RAKK dac Active 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 - an active solution.
Since the PCM1794 DAC chip provides a differential output, it would make sense to maintain the differential topology and use a differential amplifier. One example of such an amplifier is the Raleigh Audio Line Stage, which was used as the genesis for the RAKK dac active output stage. There are many possible ways to design a differential amplifier and this one has evolved as the result of many experiments on those possibilities. See the Line Stage series of articles on this website for the story of that evolution. In this amplifier a parallel feed transformer extracts an output signal from a pair of single-ended-triode amplifiers that are driven in opposite phase. Of course, a general-purpose line stage has circuitry such as a volume control and external interface that are not needed in a DAC output circuit, so these have been removed.
Figure 1. RAKK dac Active Output schematic
The resistors on the input are directly attached to the current output from the RAKK dac and provide current-to-voltage conversion. A resistance of 91Ω was chosen to provide 2VRMS output from the amplifier at a maximum 0dB digital signal. The two halves of the 6N6P triode, which are driven in opposite phase because the two outputs from the RAKK dac are out of phase, provide about 27dB of gain. Of course some of this gain is traded off for drive capability in the output transformer. In a conventional amplifier, both the signal and power follow the same path. This is not so in a parallel feed amplifier. The term “parallel feed” comes from the two parallel loops in the amplifier: a power loop and a signal loop. Power for the circuit flows from the power supply, through the constant current source, through the tube, to ground and back to the power supply. The signal flows from one tube, through the parallel feed capacitor, through the transformer primary, through the second tube and back to the first tube. The two loops are separate because the constant current source provides high impedance to the signal while allowing the direct current power to pass and the parallel feed capacitor blocks the direct current while allowing the signal to pass unimpeded. The output transformer provides good drive capability and an inherently balanced output.
It would be a nice feature to add a volume control to the active output so that it could be used without a preamplifier in a CD-only system. There are several different attenuator configurations that can be used in a differential amplifier and through extensive experimentation I have found the shunt configuration to be the most transparent. Therefore I offer the following circuit for anyone wanting to add a volume control to their RAKK dac active output.
Figure 2. RAKK dac Active Output with volume control
All that's involved is a volume control variable resistor along with a couple of series resistors, R1 and R2 in the diagram. The volume control can be any value but I have achieved the best results by using a 10K volume control. Resistors R1 and R2 should be about 1/2 of the value of the volume control. The exact value is not critical - with a 10K volume control I use something between 4.7K and 5.1K for the series resistors.
Several enthusiasts have asked if the active output stage can be configured as a combined DAC output and preamplifier. The answer is “yes it can.” We already have discussed the DAC function with a volume control so we are almost there. Now let’s look at what it would take to provide the preamplifier function separately and then we will combine them.
Figure 3. RAKK dac Active Output as a line stage
For the input to the preamplifier we want to provide isolation and unbalanced-to-balanced conversion. A transformer is perfect for this job. The volume control is the same as discussed above with one important consideration. The impedance of the attenuator is reflected back across the transformer as the input impedance of the preamplifier. When the potentiometer is at minimum volume rotation, the reflected impedance is the two input resistors, R1 and R2, in series. We want to keep this at a reasonable value so with the input transformer wired for 2:1 ratio, the 10K sum of the input resistors is reflected across the transformer as a 40K input impedance, which works well with most sources.
Figure 4. Combined RAKK dac Active Output and Line Stage
The two circuits have the same topology so it is a simple job to switch between them. You could use a switch for this function, however I have had good results using a high-quality relay here. A circuit board and parts to implement either or both of the DAC with volume control and preamplifier are available through K&K Audio.