RAKK dac Mark III Installation and Grounding
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This article covers the installation and grounding for the RAKK dac Mark III suite of products. The assembly manuals provide installation instructions for each component but there is nothing to give a system overview. There are several ways to interconnect the components and provide grounding for the system and some ways are more effective than others. We have found that the vast majority of problems experienced with the RAKK dac and its associated components are traced back to incorrect installation, particularly ground loops and faulty grounding. Do not trust your intuition - rather, follow the instructions in the manuals and the concepts presented here - we know that they work. We will start with a couple of general concepts.
First is grounding: We often think of "ground" as something we can connect power supplies, circuits and external cables to indiscriminately. However, what is connected where can lead to problems. You can categorize connections to "ground" by their function, or purpose. For example there is "safety ground" intended to protect people from electrical shock. There is "power ground," which together with voltages allow a circuit to perform its job. There is "signal ground" which together with a signal line provides a signal for the circuit to work on. There are others but we will focus on those three. There may be several instances of "signal ground" and "power ground," but there is only one "safety ground." Ground current will flow within each instance of "ground" but there is no need for current to flow between one "ground" and another "ground." All of the "grounds" must be connected in such a way that they are at the same potential voltage but no current is flowing between them. The "grounds" must also be connected such that there are no loops, which may act as antennas and cause noise in the system. The installation manuals use the term "chassis" for the safety ground, "reference" for the signal ground, and "common" or "Ground" for power ground. I will use those terms for the rest of this article.
The second general concept is cable attachment. All of the signals, both digital and analog, attaching to a system comprised of the RAKK dac and its associated components are isolated by transformers. This galvanic isolation goes a long way to eliminating grounding problems when the DAC sub-system is connected to other components such as a CD transport and a preamplifier. However, since the grounds are isolated between components, both the signal and the reference must be explicitly connected. That is, you cannot just connect the signal and assume that a ground reference is present. An external cable may or may not have its reference connected to the chassis. It should never be connected to an internal reference or common. The connection between the cable reference and chassis is optional. You may find that one way or the other will reduce or eliminate hum or noise.
This is a very short synopsis of grounding and interconnection. For a thorough understanding of the topic refer to my general article, Audio Component Grounding and Interconnection.
Now let's look at some specifics. The following figure shows the RAKK dac Mark III digital interface.
There are three inputs, all of which can accept an S/PDIF or AES/EBU signal. The circuitry is the same for S/PDIF or AES/EBU and the difference between the two types is the transformer used. The S/PDIF input uses a Lundahl LL1572 with a 1:1 winding ratio that provides the 75Ω standard impedance. The AES/EBU input uses a Lundahl LL1574 with a 1:1.2 winding ratio that provides the 110Ω standard impedance.
The AES/EBU standard allows a much wider range of signal amplitude than does the S/PDIF standard. Modern electronic chips are designed for the S/PDIF standard and may be overloaded if an AES/EBU input with an amplitude greater than 3.3V p-p is applied. In this case you will need to adjust the level of the incoming signal by inserting a resistor T-pad between the input and the RAKK dac Mark III. The details for this are shown here.
Note that both polarities of the external digital signal must be presented to the RAKK dac Mark III board. I have seen several wiring errors based upon the erroneous assumption that the external digital signal is referenced to ground and therefore doesn't need to be connected to the board. The external digital signal is not referenced to ground and should not be connected to ground.
The RAKK dac Mark III has a forth digital input that previous versions of the RAKK dac did not have. New to the Mark III is an I2S digital input.
The I2S interface is used to connect external devices, such as a CD transport or a computer. Presently there are two variations - one for connecting a USB interface and another for connecting an external I2S interface. There are a couple of things to note here. First is the galvanic isolation. In order to prevent ground loops, both the signal interface and the power are isolated. Second is the I2S connection to the RAKK dac. Since the interface is galvanically isolated, both polarities of each of the three signals must be present on the interface - there is no common signal reference shared between the two boards.
Let's now move on to the outputs. The following figure shows one channel of both a RAKK dac with a Passive Output and an Active Output.
Figure 2. RAKK dac Mark III Analog Outputs
The RAKK dac analog output is a differential current, that is, as L+ increases L- decreases and vice versa. The currents flow through the I/V resistors in the case of the Active Output or the transformer primary in the case of the Passive Output and then return through the REF. The REF is a reference that is internally connected to common on the RAKK dac. It is important that only signal current flow through the REF connection. In the case of the Active Output there must be a voltage reference between the cathode and the grid of each triode for the circuit to function. The cathodes are referenced to power common on the Active Output through the cathode bias resistor. Thus the bottom of each grid resistor must be also referenced to this same power common. (Note that the power common on the Active Output is different from the power common on the RAKK dac.) The bottom of the two grid resistors that must be referenced to power common are connected together and also connected to REF. We need to have a voltage reference but there is no need for any current to flow between REF and power common on the Active Output. We can achieve this by ensuring that there is no loop or return path. That is, there is no other path between the RAKK dac ground or its power supply common and the Active Output ground.
As with the input signal cable, the output signal cable is transformer isolated from the internal circuitry. It is not shown on the diagram but the Active Output has a transformer on its output like the Passive Output. The signal reference on the output cable may or may not be connected to chassis. One option or the other may provided reduced hum and noise.
The following figures show the ground map for a RAKK dac Mark III system.
Ground Map with a Passive Output Stage
Ground Map with an Active Output Stage
There are a couple of things worth noting on these diagrams. First, if an external signal reference is connected to the chassis, ground current probably will flow in that connection. This is because there may be an external ground loop; it all depends upon the far end connection. Controlling external ground loops is beyond the scope of this article - check here for that kind of information. The second thing to note is that there must be a single connection between the chassis and the grounding scheme somewhere in the system. Under normal conditions no current will flow in this connection. However it is there for safety, and in the case of a fault, current is supposed to flow in that connection and any audible noise is irrelevant. Third, note that there are no loops in the ground scheme.
Let's wrap it up with a pictorial diagram of the interconnection of RAKK dac Mark III components.
RAKK dac Mark III with Passive Output
RAKK dac Mark III with Active Output