There are many contentious issues in this world of high fidelity. Supports, cables, tweakery and various forms of black magic (or accusations thereof) are among them.
I wish to address capacitors and burn-in.
I have recently performed some experiments. In brief, these experiments consisted of measuring the performance of an amplifier output stage, then replacing the capacitors with new ones of the same type, and re-measuring. I then replaced the capacitors again with new ones of a different type. The same output stage was used, and had been long run-in before the experiments were performed.
The measurement taken was a simple THD sweep from full power down to fractions of a Watt into an 8ohm resistive load at 1kHz. At this frequency, the resistive load is a reasonable approximation to the behaviour of a loudspeaker, unlike at very low or very high frequencies. I do not intend to get into this debate.
I regret that I am unable, for various reasons, to publish the actual results I measured. However, a brief summary...
At very low powers, no considerable difference was observed between the capacitors.
At the 1W power level, there was starting to show a difference. The worst capacitor was the new one of the original type, with the "burned-in" capacitor showing a small but measurable improvement. The other new capacitor performed better still, but this was a more expensive capacitor (I will not say what it was, but the type has been mentioned here on more than one occasion).
The more expensive capacitor maintained a performance advantage through higher powers up to about the 40W level. Of the other two, the burned-in capacitor maintained an advantage over the new capacitor of the same, cheaper type.
At higher power levels still, things changed. The two new capacitor types started rapidly increasing in distortion whilst the burned-in items remained lower (this is before the onset of clipping).
Going even further, just before the onset of clipping, the story changed again. Now it was the brand new cheap capacitor that gave the best result, with the expensive capacitor poorest and the burned-in cap just in front.
Conclusions...
1) There are measurable differences between different types of electrolytic capacitor
2) Burn in is a very real and measurable effect (The amplifier had been characterised when first built, and that characterisation matched that measured with the new capacitors).
Explanations
So, what changes? There is some misunderstanding about how electrolytic capacitors work. An electrolyte, as anyone who has done GCSE chemistry knows, is a solution that contains a large number of charged atoms (ions) and is very conductive. Clearly this is not the dielectric material. The dielectric in an electrolytic capacitor is, in fact, a layer of aluminium oxide on the surface of the foil that forms the plates of the capacitor. This layer gets damaged and made uneven manufacture, and before it leaves the factory, the capacitor undergoes a process called "reforming", where a larger than specified voltage is applied to the capacitor in an elevated temperature environment, in order to recreate this oxide layer (or "reform" it). This process is not entirely complete, and the capacitor may spend a year or two in various warehouses before it is actually used in a production item. Thus, when equipment is first put into use, and the capacitor is given a voltage and elevated temperature, the process is slowly completed. This is the burn-in that people have anecdotally refered to. The completion of the reforming process results in a drop in ESR, increase in ripple current and general slightly improved performance in the cap.
It is also extremely well known that electrolytic capacitors have a limited lifetime. If you have a piece of kit that is 10 or more years old and contains electrolytic capacitors, you may find an improvement in performance simply by replacing the capacitors with new ones of the same type, let alone higher performance items.
