RAKK dac Active Output
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 75Ω was chosen to provide 2VRMS output from the amplifier at a maximum 0dB digital signal. The two halves of the 6N1P triode, which are driven in opposite phase because the two outputs from the RAKK dac are out of phase, provide about 30dB 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
Because of the impedances in the circuit and the inherent 6dB loss in the shunt attenuator, I have inserted a transformer between the RAKK dac and the output stage. Resistor R3 across the secondary of the transformer sets the maximum output level and should be chosen to provide a good range of control with the potentiometer. I suggest that you start with 1.5K and go larger if you need more output or smaller if you need less output. 4.3K will give you 2VRMS maximum output. The potentiometer can be any value between 10K and 50K. R1 and R2 each should be about one-half the value of the potentiometer – their exact value is not critical. Also note that the grid bias resistors have been changed to be 100K.
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 I recommend that a 50K potentiometer be used. Again, the grid bias resistors have been changed to be 100K.
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.