of the next, apart from capacitors that is, which we’ll visit in more detail shortly.

The arrangement of molecules and atoms before switch on is to an extent more haphazard than it becomes after switch on. Once switched on, these things start to move, rotate and generally sort themselves out into the correct order, and in the process of doing so generate a small amount of heat. Heat implies expansion and expansion requires “give”. I’ve heard many people talk about giving working parts time to relax in new things. Most of this “give” happens quickly – often within the first hour or two, and generally it is a one way process – once done, it is done.

Capacitors on the other hand don’t all possess this one way process. Much depends on the type of dielectric – the barrier between the plates that prevents DC current flow, but allows the back-forth AC electron behavior (AC current flow).

Film capacitors (the ones that aren’t electrolytic) comprise in their simplest form, two plates isolated by an insulator. The insulator is the dielectric. The atoms of the plates cannot exchange electrons with the atoms of the dielectric preventing DC from flowing through the capacitor. In a real capacitor however, there will be some leakage, although in film capacitors it is extremely low, it clearly shows that there isn’t such a thing as a perfect insulator, because the odd electron makes the journey through.

As electrons “push and pull” against each other, the junction between conductor and insulator – between plate and dielectric, will obviously result in some miniscule erosion at the junctions. The intimate contact of the manufacturing process is adjusted by nature to what nature decides. This process will be over and done within hours to a few days – maybe up to a week. As film capacitors tend to be used for the mid to higher frequencies then the upper end of the audio spectrum should start to sound more real within this time, and because we can retain in our memories the things that have happened in the recent past quite well, and further because our hearing is more sensitive to the mid frequencies, we can detect a change in the sound over this “first” period.

And now the electrolytic

People are prone to imagine all sorts of things and then to provide an argument to substantiate what they’ve imagined.

Therefore I have gone to great lengths to study and research electrolytic capacitors from a burn-in perspective.

An electrolytic capacitor is again two metal plates separated by some insulation. To obtain the large values of capacitance the plates are large in surface area and comprise long lengths of metal foil which are ultimately rolled up into the typical cylindrical shape of the electrolytic.

The electrolytic is assisted by the property of a battery in that large amounts of charge can be stored by the addition of an electrolyte (hence the name).

The electrolytic capacitor therefore comprises an anode plate – a foil that is coated in a dielectric (an insulator), the electrolyte – in its simplest terms: a salt in aqueous (water) solution, and the cathode – the other foil which isn’t coated. To distribute the electrolyte along the full length and width of the foils it is soaked into “separator” paper, and the whole assembly is tightly rolled-up inside the canister.

The dielectric coating is what makes the capacitor do its job. The dielectric in an electrolytic capacitor is aluminium oxide, which is like the anodizing you see on a front panel, except that in the electrolytic capacitor it has not been “sealed” and is therefore not stable like the coating on a front panel.

The aluminium oxide is grown onto the aluminium foil by the process of anodizing large sheets of foil. These foils are then slit to the size required for each capacitor, and the manufacturing process is continued to form the finished product. Before the end of production the manufacturer applies voltage treatment to grow the aluminium oxide to the correct depth to give the required capacitance and dielectric properties.

The electrolytic capacitor relies on charge to maintain its aluminium oxide dielectric. Without charge the dielectric decomposes over time.

When placed in a circuit – for example between the power rails as we discussed in the introduction to this article, the capacitor is charged by this voltage. Because it stores this charge, it will release energy when there is no charge being received by it, and it will receive energy when it is under charge. Therefore it can be seen that it will “smooth” the “twice mains frequency DC” into a near constant DC voltage.

It also conducts the signal (It doesn’t conduct DC), and is often used at other points in a circuit where the signal needs to travel between two different DC voltages.

Because of the difference of DC voltage across it, it maintains its charge which in turn maintains its dielectric layer. And this is the key that answers the long burn-in question.

Mass production?

One of the virtues of mass production, especially when coupled with JIT (Japanese Just In Time) inventory control, is that all components are delivered “fresh” to the manufacturer. The very description “mass production” implies mass demand, and so the manufactured goods are quickly sold, and between component manufacture and the time of first use perhaps only a few months have passed.

Therefore any electrolytic capacitors within the product will still have a near perfect dielectric layer.

Now take the system we have in the UK of supply to small manufacturers (which is probably similar in Europe and the United States). Small manufacturers buy from component distributors – often big ones that specialize in supplying thousands of small enterprises.

They buy in stock occasionally and when they buy, it is in extremely large quantities for the purpose of making a large profit. This stock can lay around in warehouses for several months, or in the case of the Elna Starget audio electrolytic capacitor RS Components of Corby, England used to stock, several years (I knew this because Elna UK told me so).

Even if the small equipment manufacturer buys a special batch of electrolytics through a distributor, in which case they are “fresh” upon delivery, the batch will be a rather big one to be able to obtain a sensible price (minimum order quantities apply). The usage rate will be slow, and the storage time long.

In either case, the electrolytic capacitors in any piece of equipment made by any small enterprise will have had their dielectric layers decompose more than those inside a mass produced item.

But as mentioned above, voltage treatment (it used to be called re-forming in the valve days) will restore the dielectric layer to as good as new. However, voltage forming in a capacitor factory and voltage forming in a small enterprise are two different things. In the capacitor factory a machine handles voltage forming. Such a machine has a throughput of thousands per hour (doing several at a time). It takes a full hour to voltage treat an electrolytic capacitor - you can’t do it faster because if you did, the dielectric layer would be damaged. We, like many other small enterprises, would need to voltage treat about 100 per hour. Not being able to fund the capitol investment required for a bespoke voltage treatment machine, we, like the rest, simply cannot perform this function without substantially increasing the prices of our products.

Now even if we, and other similarly sized enterprises, did voltage treat every electrolytic capacitor, the dielectric layer starts to decompose again the moment the voltage treatment stops, until the equipment gets switched-on and the capacitors receive charge in their normal course of usage. There is no way we can know how long a product will stay on the shelves of our distributors and dealers. Our turnover is good for the size we are, but the demand for our products isn’t the size of Apple’s.

Therefore, when you receive your Graham Slee phono preamp or headphone amp, the voltage treatment of the electrolytic capacitors it contains begins from the moment of switch on.

How long will this take? Well, as I said above, in the factory it would take one hour using the prescribed technique (see the Rubycon reference below for details), but few capacitors in a real circuit are exposed to the right conditions. Power supply capacitors should, one would think, re-form instantly, but more decomposition to the oxide layer is done at switch on due to the lack of a controlled charging current – it’s up to voltage in a flash and at full ripple current.

Electrolytic capacitors elsewhere in the circuit may not have sufficient charging current to be re-formed within the hour. In fact it can take days if not weeks for the tiny trickle of current dictated by the circuit’s operation to reach the specification dielectric.

This description may be leading many to wonder if they’ve been buying scrap? But the picture isn’t that bad. The capacitors are generally OK from a correct (or near correct) value point of view. It’s just that the quality of dielectric is wanting at switch on. The quality of dielectric affects the leakage performance, and although the circuit will function correctly, by which I mean it will establish the correct voltages, the sound quality at switch on clearly isn't that which the product gives after a number of weeks use.

Therefore a piece of high performance hand built equipment simply will not sound as intended at first. It may not sound as intended for a number of weeks. The sound can go through changes as one capacitor comes on-stream while the others are still getting there.

And when it’s switched off? Even for a short time there will be some decomposition of electrolytic capacitor dielectric because of the lack of charging voltage. However, it should not take anywhere as long to return to the burned-in performance level. But where equipment is stored for a year or two...

References:

http://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/CautionAlumi_Eng.pdf

http://www.cde.com/catalogs/AEappGUIDE.pdf

http://www.elna-america.com/products/pdf_files/AL/al_caution.pdf

Old Linsley-Hood circuits which applied a polarising voltage by means of diode drops



Edited by Graham Slee - 23 Apr 2008 at 12:50am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stuxter Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 2008 at 10:53am
Amen !     Tongue
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Millier Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 2008 at 1:30pm
Thank you for the explanation, Graham, very interesting.
 
I can't say I've heard (or at least can remember) the slightest change in sound quality despite hundreds of hours of burning in of the Solo but Your Mileage May Vary as they say...
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 2008 at 1:49pm
Dave, you're special mate! One in a million...Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dvv Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2008 at 9:34pm
Also Shprach Grahamthustra ...
 
... so, what small enterprises can do, and by extension should do, is to PROPERLY form a large capacitor, where "large" arguably applies to anything larger than 470 uF.
 
This is tedious process which requires little brains, some time and a simple setup rig. It consists of a single power transfomer, delivering say 20V at its single secondary, let's say with 200 mA or a bit more, 1A will not hurt, but even so, a simple 20...30 VA transformer. No need for toroids.
 
Essentially, the rig will take the transformer, a full wave bridge rectirifier, discrete or block (but in this instance, I would always go for a block for best possible diode matching), a say 2,200 uF/50V capacitor, a three pin fixed 5V, a three pin fixed 18V regulator, a 1,000uF/35V cap, and a 100nF/100V cap just before each regulator and as the last component.  Some resistors may also be in order, their job being to limit the output current, thus prolonging the charge times of the CUT (Capacitors Under Test).
 
Lastly, a very high power, 17W or better, resistor is also required, value say 2K.
 
A new cap is first connected to the +5V section. It is allowed to fully charge and should do so slowly, ideally during 30 minutes or so. After say an hour, it should be disconnected and then carefully discharged by the said high power resistor, held by pliers (because it will get rather hot - watch it!). Repeat the procedure once more with the +18V line, and you will end up with a propery reformed capacitor which I guarantee will last at least twice as long as one taken from stock "as is".
 
Graham made a DAMN valid point, people, though he forgot to mention a rather important point - different quality capacitors have very different so-called "shelf life" times. "Shelf life" refers to a capacitor's ability to be reformed to stated specifications after spending so much time sitting on a shelf, or in some warehouse. Low cost, even lower quality caps have shelf lives of a year or two at best, whereas high quality and cost types, such as Siemens Sikorel series, have shelf lives of 10 years. And they really do, I tried it twice for the hell of it and both times ended up with values exceeding those specified.
 
This treatment will do two things: 1) it will make capacitors work and generally live considerably longer than either long waiting or brand new caps, and 2) it WILL make your gear sound better by virtue of better power supplies if DC coupled, and better yet if AC coupled.
 
One more, far too often neglected point (which gets me another beer from Graham) - all this is no less true and no less important if you have fully electronically regulated power supplies, because no matter how you stabilize the power on demand, if poorly filtered junk from poor capacitors gets into the system, you will eventually end up with wonderfully stabilized junk power just the same.
 
That's telling 'em, eh, Graham? Wink
 
If anyone's interested, I can draw a schematic and either put it on here publically if Graham agrees, or mail to as personal mail to interested parties, free of charge, of course.


Edited by dvv - 23 Apr 2008 at 9:37pm
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"If anyone's interested, I can draw a schematic and either put it on here publically if Graham agrees, or mail to as personal mail to interested parties, free of charge, of course."

Yes please!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dvv Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2008 at 11:17am
 
First of all, Graham, please, look it over carefully, I am notorious for flipping diodes over. I sketched this from memory, I made mine some years ago in three samples, and they are not with me, but are with the people who assemble my products.

 

This is the basic circuit in a slightly refined form. It can be made simpler still by omitting some parts, like C7 ... C10, F1, etc, but since this is a one-off model, probably the only one you will ever make, I wouldn’t recommend it.

 

A few words.

 

After C3 and C4, you should allow space for 2W 5% resistors. They were not drawn in because they are optional, you can use them, but are not obliged to do so. Their value will vary depending on the size of the cap you are reforming, because it’s obviously not the same thing whether they are say 470 uF or say 10,000 uF. Also, their value will depend on your patience, how long are you willing to wait for it to charge. Setting a nice value of say 1 minute for a 470 uF cap will make the wait rather long in case of a 10,000 uF cap. As a general rule, you can use say 150 ... 200 Ohms for large caps (2,200 uF and over) and 1K ... 2K for 1,000 uF and smaller, but again, this depends on your patience.

 

Speaking of which, remember that capacitors are a bit like batteries, they prefer to be charged slowly, a luxury we seldom afford them, but in case of reforming, a well advised precaution.

 

CAUTION !!! Make sure the cap you are reforming is rated at 20V or more when using the +18V output!!! Remember, an overstressed capacitor acts like a shrapnel bomb, when it blows, it could send pieces flying all around and cause serious harm. Ask me about it, I still have a nice scar on my left hand from just such a case.

 

The output termination should use wires with crocodile clamps, which should make it easy to change caps as required.

 

Your are NOT advise to simultaneously use both outputs unless you have a larger transformer at work, where larger means 30VA or better. You can, but the charge times will be significantly prolonged and both the transformer are the regulators may become rather hot.

 

You are strongly advised to use reasonable size heat sinks, with thermal compound, for both regulators.

 

A proto board will do very nicely.

 

BOM (Bill Of Materials):

F1 – Slow Blow 0.5 Amp fuse (actual value will depend on transformer power rating)

C1 – 4,700 uF/35V electrolytic capacitor

C2, C3, C4 – 0.1uF/100V capacitors (recommended: Wima, Siemens, Plessey)

C5, C6 – 1,000uF/25V electrolytic capacitor

C7, C8, C9, C10 – 22 nF/100V capacitors (22 nF = 0.022 uF)

D1 – brodge rectifier, B80C1500 (80V, 1.5 Amps block)

D2, D3, D4, D5 – 1N4007 diodes

U1 – 7805 three point fixed 5V output regulator

U2 – 7818 three point fixed 18V output regulator

Rx (not shown) – 200 Ohm, 2W, 5% carbon resistor (for smaller caps)

Ry (not shown) – 2K, 17W, 10% wire-wound resistor (for larger caps, 1,000uF and over)

 

This is a basic circuit used for a very specific purpose and it does not require top notch components, although, like any circuit, it will only benefit from quality parts.

 

The net effect of this circuit will be properly (“by the book”) reformed capacitors. You may well find that when formed or reformed properly, most will exhibit values in excess of nominal; for example, larger capacitors are typically declared as “x,xxx uF / +10, -40 %” and with this method, more often than not, you will make it to the “+” part of the declared range. In addition, capacitors will last at least typically twice as long as they would if just soldered in “as is”. By this, I don’t mean that they will simply go on working, most will do that for 30 years anyway, I mean they will be up to their spec much longer, they will maintain their quality as well as their quantity for longer periods.

 

Why is this not generally done, as all this has been known for at least 40 years and probably longer? Because it’s time consuming and most unpractical for mass produced items, where a manufacturer is looking at something like several hundred thousand capacitors in just one production run. It’s too complicated and expensive for them.

 

Graham, in your book, am I good for a pint?Tongue

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