RME Audio ADI-2 Pro, User Manual

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User’s Guide ADI-2/4 Pro SE – v 1.0
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34.6 True Balanced Phones Mode
Headphones usually share one wire between left and right channel: the common ground, hence
operation is unbalanced. A different way to build a powerful output stage uses a balanced design.
Both wires to the speaker are ‘phase’, there is no ground connection. This technique is mostly
used in car audio, as the operating voltage is limited to 12 Volt, and balanced operation, here
called bridging, delivers double the output voltage and four times the output power to the speaker.
In balanced operation, two identical power amplifiers are connected to one side of the phone's
speaker each, and the input signal of one of the power amps is inverted in polarity (180°). When
one power amp sends out a positive voltage, the other one sends out the same as negative. Thus
the voltage seen by the speaker is double as high.
With the comparatively low power required by headphones, balanced phones mode still has some
interesting aspects:

Output level is doubled. With the ADI-2/4 Pro SE +19 dBu would rise to +25 dBu (a gain of +6
dB). Now only few might have a headphone that requires that level, 13.8 Volts in output volt-
age. But when the phone is driven at the same volume and effective level as before, the driver
stages now operate at a 6 dB lower level. This can have a positive effect on THD and linearity.

Output power is quadrupled. A driver stage that was designed for 1 Watt will deliver 4 Watt
when a second such stage is used with inverted input signal. That is a significant rise, which
would allow to either achieve output powers that even bassheads would fear, or allows for the
use of less powerful or simpler output stages than usual.

Complete ground-free operation will prevent EMI and floating voltage level problems in certain
(seldom) cases.
One often mentioned aspect is questionable: having no common wire anymore the separation
between left and right is optimized. That is correct in theory, but has not effect in real-world, unless
the headphone cable is in a state that would have to be called defective.
The below block diagram shows the standard way to turn normal phones output stages into bal-
anced mode. As mentioned two stereo outputs are necessary, and one side of them has to be
fed by an inverted signal.