DAC1 USB
Instruction Manual Revision D
Page 17
Benchmark
Technologies
HPA2
™ Headphone Amplifier
The
DAC1 USB
headphone output is driven
by Benchmark’s signature
HPA2
™ headphone
amplifier. This high-current, high-output
amplifier has an output impedance of near 0-
Ohms. It is designed to drive loads as low as
30 Ohms without any increase in distortion.
It also has sufficient amplitude to drive low-
sensitivity 600-Ohm headphones.
The
HPA2
™ includes current-limiting circuits
that fully protect against damage from short
circuits. This is important because the right
channel of a headphone amplifier will
experience a short whenever a mono phone
plug is inserted into the stereo headphone
jack. Shorts may also occur when a plug is
partially inserted.
0-Ohm Output Impedance
Most headphone amplifiers use series
resistors to maintain stability and protect
against short-circuit conditions. These
resistors are usually at least 30 Ohms, and
have a negative impact on performance. A
headphone amplifier with series resistors may
measure very well when driving resistive
loads. However, the same amplifier will
measure very poorly when driving a
headphone load. Unfortunately, most
manufacturers do not specify headphone
amplifier performance with anything other
than ideal resistive loads. Our measurements
show that headphones do not behave like
resistive loads.
Headphone Performance
In our tests we have measured substantial
distortion across resistors that are wired in
series with headphones. We conducted
measurements with a variety of headphones.
In general, distortion increases as headphone
impedance decreases. This distortion can be
eliminated with a properly designed 0-Ohm
headphone amplifier.
The performance of the
HPA2
™ does not
change when headphones are driven. THD+N
measurements for no-load, 30-Ohm resistive
loads, 30-Ohm headphone loads, and 600-
Ohm headphone loads are virtually identical.
The
HPA2
™ will substantially improve the
sound of 30 and 60-Ohm headphones. It will
make very noticeable improvements with
600-Ohm headphones.
Headphone Gain Range
Because of the variations in headphone
sensitivity, the
DAC1 USB
features two gain
ranges for the headphone outputs. Our
experience with the classic
DAC1
showed us
that most users cannot use the full output of
the
HPA2
™. These users benefit from a
reduced maximum gain. Internal jumpers
reduce the input to the
HPA2
™ by 10 dB and
place the
Volume Control
near 12 O’clock at
a comfortable listening level with most
headphones. These jumpers are factory
installed, but can be removed if a user needs
more output. Removing the jumpers
increases the gain to a level that is equal to
that of the classic
DAC1
.
TIP: For optimal L/R balance, the
headphone gain jumpers should be set
so that comfortable listening levels occur
when the ‘Volume Control’ is set above
the 10
th
detent.
Instructions for setting the headphone gain
range and headphone mute switch are
detailed in the ‘Internal Settings’ section of
this manual.
