The ezDAC (pronounced 'easy-DAC') is a very nice do-it-yourself high-end 24bit/96kHz upsampling audio D/A converter, designed by Evan from ezDIYaudio.com. The design features a Crystal CS8416 S/P-DIF input receiver that sends the digital data by means of I2S to an Asynchronous Sample Rate Converter (ASRC). An AD1896 from Analog Devices or a SRC4192 from Texas Instruments is used for this. This part in turn upsamples the 16bit/44.1kHz data to a 24bit/96kHz data-stream, that is processed by a TI PCM1794 or 1798 high-end D/A-converter. The two output currents of this D/A-converter are passively converted to a voltage by means of two resistors and summed by an opamp (in this case an AD8610A). Minimal output-filtering is used. The DAC has an 8x oversampling filter, so the high frequency residue is located far outside the audio-band. The stopband attenuation is 100dB minimum.
The official ezDAC homepage can be found here: www.ezdiyaudio.com
The trick with the ezDAC is that the ASRC and DAC chip are clocked by their own stable 24.576MHz on-board clock oscillator (or an off-board Flea :-), and not by the recovered clock signal generated by the input receiver's PLL, which is normally used. So the input clock and on-board clock are not related in any way, hence the term 'asynchronous'. The jitter present in the input data signal is better attenuated by the ASRC chip, because its internal Polyphase Locked Loop has a much lower bandwidth compared to the PLL in the CS8416.
All this wisdom can be found in this thread about Asynchronous Sample Rate Conversion, on the diyAudio forum. Big thanks to 'Werewolf' for his very extensive explanation!
There's also an excellent piece of theoretical background information about the ASRC principle in the datasheet of the AD1890 from Analog Devices.
The SSOP28 IC's
The hardest part is soldering the ASRC and the DAC IC. They come in a SSOP28 case, with only 0.35mm space between the pins. All against the general rules of electronics ('first mount the resistors and capacitors') these two IC's are mounted first, so other components won't be in the way during this precise job.
Line-up the IC's on the pads and first fix them on the PCB by soldering a pin on two opposite corners. Then solder the rest of the pins, without trying to avoid shorting them. They will probably end up being soldered all together, but that's not a problem, as long as they are connected to the PCB. Next, take a piece of desoldering-wick and place it alongside one side of the IC. Move the iron along the pins without sliding the wick. All excess solder will now be sucked up. Simultaneously lift the wick as soon as the iron passes so it won't get soldered to the IC or the PCB. If this doesn't work the first time, make another pass along the pins to remove the wick. Use a tip that's not too small, so there is enough heat transfer to the wick. The excess flux can be easily removed with some isopropyl alcohol.
The other components
After mounting the two SSOP28 IC's, the rest is not difficult anymore :-). The 0805 parts are among the smaller ones, but with a pair of good SMD tweezers and some patience, soldering them is not too difficult. The S/P-DIF receiver and the opamps in their SO-case can be soldered using the ordinairy pin-by-pin method. In general, for the SMD parts applies: first fix it on the PCB straight and level by temporary soldering one side, then solder the other side and finally re-solder the first side.
The power supply
For the ezDAC circuit, a total of four different supply voltages are needed: 3.3V and 5V for the digital logic and +/-12V for the opamps. These voltages are regulated locally with four LM317/337 voltage regulators. The PCB has one connection for the positive and one for the negative supply, so strictly speaking it can run on a simple symmetrical power supply, as long as it delivers a minimum of about +/-15V.
The 3.3, 5V and +12V voltages are all generated out of the same positive 15V voltage. This leads to the fact that especially the 3.3V regulator has a lot of heat to dissipate. Also, it is not recommended to supply the opamps out of the same voltage that's used for the digital section. High frequency interference is given a path to penetrate into the analog output-section. By cutting the +12V supply-track this voltage can be isolated from the 3.3 and 5V, and a separate supply can be used for the analog section.
In this DAC, a total of three supply-modules are used to feed the PCB. One of them supplies a stabilized 9V for the 3.3 and 5V connection, and the other two supply +/-15V for the opamps. This way, heat dissipation is reduced and spread, and only a small heatsink can be used for the 3.3V regulator. Two small toroidal transformers of 2x12V and 2x15V provide power to the modules. Here, for the symmetrical +/-15V two identical universal modules are used, based on an 7815. The one for the -15V is tied to the PCBs ground with its positive output-terminal. This way, the required -15V is available at the negative or null-terminal of the module. The advantage of this is that each module can be placed on one side of the PCB, close to the supply-connection. In this case a transformer with two separate windings, without a center-tap, must be used. A standard symmetrical supply, based on a 7815 and a 7915, can also be used of course.
Building a case
To house all this, an aluminium profile-based Hammond case is used, model 1455T2201. In the rear panel, the RCA connectors for the in- and outputs and an IEC socket with built-in fuse are mounted. The ezDAC PCB is situated close to the rear panel, to keep the distance to the connectors short. The supply modules have been placed on each side of the PCB, close to their corresponding connections. The toroidal transformers are placed in the front. This way, they can be easily connected to the mains switch, which is fitted in the front-panel. Another option is an IEC-socket that also has the mains switch built-in.
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