RAKK dac Passive Output
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The RAKK dac digital-to-analog converter uses the excellent
Burr-Brown PCM1794 DAC chip, which has a low level differential current audio
output that is not suited for direct connection to a stereo amplifier or
preamplifier. Therefore, circuitry is needed to condition the signal for use with
stereo components. This circuitry needs to provide three functions: gain,
isolation and current-to-voltage (I/V) conversion. There are many ways that
these functions can be implemented and we will address only one of those here -
a passive solution.
A transformer is the perfect device to provide both isolation and gain therefore we have chosen the superb Lundahl LL1674 amorphous core line output transformer for this function. I/V conversion is accomplished simply with a high-quality resistor.
Figure 1. RAKK dac Passive Output schematic
The resistance of 3KΩ was chosen to provide 2VRMS output at a maximum 0dB digital signal. This resistance sets the output impedance of the output stage. For a typical component, an output impedance of 3KΩ would be considered too high and compromise the sonics of that component. However, the RAKK dac Passive Output board is not a typical component. In the typical component, the output impedance is in series with the output, while in the RAKK dac, the output impedance is in parallel with the output. With the resistance in series, the component is a voltage source; with the resistance in parallel, the component is a current source.
Figure 2. Series versus parallel output impedance
The significance of this difference is that when the output impedance is in series, external capacitance, for example in an interconnect cable, will form a low-pass filter with the series output impedance. Lowering the output impedance will raise the roll-off of the filter above the audible region. When the output impedance is in parallel, a filter with its sonic effects is not formed; therefore the output impedance may be as large as needed to produce the desired output voltage. In fact, the output impedance may be adjusted with a potentiometer to provide a volume control. The additional parallel resistance is actually a benefit. Although the PCM1794 current output can operate into a moderate resistance, it was designed to operate into a very low resistance: ideally zero Ohms. In the RAKK dac Passive Output, the PCM1794 current output operates into the resistance of the primary winding of the transformer (33Ω) in series with the reflected secondary impedance. When a volume control potentiometer is added in parallel with the output, the reflected impedance is lowered. To maintain a 2VRMS maximum output signal, change the I/V resistor to 4.3KΩ for use with a 10KΩ volume control. This provides an excellent single-source solution for audiophile with a CD-only system, eliminating the need for an external volume control.
Speaking of volume controls, the Transformer Volume Control (TVC), a transformer that has many selectable secondary taps, has become popular with audio enthusiasts. The TVC is an ideal accompanist to the RAKK dac Passive Output. As long as the interconnect cable is not too long, the TVC extends the transformer output of the RAKK dac Passive Output by providing selectable output taps. You may wish to adjust the value of the I/V resistor to provide 2VRMS at the output of the TVC. This value will vary depending upon the model of TVC that you have.
Figure 3. Resistive and TVC volume controls