The classic implementation of a headphone connection is that of "single ended driving". However, over the past few years other implentations have been developed that have major advantages in improving soundquality. In this chapter these implementations are compared on a technical basis.
"Single ended driving"
Each driver of a headphone is connected to the amplifier by two wires. One of these wires (the ground wire) is directly connected to ground and thus carries a constant zero-Volts signal. The other wire is connected to the active part of an output stage and carries a varying voltage, the electrical audio signal. The driver is driven by the voltage-difference between both wires.
The voltages of the two active wires change continuously and the external electrical fields generated by the electrical charges on the wires change accordingly. There is energy associated with these fields and thus energy is constantly stored and released. The dielectric properties of the insulating material have a strong influence on this process and thus effect sound quality.
With a balanced setup the wires of each driver are both connected to an active output stage. Both wires carry the same signal but on the "ground" wire the signal is reversed in sign. The amplitude of each signal is half that of the signal in a single ended setup. The driver is driven by the voltage difference between both wires and is the same as in a single ended setup.
As the voltage signals on both wires are reversed in sign, so are the electrical charges on these wires. The total electrical charge thus becomes zero. The external potential field is strongly reduced and much less energy is stored and released when an audio signal is applied. The purity of the sound signal is much less effected by the properties of the insulating material.
Principally a balanced setup therefore does have a clear technical advantage over a single ended setup. However, there also is a major practical drawback. The "ground" wires of the two drivers of most headphone are electrically connected to each other and thereby are forced to carry the same signal. This is not allowed in a balanced setup and therefore such a setup requires recabling of every headphone that is to be used.
"Active balanced ground driving"
With "active balanced ground driving" the ground wires are still connected but these wires are not kept fixed at Zero-Volts potential. An extra signalcomponent is added to all the four wires (both wires for both drivers) such that the sum of the voltages of the 4 wires is constantly zero. This extra signalcomponent is the same for all wires so the voltage differences between the two wires of each single driver do not change when compared to a single ended or balanced setup. The effective audio signal is still the same.
However, since the sum of the voltages is zero, the total electrical charge on the wires is also zero, like in a balanced setup. Again, the external potential field is strongly reduced and much less energy is stored and released when an audio signal is applied. The dielectric properties of the insulating material have much less effect on the quality of the sound signal.
The concept of "active balanced ground driving" thus offers the same advantages as "balanced driving". However, no costly recabling of any headphone is required.
At the left picture below we see the signals as generated by a conventional headphone amplifier. The ground signal is kept constant. Left and right channel signals vary independently.
The middle picture shows the four signals involved in a balanced setup. The two signals for each driver are the same but have opposite sign.
At the right we can see the signals generated by an "active balanced ground" setup. The ground wire now carries a signal that is also added to the left and right channel signals. The difference between Left and Ground and between Right and Ground is the same as in the other two setups.
The left picture below shows the electrical potential field of the positively charged headphone cable when both signal wires carry a positive signal at the point of time indicated by the yellow bar at the left picture above.
The right picture shows the potential field for balanced driving and for active balanced ground operation. It can be easily seen that field potentials are much lower than at the left picture.
The differences are even more pronounced in the far field, as shown in the next pictures. Since the "balanced" headphone cable has zero netto charge there is hardly any energy stored in the far field.
Apart from these principle benefits there are also other technical advantages offered by both balanced concepts.
When a positive signal is applied to a headphone driver of a single ended headphone amplifier current passes from the output stage of the amp, through the headphone driver towards the ground plane of the amp. During this process the current is "taken" from the positive power supply line whose voltage, as a result, will not be entirely constant. (In the pictures below the positive and negative supply lines are depicted as two large storage capacitors.)
The currents that run through the ground plane do induce small voltage differences. Ground is thus polluted and since it acts as a internal reference for all signal amplification processes sound quality is impaired.
Also variations of the powersupply lines are not entirely rejected/neglected by the output stage and are partly transferred to the output signal. This reduces sound quality even further.
When a signal is applied to a "balanced" or "active balanced ground" setup current passes the signal output stage, the driver, and the "ground" output stage. Both positive and negative powersupply lines are "loaded" simultaneously. No driving currents pass through the internal ground plane. There is no pollution of the internal referfence.
The voltage variations of the two powerlines are mirrored. Their mean value is roughly constant. This strongly increases their rejection at the output stages.
If voltage variations are found in one of the powerlines only, then these have a similar effect on the signals of both the signal output stage and of the ground output stage. However, it is the difference between the two output signals that drives the headphone. This difference is not changed and the effective audio signal is not influenced. PSRR (Power Supply Rejection Ratio) is very high.
"Balanced driving" and "active balanced ground driving" thus offer many advantages for a faithfull reproduction of your favourite music.
"Active balanced ground driving" has the advantage over "balanced driving" that no expensive recabling of the headphone is required.
"Single ended driving" versus "balanced driving" versus "active balanced ground driving"