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The tool that I made and I discuss in this article I give the name USB SPDIF DAC Every word in the name corresponds to the function of the tool that I made this.

USB, this device serves to take the music information in digital form from a computer via the USB port
SPDIF, this tool can output digital music information received through the USB port, through the SPDIF output terminal that is also present in this device, so if you have a DAC that does not have a USB input, you can still use and do not need to buy a new DAC that has an input USB.
DAC, this tool can convert the digital signals derived from the USB port, an analog signal, which you can connect to the amplifier to the ahirnya you can hear the speaker

Scheme of the USB DAC can be seen in Figure 1 below.
Click to view larger. | Figure 1

As you can see on the schematic in Figure 1 above, which is the heart of the circuit is IC USB DAC PCM2902 made ​​by Texas Instrument, other than that I also added a digital SPDIF output isolation transformer on from this series. 2902 PCM encoding decoding work on the principle that generally abbreviated as word CODEC. 
The new circuit will work when the USB input port terminal in the circuit connected to the computer, at this voltage 5V on the USB port of any computer will be triggering the circuit to begin work, and vice versa when the computer is turned off then the circuit will be in standby mode to wait until there is more tension 5V from USB port of computer. 
When the received voltage is 5V USB port the computer then the circuit will begin to receive the digital data files during the grace period of 1 ms, all data received during the period of time of 1 ms is called a frame, the data in the first frame is stored in a memory buffer that is in the PCM2902, then the circuit will start the second frame as well as long lead times for 1mS, 1mS a second after the first data from the file will be converted into analog signals for analog terminal is then removed through out the pin 15 and 16 of the PCM2902, other than that this digital signal simultaneously will also excreted through the DOUT terminal pin 25 of the PCM2902 DAC to be connected to older products that do not have a USB input terminal. In addition to functioning as a DAC, PCM2902 actually also has other functions such as setting volume level and ADC, but the circuit that I developed is only aimed for the ADC and the conversion from USB to SPDIF.

Proposed Power Supply For USB SPDIF DAC
Power Supply that I use in this project using the configuration has been proven able to tame the hum in the tube 26, the power supply has karakater very stable, low noise and low impedance, but complicated to make, so for the purposes of this project I recommend another series that is not as complicated Heater power supply 26, but the quality is still better than most of the power supply kit used in the DAC in general, Figure 2 below is the recommendation power supply for USB DAC this project.
Click to view larger. | Figure 2

Power supply circuit in Figure 2 above is composed of four main parts, namely
 1. Diode bridge and capacitor input filter
 2. TL431 as the reference voltage
 3. OP Amp OP27 as the error amplifier
 4. BD139 transistor as a pass transistor This configuration is basically the general configuration of a linear regulator series, very much the regulator IC using this configuration, but the circuit in Figure 2 above has several advantages that are not owned by the regulator circuit in the form of IC a) high stability, since it uses a reference voltage source of good quality is TL431, and therefore not in a package with a pass transistor, the heat of the pass transistor can not seep into the source of reference voltage and makes it unstable, this sort of thing happens in IC voltage regulator. b) Using the Op Amp high quality, which in this case OP27, so it is not easy to oscillate as the OpAmp used in the regulator IC. c) If there is a chance you could also replace the TL431 or OP27 Op Amp voltage source and another that is expected to be better than either of these components.
Series of the finished PCB, top view

IC PCM2902

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