The γ2 compact high performance DAC

Technical Highlights

Compact size, high-end performance

  • 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 C impulse response Filter C square wave response
 
Filter B impulse response Filter B square wave response
Filter B impulse response Filter B square wave response
 
Filter A impulse response Filter A 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:

Integrated circuit complement

  • Wolfson WM8741, WM8742 or WM8740 - 24-bit 192KHz high-end stereo DAC
  • Texas Instruments/Burr-Brown SRC4192 or Analog Devices AD1896 - 192KHz asynchronous sample rate converter
  • Texas Instruments/Burr-Brown OPA2365 or Analog Devices AD8656 - CMOS rail-to-rail dual operational amplifier
  • Microchip MCP100 or Maxim/Dallas DS1818 - reset manager for power-up sequencing
  • Texas Instruments TPS79333, TPS793475 or LP2985AIM5-3.3, LP2985AIM5-4.5 - LDO voltage regulators for independent digital and analog supplies.

Designed to be easy to build

  • 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.




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