Buy Boards & Parts:
The Mute Audio GrubDAC

Technical Highlights

The mute audio grubDAC is based on the Texas Instruments PCM2707 and the Wolfson Micro WM8524. The PCM2707 is used as a USB to I2S convertor which then feeds the signal to the WM8524 DAC chip. One feature not commonly seen is that the PCM2707 is clocked by a quality crystal oscillator (Crystek C3392-12), as opposed to a simple crystal. It possesses very good performance for low cost. Some of the primary features of the PCM2707 are listed below:
  • USB 1.0 specification compliant
  • Supports 12MHz data tranfer rate
  • Accepts 16-bit stereo and mono data streams
  • 48KHz, 44.1KHz, and 32KHz sampling rates
  • Clock Generator with 12MHz clock source
  • +3.3V Digital, +5V Analog operation

(prototype pic)
The WM8524 is of a set of DACs that create a ground referenced signal. As opposed to the similar BantamDAC which requires output capacitors to filter out the midrail DC signal, the WM8524 on the grubDAC generates it's output relative to 0.0V ground, allowing for it to be DC-coupled. DC offset is specified to be within only +-1mV maximum. The only filtering requirement after the DAC is a low-pass filter to help deal with some of the ultra-high-frequency noise (well above the audio band) that a sigma-delta convertor can produce. On the grubDAC, the post DAC R-C filter is designed with through hole components. The resistor is specified to be a high quality Vishay-Dale RN50 with the capacitor being a Wima film cap with the option of using a SMD ceramic resistor. Note that this RC low-pass filter is bypassed to ground, it is not directly in the signal path output. Those that have built the GrubDAC prototype report that it creates a full, lively sound with this output stage. Some of the primary features of the Wolfson WM 8524 DAC are listed below:
  • High performance stereo DAC with ground referenced line driver
  • Audio Performance:
    -  106dB SNR (‘A-weighted’)
    -  -89dB THD @ -1dBFS
  • Maximum 1mV DC offset on Line Outputs
  • All common sample rates from 8kHz to 192kHz supported

(Wolfson block diagram)
Supporting the simple design of the board is the fact that the PCM2707, WM8524. and the clock require only a single 3.3VDC supply. This requires only one voltage regulator on the board. The use of a TPS79333 provides a quality low dropout regulator with a high Power-Supply Rejection Ratio (PSRR) to provide clean power to the ICs. The board also uses the optional noise bypass pin on the TPS regulator to help reduce noise even further.

Like the BantamDAC, the grubDAC is only 1" long x 2" wide. In a similar vein as the Bantam, an effort was made to keep the SMD parts to 0805 and 1206, requiring less skill in soldering the SMD parts. The resistors on the board were kept to 1206, which due to their low profile makes them easier to handle. The only 0805 parts (smaller than 1206) are a set of the capacitors. Due to their cubic shape, they are not as hard to solder as an 0805 resistor.

The output of the WM8524 is specified to be able to drive anywhere between a 1KOhm to 50kOhm load with a output of 2.1Vrms at 0dBFS. You can drive headphones directly off the GrubDAC, but expect a degradation of sound quality - especially noticable in the high end.
Manufacturer data sheets:
WM8524 PCM2707
TPS79333 Crystal Oscillator
file last changed:Tuesday, December 20, 2011 6:00:00 AM
Please contact the GrubDAC webmaster for questions about these web pages.