γ2's enclosure footprint is about two credit cards
side by side, and a little over one inch thick. The compact size
makes it DAC very portable and uses minimum desk/shelf space.
The extruded aluminum Box Enclosures B2-080 series case,
available in black, clear, gold, red, blue and green anodized
finishes, is attractive and sturdy.
γ2 is based on Wolfson's top-of-the-line WM8741 or WM8742
ultra-high performance DAC chip. The WM8740 is also
supported. These DAC chips are found in some of the most
expensive commercial DACs and CD players, and are reknowned for
their superior sound quality.
With the WM8741 and WM8742, front panel switches allow the
selection between three different digital filters (see details
below), and to enable or disable an anti-clipping mode.
The anti-clipping mode causes the DAC to attenuate
the signal amplitude by 2dB.
An optional asynchronous sample rate converter (ASRC) chip
(Texas Instruments/Burr-Brown SRC4192 or Analog Devices AD1896)
up-samples all input digital streams to 24-bit 96KHz. The
ASRC and DAC are both fed with an I²S master clock generated
by an onboard ultra-low jitter CMOS oscillator. Together with
careful circuit board layout, the result is an extraordinarily low
overall jitter.
The ASRC and CMOS oscillator may be omitted and bypassed with
jumpers to save cost. It can be installed in the future as an
upgrade. Without these parts, no up-sampling would occur.
γ2 plugs into the
γ1
USB and DAC board which provides USB and S/PDIF (coaxial and
optical) input connectivity, as well as supplies power to the
γ2 board.
γ2 is compatible with γ1 Full++ (F) configuration.
It is also compatible with γ1 Full (A), Lite (B),
USB-to-I²S (D) and S/PDIF-only (E) configurations if the two
γ1 board sections are joined back together. See the
instructions section
for more details.
The γ2 board taps the I²S digital audio stream from
the γ1, and allows the the DAC on both boards to function
simultaneously.
γ2 could also be used standalone as an upgrade DAC for
devices that already has an I²S bus. Just connect the
I²S bus to the γ2 board, and provide γ2 with
5V DC power. See the
instructions section
for more details.
The DAC chip's balanced analog outputs are sent to a low-noise,
precision rail-to-rail CMOS dual opamp which serves as a
balanced-to-unbalanced convertor, analog low pass filter and
output buffer.
The opamp choices are Texas Instruments/Burr-Brown OPA2365 or
Analog Devices AD8656. Both are excellent performers and meets
the stringent rail-to-rail swing and low noise requirements of
this DAC design. The OPA2365 offers superior CMRR performance
and is free of zero-crossing distortion. The AD8656 has very
low noise and high output current capability, which makes
it possible to drive low-impedance headphones directly
from the γ2 (see the parts list
section for further details).
Using a true balanced-to-unbalanced converter (rather than just
taking half the DAC's output) retains a high level of common-mode
rejection (CMR) and maximizes the DAC's signal-to-noise ratio.
The analog low pass filter has a corner frequency of
100KHz and removes out-of-band digital artifacts which may
cause in-band signal degradation and affect the performance of
some amplifiers. The second-order filter (12dB per octave)
uses precision polypropylene film capacitors.
The output buffer decreases output impedance and increases output
current drive capability. These in turn reduces susceptibility
to noise interference and stereo crosstalk, as well as allow
the use of long interconnect cables without signal degradation.
Both RCA phono jacks and 3.5mm stereo mini jacks are provided
for the analog output. You may use either depending on which cables
you have, or use both simultaneously to drive two amplifiers.
The digital and analog section power supply rails are each
independently regulated by separate high-performance Linear LDO
voltage regulators.
Ferrite beads and liberal use of decoupling capacitors further
assure clean, low-impedance supply rails.
Big, high-end sonics from a small package -- the γ2 is
carefully designed for excellent performance, rivaling top-tier
DACs many times its size and cost.
Digital filters
Most modern DACs employ a digital filter for interpolation and decimation.
In pursuit of improved sonic performance, recent research and development
have focused on the filter's time-domain characteristics in
addition to the traditional emphasis in the frequency-domain.
The Wolfson WM8741 and WM8742 DACs have several built-in digital filters
which could be selected on-the-fly. The γ2 front panel switch
marked A, B and C corresponds to the following filters:
A: Linear phase, slow rolloff
B: Minimum phase, slow rolloff
C: Linear phase, brickwall
The C filter is found in most ADCs and DACs, and offers no phase shift
and the widest passband. The respond drops like a brickwall beyond the
passband. While this type of filter exhibits excellent frequency-domain
performance, it has "pre-ringing" and "post-ringing"
behavior in its impulse response. This is also reflected in the
square wave response's rising and falling edges, also known as
Gibbs Phenomenon.
In real life, musical instruments, voices and naturally-occuring
sounds never produce pre-ringing effect (except for something
intentionally concocted on a computer to do that).
The B filter is a new type which is appearing in some high-end
commercial DACs. It trades some phase response deviation for a complete
elimination of the pre-ringing effect. Its post-ringing is more severe
than the linear phase filters. Since the post-ringing is usually
masked by ambience, reverberation and natural acoustic decay,
this filter is said to be more natural-sounding and pleasing to
the ear.
The A filter is a "compromise" between the two other
filters. It has the excellent phase response of the brickwall
filter but reduces the pre- and post-ringing to much fewer cycles,
because the amount of ringing is directly related to the sharpness
of the stopband rolloff.
By allowing the three filters to be selectable on-the-fly,
γ2 offers the user a choice. You may experiment to
find your preferred filter for the music content you're playing,
and to hear the subtle differences between them.
The frequency response, impulse and 1KHz square wave response of
all three filters are shown below. Note that these are measured
on a γ2 equipped with the ASRC. All three filters in
this configuration have flat response to well beyond the audible
band regardless of the input stream's sampling rate. The A and B
filters' corner frequencies are above 30KHz and the C filter goes
beyond 40KHz.
Frequency response (ASRC, all sampling rates):
Filter C impulse response
Filter C square wave response
Filter B impulse response
Filter B square wave response
Filter A impulse response
Filter A square wave response
For a γ2 without the ASRC, the A and B filters exhibit early
rolloff (corner frequency around 14-15KHz if the sampling
rate is 44.1KHz or 48KHz), and could lead to audible dulling of
the treble (See response curve below). Thus, only the C filter is
recommended for this configuration.
Frequency response (no ASRC, 44.1KHz sampling rate):
For more information about the different filters, the following
whitepapers are worth reading:
A specific target case, input and output jacks,
switches and other parts make this a highly integrated design.
All parts are through-hole except for the integrated circuits,
minimizing the number of SMD parts and making the γ2
easy to solder and assemble.
All parts are board-mounted, there are no wires to connect.
There are no trimpots to adjust after assembly.
All parts are easy to obtain (from AMB audio shop and from various
online vendors, see the Parts list section).
High quality circuit board
FR-4 glass epoxy 3.1625"x3.9375"
printed circuit board, three copper layers with plated-through holes,
silkscreen and solder mask.
The layout of all parts and traces have been carefully
considered for optimum fitment and maximum performance.
Double ground planes are employed, with particular attention
paid to make the both ground planes as contiguous
as possible to minimize current loops. In addition, a array
of vias tightly couple the two ground planes to minimize
impedance.
A strip of the ground plane is exposed along each side (both top and
bottom), allowing the DAC's signal ground to be electrically connected
to the case (via the board slots) without the need to add any wires.
The grounded case improves RFI shielding without additional wiring.
The γ2 and γ1 board "sandwich" is designed to
slide into a Box Enclosures B2-080 series extruded aluminum
enclosure with no additional mounting hardware, making the casing
of this DAC very easy.
A 3D rendering of a partially-populated γ2 circuit board.
