For those who are interested, in Figure 3 we have drawn the internals of the PGA2311. It is immediately clear that we have two (because it is stereo) amplifiers, the amplification of which are set by a set of (serial) control inputs. The control signals (CS, SCLK, SDI and MUTE) come from microcontroller IC1.
A brief remark about the connection of ZCEN (Zero Crossing Enable). The PGA2311 has a zero-crossing detector;
the idea behind this is (when this function is enabled) that the change in volume only takes effect after the next positive-going zero-crossing of the audio input signal. In this way spurious audio artifacts as a consequence of the change in volume can be minimized. On the circuit board ZCEN can be connected via a jumper to either +4.7 V or ground, so that you have the choice whether to enable or disable this functionality.
For a more detailed description of the inner workings of the PGA2311 we refer you to the datasheet . Between the microcontroller and the PGA2311 we provided test points on the circuit board for the serial control signals (CS, SDI, SCLK and MUTE).
The input and outputs go via resistors of 47 Ω (R1 through R8) to two different connectors. A pair of so-called stack-through-connectors (K2 and K3) connect the inputs and outputs of the DAC and the volume control (in the same was as is done for the power supply).
The inputs and outputs of IC2 also go to two 3.5-mm audio connectors (K4 and K5) on the circuit board. This means that the unprocessed DAC signal is available using a 3.5-mm jack (K5) or via the cinch connectors on the DAC board. The output signal from the volume control is available on K4.
When the board is used as an ‘independent’ volume control, K5 is the input for the PGA231 and K4 is the output. The 47-Ω resistors protect the outputs from capacitive loads and create a separation between the various connectors.