9
Headphone Amplifier
DAPC
Volume Control
LEF Power Driver
Pure Power
B.M.C. recommends balanced headphone use.
The headphone amplifier section of the PureDAC is a unique and advanced design. A precise description of
its function would be “Digital / Analog-Power-Converter” (DAPC). This means there is no additional
amplifier stage and the headphone amp’s voltage is generated from a special type of I/V converter, which is
an integral part of the DAC. Eliminating an entire amplification stage results in a shorter and cleaner signal
path and the sonic degradations which result from additional and unnecessary circuitry.
The PureDAC applies B.M.C.’s proprietary CI (Current Injection) circuitry for a virtually lossless current-to-
voltage (I/V) conversion and the filtering of ultrasonic digital sampling noise. The CI stage provides very low
distortion and very high stability without the use of a negative feedback loop.
Normally an additional amplifier stage would be required for managing a volume control in the analog
domain. The PureDAC provides a superior solution by applying B.M.C.’s DIGM (Discrete Intelligent Gain
Management). DIGM makes the I/V conversion ratio variable, so the volume can be set without this
additional circuitry. By reducing the volume DIGM also reduces noise and distortion while increasing stability.
It also resolves the very different gain requirements of different headphone models. This way the PureDAC is
an optimal match for a very wide range of headphones. DIGM also provides a perfect channel balance with a
deviation of just about 0.5% - potentiometers achieve a 20% channel balance in the best case.
A clean signal voltage from the CI-IV converter does not drive a headphone directly and thus a driver with
exceptional control ability has to be utilized. B.M.C.’s “Load Effect Free” circuit has a unique structure: The
voltage source controlling the headphone with a very low impedance generates no distortion since it does
not have to deliver any current to the headphone; thus the output device’s non-linearity doesn’t matter. The
current is coming from a separate source and is not directly related to the music signal rather than the actual
current demand, which is the music voltage divided by the complex and moving impedance of the
headphone. LEF delivers current regardless of the phase and is superior in controlling a dynamic
headphone. Due to the very low distortion of this circuit a feedback loop is not required.
The PureDAC uses the input capacitance of the driver device as part of the I/V ultrasonic filter. This allows
powerful output devices to be used and turns the disadvantage of their higher capacitance into a useful part
of the I/V converter. The PureDAC’s output devices are 8A lateral MOSFETs, and the entire circuit is fully
balanced. This way the PureDAC can provide a close to 0 Ω output delivering the best headphone control
possible. The result is a new standard of definition and punch.
To protect the headphones the PureDAC limits the current to 200mA.
The unbalanced headphone jack cannot be driven with 0 Ω due to short circuiting plug design. The PureDAC
uses a unique quasi-balanced solution with a floating mono-ground and cancellation of the opposite channel.
This way the quietness and low distortion of the balanced drive can be preserved, with a slight reduction in the
power and dynamics that is available when using headphones wired with balanced connectors.
