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The pot is followed by the power amplifier, which is made with special
operational amplifiers which are famous for their power and their
transmission speed. To enable balanced operation, each stereo channel
is equipped with two amplifiers. These are operated at +/-18V supply
voltage. Overall gain is set to 8dB (factor 2.5).
In balanced mode this amplifier delivers one of the highest voltage
swings available which is useful for high impedance phones. For low-
impedance headphones on the other hand there is power in abundance,
with a damping factor of 250 !!
At the same time, distortion and dynamic range represent the limits of
the technically feasible.
Why are balanced amplifiers essential ?
Regarding common amplifiers with phone jacks (with unbalanced
outputs) the remaining energy from the headphones will find its way
through a shared ground cable back to the internal ground which both
have specific resistances. Whilst doing so, the ground - which should
remain calm - is “polluted” by remains of left and right signals.
This means crosstalk between both channels and it is hearable and
measurable !
In balanced mode the voice coil from the headphone is driven by two
amplifiers. One is operating the normal signal, one the 180
0
phase shifted
(inverted) signal what results in a “push-pull” operation. Whilst one amp is
“pushing” the voice coil, the other amp is “pulling” it. Doing so, a double
as high output voltage is accomplished, creating a much higher volume.
As the wanted additional effect now the ground is free of any influences
as the energy flows only between both operation voltages.
Why makes it sense to make such huge efforts ?
A headphone amplifier is a device designed to condition audio signals
with regard to the very specific requirements of headphones. This doesn't
sound too spectacular at the first glance and can be achieved relatively
easily. As with many things however, the devil is in the details and much
more effort is required to design
one
amplifier for
all
current headphone
models.
