1.1.3. D/A Volume Control:
The revolutionary volume control is accomplished without
any added parts
to
the line-output signal path. It is not an entirely digital or entirely analog volume
control. It is a little of each actually. The volume is adjusted by changing the
reference voltage of the D/A conversion. It does
NOT
adjust attenuation of a
resistor divider, change the gain of an amplifier stage or truncate bits in the digital
data. All of these would add noise and/or distortion to the signal. This volume
technology actually reduces S/N ratio with lower volume.
1.1.4. Ultra-Linear Output Stage:
The output stage is a single transistor in Class-A mode. It is well known that this
simple configuration results in some compression distortion, but also sounds
most natural. In order to effectively eliminate this compression distortion, we
incorporated a compensation circuit in the Overdrive. The output stage
impedance is low enough and the power output high enough to drive most high-
impedance headphones easily, so we provide an adapter. It can drive either
normal or balanced headphones such as Sennheiser.
1.1.5. Fully Balanced Analog Path:
There are 4 analog paths, all identical. They connect from the D/A converter chip
to the outputs. All 4 are used for the balanced outputs, 2 of the 4 are used for
the Single-Ended outputs. Resistors provide some isolation, so all outputs may
be used. We recommend low-capacitance interconnects if all 4 outputs are used,
and shorter is better. The output drive is sufficient for all preamps and amps, and
even high-impedance headphones like Sennheiser.
1.1.6. Selectable Digital Filters:
A front panel switch selects 3 digital filter responses, low, medium and high. For
most DACs these are automatically selected, low for 44.1, medium for 96 and
high for 192. However, the Overdrive allows these to be selected manually in
order to improve audio quality. This makes the Overdrive closer to a NOS DAC.
For sample-rates of 176.4 and 192, the high position MUST BE SELECTED.
It is recommended that the high position be selected for all sample-rates,
however some speakers with Supertweeters may render ultrasonics audible at
44.1. For these, the medium or low settings can be used. The settings do not
engage until the track is stopped and restarted.
1.2. Design Choices:
A number of design decisions were made in order to optimize performance and sound
quality over all other criteria:
1.2.1. Small Chassis size
This was critical to reducing both digital and analog signal path lengths. Long
signal paths add distortion due to circuit-board dielectrics and also introduce
more noise due to ground-plane coupling and crosstalk.
1.2.2. Op-Amp I/V conversion
This was necessary in order to optimize the loading of the D/A converter. Some
D/A chips require certain loading and voltage on their outputs in order to
guarantee best linearity and low-distortion.
1.2.3. AC-coupled output
This was chosen because the alternative of DC-coupling would not allow a pure
Class-A operation throughout, which delivers lower distortion. The coupling
capacitors available now are very close to a copper wire in performance.
