24
Headphones per se are quite diverse, and there are two essential
parameters: impedance and sensitivity.
In general, headphones with higher impedance can be regarded as less
sensitive than headphones with low impedance (which is not generally
true, but in the majority of cases). The sensitivity of headphones is
usually stated in dB (sound pressure level) per milliwatt.
Extremes in this sense are the AKG K1000 with 74dB/mW on the one
hand, and the Sennheiser HD25 with108 dB/mW on the other hand: The
K1000 requires 2500 times the power to achieve the same sound
pressure as the HD25.
There is also the fact that headphones with high impedance usually
require much higher voltage to achieve high loudness. Thus the amplifier
must
be designed with high internal supply voltages.
Why is a high damping factor essential ?
When actuated, electro-dynamic systems respond with a counterforce.
When the voice coil of a headphone has been displaced by the signal, an
(error) current will be induced when it swings back to its initial position.
This current must be suppressed as far as possible, which is effected
best if the amplifier's output impedance is the lowest possible. The
damping factor describes nothing but the ratio between output impedance
of an amplifier and a given load.
Since there is no known technical specifications, we define the load
(voice coil impedance) as 50 ohms. This results in an output impedance
of <0.2 ohms for the V181.
The power supply with its supersized toroidal transformer provides
about +/- 24 Volt AC. After rectifying and smoothing by 16.000 uF the
above voltage is stabilized to +/-18 volts.
Why are high supply voltages essential ?
A headphone doesn't really require high power, but from the equation
P = U
2
/ R we can see that the square of the supply voltage determines
the power into a given load resistance. The higher the headphone's
impedance, the more voltage will be needed. But this deals with the
achievable loudness to a limited extent only: Technically spoken, music
lives on fast transients which put high demands on signal processing.
