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User’s Guide ADI-2/4 Pro SE - v 1.0
This design, as common as it is, has several disadvantages:

an analog inverter stage has to be added to the signal path

the common mode situation of the signal at the phones is compromised by the difference
b and
– phase, caused by the analog inverter

multiple relays and laborious cabling with wires running back and forth from/to the PCB are
typical for such a design
The base version of the ADI-2/4 Pro SE, the ADI-2 Pro, uses a different, better method - the
Advanced Balanced Mode. It uses both DACs, i.e. all 4 channels, and thus even achieves a fur-
ther increased signal-to-noise ratio of 3 dB. However, using both DACs also means that in Bal-
anced Phones mode the rear outputs are no longer available simultaneously or independently -
they must therefore be muted.
This limitation is elegantly solved by the
True Balanced Mode in the ADI-2/4 Pro SE, and also
explains the name of the unit, because here there are 2 balanced input channels, but 4 balanced
output channels. Balanced headphones only use DAC channels 3/4, the rear outputs can still
output an independent signal via DAC 1/2.
The True Balanced mode takes advantage of an important feature of the ADI-2 (Pro as well as
DAC): the symmetrical (balanced) circuit design. From the output of the DAC chip via the level
adjustment to the input of the Extreme Power output stage, the entire signal path is doubled, with
positive and negative phase. Such an elaborate design improves almost all technical data, from
SNR to THD to crosstalk. And it allows a simple trick: an inverted signal can easily be generated
by swapping positive and negative connections - which is possible via a relay without performance
loss.
The double inversion and the switching of the input of the Extreme Power stage Ph1/2 to the
output signal of DAC 3/4 requires three relays, but thanks to the balanced signal routing there is
no degradation of the audio signal. The usual active inverter is not necessary. The DAC channels
1/2 remain with the rear XLR and TRS outputs. And the previous concept, which offers the Phones
TRS outputs also as Balanced Line outputs, has been retained, and successfully extended with
the Pentaconn jack for balanced use without an adapter.
The block diagram below shows the balanced analog routing in True Balanced mode.