It refers to the output stage, which is voltage only (no current outputs).
"4. Diamond differential output circuits : Benefit from to high speed diamond differential output circuits , excellent linearity and low phase distortion, diamond differential circuit is being loved and emphasized by more and more audio fans. Although the circuit is complex, the sound quality is impressive. We’ve done a lot of study and experiment on diamond differential current these lot years, aiming to get a better solution of circuit design.
For a DAC, high analyzing ability is the most important requirement. The detailed recurrence of resolution lies in the upward and downward characteristics and the noise level of the output amplifier. Because output amplifier is the bottleneck, if the upward characteristic is not good enough, the signal cannot be followed correctly on time and amplifier can only catch half or even lower of the signal level when it disappears. This leads to a downsizing of the original range of signal output from DAC, which directly causes obscurity or even loss of sound effect and a low analyzing ability. A lot of DACs, therefore, even with a highly analytical D/A converter in the digital section, cannot play most of the details due to its low analog circuit speed. To get a good upward characteristic, the only way is to use a high-speed low-noise transistor amplifier. Even operation amplifier and tube cannot compare with high-speed low-noise transistor amplifier in effectively lowering the TIM distortion which has a big impact on the sound quality and promising a pure and pleasant sound delivery. "