RAKK dac Mark II Installation and Grounding
This article covers the installation and grounding for the RAKK dac Mark II 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. Figure 1 shows the RAKK dac Mark II digital interface.
Figure 1. RAKK dac Mark II Digital Interface
There are two inputs, either of which may support a S/PDIF or AES/EBU signal. The signal is connected to pad 2 and the reference is connected to pad 3. Pad 1 is a chassis connection for an AES/EBU signal which uses an XLR connector and is unused for a S/PDIF signal which uses either an RCA or BNC connector. Provision is made on the board to optionally connect the reference to the chassis. This connection may either be direct ( J1, J4 closed) or lifted (J1, J4 open) for an RF connection between chassis and reference. Alternately the shell of the connector may be directly connected to the chassis. If that is the case, the chassis connection should not be made on the board. One example of a poor practice that we have seen is both the connector shell and the on-board reference was connected to the chassis and no wire was present between the shell of the connector and pad 3. This caused the signal return current to flow through the chassis and sort-of worked but caused a large amount of digital errors. Another person connected the shell of the connector to common, and thinking there was some kind of internal ground for the input made no connection to either the chassis pad or pad 3 on the board. This did not work at all.
Note: After two years with this arrangement, I no longer recommend grounding the digital signal input because it causes reflections which result in jitter. Also, I have never seen a problem of a ground loop here, and if there were one, I recommend fixing it.
Let's now move on to the outputs. Figure 2 shows one channel of both a RAKK dac with a Passive Output and an Active Output.
Figure 2. RAKK dac Mark II 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.
Figures 3A and 3B show the ground map for a RAKK dac Mark II system.
Figure 3A. Ground Map with a Passive Output Stage
Figure 3B. 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. 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 II components.
Figure 4A. RAKK dac Mark II with Passive Output
Figure 4B. RAKK dac Mark II with Active Output