Balanced Phones Mode - RME Audio ADI-2 Pro User Manual

34.3 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 de-
sign. 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 Pro +22 dBu would rise to +28 dBu (a gain of +6 dB).
Now only few might have a headphone that requires that level, a scary 19.5 Volts in output
voltage. 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 cer-
tain (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
balanced mode. As mentioned two stereo outputs are necessary, and one side of them has to
be fed by an inverted signal.
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User's Guide ADI-2 Pro © RME