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Repair Gallery......

Why amplifiers fail!

By ARS, Aug 30 2012 04:45PM

In the vast majority of cases this is usually down to the failure of either output or driver semiconductors or electrolytic capacitors. In particular heat is the enemy of such devices and poor layout by the designer is often a contributary factor. For instance a number of manufacturers mistakenly locate electrolytic capacitors alongside heatsinks or power resistors. An average quality 85C capacitor probably has an estimated lifetime of 3000 hours at that temperature. This is generally accepted to double for every 10 deg C reduction in temperature. In a well known Class A amplifier from the 1980's the heatsink is happily running at just over 65C with several capacitors within a few mm of this meaning lifetime is probably 12000 hours or so before they are outside the specified tolerance. If the amplifier is used for 3 hours daily then 12000 hours equates to about 11 years before such components are nearing end of life. Of course that is an extreme case afflicting only a few particular models.


Even worse are those parts of amplifiers that are left powered 24/7 when owners think a standby switch is turning it off. Several capacitors in a popular range of subwoofers are probably subject to an ambient temperature of at least 45C but with a 3000 hour life and unwittingly left powered 24/7 by owners this leads to just over a 1 year lifespan (and I believe there were many warranty repair claims thus confirming the maths!)


At the other extreme the very high quality main PSU smoothing capacitors (Slit Foil - presumably BHC) used in a Cyrus One or Two almost certainly had a specified life of at least 12000 hours plus. Located far away from any heat source they probably endure an ambient temperature of no more than 45C even taking into account any ripple current heating leading to a 48000 hour anticipated lifetime or ~ 45 years at 3 hours use daily. This is not as unreasonable as it sounds as a 40 year old Quad 303 in the workshop recently had it's original smoothing capacitors removed and tested fine for capacitance and ESR.


By their design electrolytic capacitors do have a finite lifetime. They are basically two rolled up aluminium foils usually etched to increase surface area with a paper dielectric in between impregnated with an electrolyte and then all sealed in a can. The electrolyte typically consists of ethylene glycol with aluminium borate as the main ingredients and a small percentage of water. The main difference between expensive long life capacitors and budget hobbyist ones is mainly the quality of the foil, seals and electrolyte. More heat means the electrolyte dries out over a shorter time scale and eventually capacitance falls and ESR rises. Several well respected manufacturers still routinely use some really nasty brands in their equipment probably knowing that 10 years on failure is not an issue when any warranty is long gone.


Semiconductor junctions although rated at quite high temperatures (e.g 150C and that is the junction - not heatsink!) will eventually break down under conditions of excess heat whether by poor design or owner abuse. The latter is often through placing other equipment on top of vents, hiding amplifiers away in enclosed cabinets combined with sustained running at high volume levels. Some failures are caused by those owners that accidentally short loudspeaker terminals together usually through poorly terminated speaker cables and exposed conductor strands - for those amplifiers without protection it usually spells instant death to an output device.


Frequently failure of a single transistor (output or driver) will lead to burnt out resistors and other failed transistors upstream or downstream (resistors usually cause the smoke you will see rising through vents as your amplifier is on it's death bed). It's no good replacing one failed device without having properly checked the others unless you wish to be quickly back at square one. It's a fact of life and probably always will be that your amplifier will be one of the least reliable parts of your HiFi setup (closely followed by cd players which can also be regarded as consumables!)