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.
