Refurbish capacitors inside speaker crossovers?

[fred40]

The manufacturer of my audio (electronics) equipment advices to refurbish the capacitors inside the amplifier every ten years or so. To let the amplifier sound again just like "new-recap" they call it-

Now I wonder since there are caps to be find inside the crossover filter of Harbeth speakers -I guess-is it wise to change these caps also once in a…? Or does this not apply to speaker crossover filters?

[gemincor]

I would not advice it at all.

[fred40]

And that"s because?

[BAS-H]

Possibly something to do with the fact that amplifier caps tend to be large and, particularly, do a lot of work. They get warm, which limits life due to electrolyte loss. I don't think speaker crossover caps do nearly so much work.

[A.S.]

Before we can begin to give a worthwhile answer to this question, we have to understand something about how a capacitor is constructed. Otherwise we may become a victim of half-truth and spend good money "upgrading" what cannot be upgraded.

Almost all references to 'capacitors' on the internet dive in with lots of complex maths (which I don't understand or need to know) such as here. Forget all of that stuff and just skim down the article, note that capacitors come in many shapes and sizes (and colours and voltage ratings too). The key words to look out for concerning how they are constructed are plates and dielectric.

Then have a look at this much more digestible article here.

Now with those two crucial words plates and dielectric we can see from this article that there are many types of capacitors made from different combinations of plates and dielectric, each with particular advantages (and disadvantages) such as size, cost, availability, case style, voltage capability, ageing, tolerance etc. etc.. There is no perfect capacitor and the equipment designer must balance cost v. size v. those other factors in deciding what exact type of capacitor to use and in what capacitance value.

As a general rule, the physically larger the capacitor the more difficult it is for the manufacturer to control the exact capacitance value hence the tolerance becomes greater. For example, as new a small thumbnail capacitor may be supplied as +/- 5% tolerance either side of the value printed on the case. A new big power supply capacitor the size of a yoghurt pot may be +/- 50% of the value printed on the case. If the true value is +50% that's probably a good thing as the capacitor will have more reserve and smooth out power supply hum better. But if the true value is -50% that particular amplifier will (perhaps) have a weaker bass and more audible psu hum.

[garmtz]

And another answer...: the electrolitic capacitors in the power supply of an amplifier are prone to deterioration over the years, because they are that: electrolitic.

Any 'dry' capacitor does not really need to be swapped out, just the electrolitics in power amplifiers and then only after 20 years (or 10 years if the equipment is always on, like NAIM). Still a NAIM amplifier will still work after 20 years, but possibly slightly below spec.

[A.S.]

Ok, that may be so but you've jumped ahead a little. Most of our readers are non-technical and you need to work progressively towards the answer you gave to carry the audience with you. We only want to be covering this subject once here on the HUG so our explanation should be comprehensive for all, yet simple. So if I may suggest a step back ...

I mentioned in my previous post the key words of plates and dielectric. That's all you need to make a capacitor. You can make one on the kitchen table, and indeed, the first capacitors were nothing more that two metal plates separated by an air gap - or technically, an air dielectric. The plates have to be something which will conduct electricity but the dielectric absolutely must not conduct electricity otherwise the opposite charges that builds up on the two plates will leak through the dielectric and cancel themselves out: the capacitor will become a resistor. Air is an excellent dielectric even up to frighteningly high voltages - which is why our mains supply carried overhead on pylons doesn't short together.

Anyone want to take up the explanation of exactly what dry and wet dielectric means and why capacitor inventors moved from the air dielectric of the early capacitors to the modern types?

[Supersnake]

The manufacturer of my audio (electronics) equipment advices to refurbish the capacitors inside the amplifier every ten years or so. To let the amplifier sound again just like "new-recap" they call it-

Now I wonder since there are caps to be find inside the crossover filter of Harbeth speakers -I guess-is it wise to change these caps also once in a…? Or does this not apply to speaker crossover filters?

Shall we please return to the original question and have a qualified person answer it directly without delving through matters of design?
Alan, as the manufacturer of Harbeth speakers "is it wise to change (Harbeth capacitors) once in a…?"

[A.S.]

I've been directly asked a reasonable and serious question. I have freely given up my time to build up to an answer which, in the spirit of this Harbeth User Group will stand the test of time i.e. only needs to be asked and answered once.

If you think it can be answered with a simple yes/no answer then you better look elsewhere. Nothing in audio is black/white, yes/no. Everything is in the grey areas. Unless you appreciate the variables that operate in the grey areas, you may as well flip a coin.

Is your need for an instant answer so pressing that the knowledge that would enable you to make your own decision (not take my word for it) just too much bother?

[EricW]

I agree with Alan.

Look, we have here a well-respected and experienced speaker designer who is taking time he does not need to take in order to answer a question in a detailed and serious way, and in such a way that people will not only know the answer, but will (hopefully) understand it.

I'm sure it would be the easiest thing in the world for him to say "yes it matters" or "no it doesn't" and leave it at that. But how would that advance anyone's knowledge or understanding one bit? All you know at that point is what somebody else thinks, without any idea of why they think so. And it's that type of "knowledge" that renders us all vulnerable to the agendas of those who speak not because they want to impart truth, but because they have something they want, and their communication is intended to manipulate people in that direction. Alan Shaw may be someone you can rely on, but others will not be, and how will you tell the difference? Unless, of course, they go step by step and tell you why what they believe is the truth.

So I for one am grateful for Alan's time and energy and will do my best to follow the discussion however he wants to present it.

[A.S.]

OK, back on the subject of capacitors, and how this relates to crossovers. Shortly, you will be able to identify the various types yourself and make your own decision about this issue just by physical examination of a speaker crossover.

Capacitors come in many shapes, sizes and colours. The next think we should dismiss to avoid confusion is colour. The case colour of the capacitor has no technical correlation with its construction, usage, durability or longevity. It does seem to have a connection with its marketability, since certain colours (yellow, purple, red) seem to add more perceived value in audio circuits. But if you are a big enough customer you can have any case colour you want: sparkly pink if you think that will add some value to your product and you order a million or two pieces. So forget colour.

We have to make a solder connection to the two plates of the capacitor, so the fixed value capacitor must have two solderable wires. The next issue is how those two wires exit the capacitor's body. There are a few alternative methods. The super-miniature capacitors used in mobile phones are a special case, but for normal consumer electronics, and certainly all speaker crossovers, there really are only two choices: the capacitor will be either the so-called radial or axial construction. The significance of this will become apparent later.
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P.S. The choice of either the radial case (wires coming out of the bottom) or axial case (wires coming out of opposite ends) is down to the equipment designer. For a given capacitance value, axial cases are physically larger. This means that for a given area of circuit board, you can fit far more radial cased capacitors than axial ones. We prefer radial cased capacitors for that reason.

[kittykat]

Happy Birthday Alan!

[Supersnake]

OK, birthday morning and back on the subject of capacitors, and how this relates to crossovers. Shortly, you will be able to identify the various types yourself and make your own decision about this issue just by physical examination of a speaker crossover.
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Hear Hear! After reading your latest installment you've made me a devotee of your splendid exposition Alan.

[Gan CK]

Happy 54th birthday Alan!

[A.S.]

Before we try and crack open a capacitor or two, I'd like to think about how a capacitor can fail, be it a slow gradual degradation or an instantaneous one. We know that electronic systems are generally very reliable. There are electronic circuits in space probes hundreds of millions of miles away from earth sent many years ago which are still working perfectly. My guess is that those circuits, designed for longevity and reliability as the primary requirements (regardless of cost) resulted from one or two key decisions at the design stage. And undoubtedly one of those decisions would have been to use as few capacitors as possible in the circuitry because capacitors do fail.

Before we look inside one, I want to show you some macro photographs I took today of the point at which the metal solder-leg actually enters the body of the capacitor. The quality of the junction between the tinned-wire and the case material is extremely critical and all-important to a reliable long service life. The reason is that with time the wire will, under the microscope, reveal that it has tarnished (a layer of oxide build-up) and as it oxidises it swells. That swelling will prize-open a very small air-hole in the case around the lead wire. And that microscopic hole, vastly bigger than an air molecule will allow the atmosphere to penetrate the inside of the capacitor's body - and conversely, for whatever is in the capacitor to slowly escape. Remember that we said in an earlier post that the capacitor is made of two conductive metal plates. And metals, just like the copper lead wires, corrode. So if there is even the tiniest air leak in the case - and the most likely place is around the lead-out wires, not only with the leads tarnish but the plates too. And given enough time the oxide build-up on the plates will grow towards the opposite plate and bang! The end of the capacitor.

From the attached pictures you should be able to identify radial and axial capacitor bodies. Attached are some close-up pictures of the point that the lead-wires actually enter the case. The outer case is a hard material cylinder or box: the ends of the case are open to permit assembly of the capacitor. The final assembly operation is to seal the open ends of the case. In these four examples, a thermosetting liquid has been poured into the open end(s), and this then dries to seal the lead wire. The pictures shows the resulting seal.

As we will see later, sealing the case ends with a runny and expensive liquid is the most expensive way of making a quality capacitor. There are far cheaper and faster curing methods in common use ....

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[DSRANCE]

Alan, I do appreciate your need to explain in a series of posts what capacitors do, how they're made and how some may age more than others - please continue.....

Post edited while this fascinating subject unfolds :-)

[A.S.]

Moving on.

The capacitors we have looked at so far have been of the dry-film type. That means that when we cut them open (next step) and examine the metal plates we'll see that they are formed from a long, wound ribbon of extremely thin metal foil or film. When we touch the film and the dielectric that holds the film apart, they will be dry to the touch. But that class of capacitor is only one of the two types that we'll find in loudspeaker crossovers: the other one is the wet-film or electrolytic capacitor. And this is the point where the whole business of crossover quality, longevity, size, cost and performance becomes complex and intertwined.

In the dry-film capacitor market there are two basic types: the dielectric is made from non-conducting polyester or polypropylene plastic - a good insulator. There are strongly held views that one may 'sound better' than another: it's hard to believe that. Technically their characteristics are very similar at audio frequencies, and any technical differences that may exist start to show up beyond audio frequencies. So for all practical purposes at audio frequencies, they are interchangeable if they have the same capacitance value, although the polypropylene ones tend to be physically larger. Harbeth uses polyester dry-film capacitors out of preference.

Now, the electrolytic capacitor was designed for use in equipment power supplies as the main smoothing or reservoir charge store where one terminal is always positive and the other negative. At some point - I think in the 60s - a far east supplier developed a reversible electrolytic capacitor. This could be used in the signal path and before long this started to appear in loudspeaker crossovers.

The 'revelco' has two advantages compared with dry-film capacitors: low cost and small physical size for any given capacitance value. The advantages of cost are obvious - that of small size not so obvious. In practice, the largest dry-film polyester capacitor that can be reliably manufactured is 10uF - ten micro farad. In the same case size as just that one 10uF polyester capacitor you could have a 100uF reversible electrolytic. Yes, that's the remarkable thing about the wet dielectric inside electrolytic capacitors: it hugely magnifies the capacitance value for a given case size: let's say ten times the capacitance for one tenth of the cost - surely a win-win? And very rapidly, reversible electrolytics became the standard crossover capacitor, even in quality loudspeakers in the 70s*. You can see here a typical crossover - the three tubular black components are reversible electrolytics. And here is a crossover using both a reversible electrolytic (shiny blue) alongside dry-film (big black) capacitors.

So, is the reversible electrolytic is the best capacitor solution with its large capacity, small size and low cost? Maybe. Next we better look inside one.

* I believe that it was KEF that championed the use of reversible electrolytic in the 60s. The Concerto and later 104(AB) certainly used them.

[A.S.]

Now we're entering the final phase of this subject - and one that I hope will give you something to think about if not a shock! But first, a look inside these capacitors.

I took a pair of pliers and using minimal force squeezed and crushed the case of a blue-sheathed 150uF electrolytic capacitor. Whatever working parts were inside did not fill the entire case and the case being made of soft aluminium, was easy to bend. Then I took a pair of tin snips and cut open the case to reveal the inner workings - see pictures A, B, C, D and E. The ends of the electrolytic were sealed with some rubbery compound through which the lead-out wires passed as you can see.

What did surprise me having not actually looked inside one of these reversibles was just how little material they contain. Unwound fully as you can see the plates cover an astonishingly small area, rather less than that of a plastic ruler. Yet the value of this capacitor - 150uF - is, as we'll see later one of the largest used on any crossover network.

There are several interesting features of the reversible electrolytic capacitor which will become apparent when we compare it with the dry-film polyester capacitor ....

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[A.S.]

We can directly compare the inner workings of the 'modern' economical and small reversible electrolytic capacitor with a traditional 'old fashioned' polyester capacitor. The electrolytic in the previous post was one of the largest reversible electrolytic values available (150uF) but this time we have a 10uF (fifteen times smaller capacitance) which is one of the largest polyesters values available. Despite the 15:1 difference in capacitance, the case sizes are similar.

I have to admit that after several minutes I gave up trying to completely deconstruct this polyester capacitor - it beat me. Whilst fundamentally the same two-plates-and-isolating-dielectric as the reversible electrolytic capacitor, the entire construction was entirely different. As soon as I took the pliers to the red case it was obvious that even if I hit it with a hammer, it was not going to break, and even to make a small crack in the hard, epoxy resin coating was quite a challenge. This epoxy coating very effectively seals and protects the working core of this capacitor.

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[A.S.]

As noted, one 10uF polyester capacitor is more expensive than one 150uF reversible electrolytic. So the economic argument to use the reversible electrolytic in every circuit position possible is overwhelming. It dramatically reduces component cost, and, because the reversible's case or 'footprint' is so small, it saves on the cost of glass fibre PCB because the cost of a circuit circuit board is directly proportional to its area, not its track complexity. And for these reasons the reversible electrolytic has been for many years the main capacitor used in loudspeaker crossovers. And why not? The speaker company saves cost and as the circuit is not seen by the consumer, why should he care*? And indeed why should anyone care? After all, a capacitor is just one (small) constituent part of the entire speaker system; one of the cheaper parts in fact. Let's look at the historical position from the BBC's perspective.

Most of the BBC's loudspeaker development history (LSU10 - LS5/5) had been undertaken in an era when reversible electrolytic capacitors had yet to become the mainstay of speaker crossovers. Look inside a 70s LS3/6 (see picture) or LS3/5a and you will not see reversible electrolytics: you will find polyester dry-film capacitors. Relative to the development cost of a BBC monitor speaker the component cost saving would not have been even considered; nor the size eduction in the pcb. But revelcos have been used in BBC monitors: the active LS5/8 uses them in-line with the tweeter to protect from amplifier thumps. Also, the early generations of reversibles were notoriously prone to leak and high resistive losses (so they became hot); they were (and still are) offered as 'standard loss' and 'low loss' where there is a trade-off between internal electro-chemical construction and the voltage they can tolerate. They are by now, fifty or so years on, billions of components later, the normal crossover capacitor around the world.

One unarguable advantage of the reversible is size and weight reduction compared to the polyester type. As an example, we can directly compare one 150uF reversible with fifteen 10uF side by side - see picture. Functionally they will perform similarly at audio frequencies. We could in theory, chose either the one piece of 150uF electrolytic or the bank of fifteen 10uF polyester capacitors to perform our crossover filtering role. What sort of business case could we make for using the ten polyesters, hugely increasing the cost of the circuit for perhaps negligible technical advantage today? If I was a junior engineer in a large speaker company I'd be told to use electrolytics on cost ground and that would be the end of the discussion.

*I mentioned recently that audio reviewers should always take a look inside the speaker. I was ridiculed. But even a cursory examination of the crossover board can tell much about the design, the manufacturer's approach to cost saving, and the expected service life of the speaker before the crossover capacitors degrade.

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