should ideally apply at all temperatures and at any rate of change of
Unfortunately other parameters exist
on a real capacitor that mars this relationship. Series resistance,
leakage resistance, inductance, value stability with temperature and frequency,
all play their part in destroying the ideal current/voltage relationship. At
radio frequencies the capacitor type used is crucial for the circuit to function
at all. In
audio circuitry, a poorly chosen capacitor type may effect the performance to a
Amplifier design engineers know what types not to use for certain applications.
Even if cost is a major consideration, they will not use a wrong
type if there is a chance it will mar the sound.
For example, instead of using a
polyester or polypropylene capacitor for coupling two circuits, cost say 30
pence, a 5 pence tantalum or electrolytic might be used instead. It is well know
that the latter types are not so good electrically and can add a tiny amount
of distortion. The crucial questions are 1/'how much?' and 2/'is it audible?. In a well designed amplifier
where the capacitor types are chosen sensibly, the answers are, 1/'a relatively small
amount' and 2/ 'no, its well below the noise floor".
Why is this the case?
To illustrate this, lets follow the
signal on it's path from the CD player through to the loudspeaker system, and
ultimately to the human aural system. We will assume a reasonably respectable
amplifier with less than 0,01% total harmonic and less than 0.01%
The voltage level of the audio
signal from the CD player appears at the input leg of the first capacitor in the
amplifier chain as several hundred millivolts rms. Lets say, 440mV r.m.s., the old
domestic line level. (Nowadays its usually a lot higher at around 2V r.m.s.) This originated in a recording studio from
electrical signals fed to a mixer desk via a set of microphones. Already the
signal has passed through scores of capacitors. The composite
signal is recorded onto a master tape or disc and then stamped onto your CD as a
series of miniscule digitally encoded bumps and troughs. The undulations are
read by a laser and converted from digital form to analogue form as a varying
voltage. At the input of the amplifier , the harmonic and intermodulation distortion level in the source
music is well above the distortion and background noise level of the amplifier.
Now let us consider the distortion caused by a 'non-audiophile quality,
capacitor' in the signal chain. The level of the capacitor distortion components
relative to the source distortion is hundreds of times lower. In fact,
in our decent amplifier, it is well below the noise floor. You couldn't possibly
hear it. Looked at another way. If the total harmonic distortion in the signal
is 3% (a very conservative figure) and the total harmonic distortion (THD) of
the amplifier is a mere 0.01%, (which includes the much lower
distortions from the capacitor), it doesn't take a genius to see that mucking
about changing capacitors for the sake of it, won't make any audible difference.
Now let's follow the signal out of
The signal travels from the
amplifier as an amplified voltage and current through the speaker cable to the
loudspeaker's speech coil. This causes a changing magnetic flux in the coil
which interacts with the fixed flux in the gap between the coil and speaker
magnet to create cone motion. This in turn creates sound pressure waves. Here
the total harmonic and intermodulation distortion leaps up even further. A
figure of 5% would again be very conservative. The non-linearities
inherent in the loudspeaker to human aural system, makes any capacitor
distortion look like a nit on a gnat's eye lash. First in the loudspeaker,
the non-linear conversion of changing current to changing magnetic flux
adds appreciable levels of distortion. Then the non-linear conversion of
magnetic force to motion in the cone adds even more. The conversion of cone
motion to sound pressure waves adds more still. Finally your own set of aural
receptors, non linearly convert the sound pressure waves into electrical nerve
impulses in the brain. You own ears add considerably more distortion than
the difference between two capacitor types.
A much more serious distortion, more
like an interference, occurs in some male brains. This blurs the original
thought of inserting a CD, relaxing, and listening to the music. Instead
thoughts of that nasty non-oil filled capacitor, loom in the brain.
The truth is, the distortion, compression
and background noise of most CD & Vinyl recordings plus the non-linear
distortions in the loudspeaker-aural system, has hundreds of times greater effect
on the original sound than the effect of changing a resistor or capacitor to an
audiophile type. That non-faulty component is innocent ! Leave it alone (unless
of course its faulty) and enjoy the music! In some cases, changing to audiophile
capacitors can actually decrease performance see polypropylene versus oil filled
blog polyprop V OF.
When we service a typical vintage
amplifier we often replace all the electrolytic capacitors and several
resistors, not because they are 'better' so called, 'audio quality types'
types but because many have deteriorated. i.e.,
either one or more of , low value, o/c, s/c, leaky, high ESR etc, etc.
We insist on using high quality
electronic components in all our service work for long term reliability and