They may last forever in so-called “textbook conditions”, but do capacitors used in real-world situations really have a finite lifespan?
By: Ringo Bones
The topic of capacitor mortality entered the conversation radar of my audiobuddies late last year when Sunfire founder Bob Carver voiced his concern after many of his Tracking Downconverter / PWM power supply equipped power amps “flooded” the online second hand market. And given a majority of these are from the mid to late 1990s and on average are 20 years old – or a bit older, Carver cautions the potential buyer to first service and / or troubleshoot these amps, especially check the power supply capacitors since given the pulse-width-modulation type power supply nature of these amps, the main electrolytic capacitors have a theoretically much shorter lifespan than power amps using conventional linear power supplies. Another power amp that’s very popular in the online second-hand market is the Musical Fidelity A1, which requires a careful main power supply capacitor examination since it runs hot at 50 to 65 degrees Celsius and the inside where the capacitor lies could get much hotter.
The power amplifier’s overall layout and operational temperature plays a very important part on the “mortality” of its main power supply filter capacitors since the leakage current of insulators and consequently all capacitor types, increases with temperature due to Arrhenius’s Law. The power dissipated in a capacitor subject to alternating current stress or ripple is a product of effective series resistance and RMS current. The effective series resistance and impedance of capacitors are both dependent on which frequency they operate and hence must be ascertained at the relevant frequencies. Assuming sinusoidal waveforms – as in a typical linear power supply – RMS current can be easily determined. With other waveforms – as in those Bob Carver power amplifier designs equipped with Tracking Downconverter / pulse width modulation type power supplies, the only accurate method requires the use of the Fourier analysis of the capacitor voltage waveform to determine the frequency components and hence permit calculation of the current for each harmonic involved. For those preferring to avoid the complications of Fourier analysis, a workable rule-of-thumb is to check the temperature rise of the capacitor case. If less than 5 degrees Celsius above the ambient temperature of inside the power amp’s casing of say 55 degrees Celsius maximum, leave well alone; if greater (as in hotter) – investigate immediately.
The normal failure mode of aluminum electrolytic capacitors is gradual parametric degradation after years of operation. But I’ve noticed to cool running power amp designs whose insides are only 3 to 5 degrees Celsius higher than the ambient room temperature, the main power supply electrolytic filter capacitors seem to measure within its parameters after 25 years of regular operation –i.e. 4 to 5 hours a day being played; Whereas for other capacitor types, normal failure mode is by short circuit. This parametric degradation results from the finite volume of electrolyte and the consumption of available oxygen.
The leakage current of normal capacitor operation consumes minute quantities of oxygen from the electrolyte, releasing a corresponding amount of hydrogen which is either reabsorbed in the electrolyte or escapes to the atmosphere. Excess quantities of gas result if: 1) the said capacitor is subjected to DC voltages way beyond its rated capacity. 2) The capacitor used in power supply filtering is subjected to alternating current for several minutes perhaps due to a short circuited rectifier diode and the power supply fuse not blowing. 3) The DC bias is externally reversed by circuit action and / or alternatively internal reverse DC bias due to excessive ripple or discharge currents. 4) Series or parallel connected capacitor banks not having suitable voltage or current sharing provisions.
When considering capacitor failures, it is essential to remember that all electrolytic capacitors comprise of two polarized capacitors back-to-back connected in series. This is implicit in the construction methods, both for polar and non-polar devices, which differ only in effective working voltage of the two capacitors. So if that Rubycon Black Gate electrolytic capacitor you installed in your power amp back in 1988 sill makes your low-power solid state design sound as if it is a single-ended triode tube amp, then it consider yourself lucky and keep it in place.