1. Design Concept
As mentioned in the previous section, the first goal was to adopt a differential-input ADC and achieve sufficiently satisfactory ADC performance at a reasonable cost. When selecting devices, even a single low-performance component in the analog signal path limits overall performance, so careful selection is necessary.For the key analog differential circuit, I adopted the low-distortion fully balanced OPA1632. To maximize performance, the surrounding circuits must be designed with as low impedance as possible to avoid S/N degradation due to thermal noise. However, for RCA inputs (mostly consumer devices), low input impedance can cause degraded sound, thin sound, or even distortion. Therefore, I attached OPA1612, which is suitable for low-distortion buffer circuits and stable under full feedback.
The S/PDIF input circuit follows the design from FreeDSP OCTAVIA, allowing both optical and coaxial inputs without switching. The DSP mainly performs band division and volume control, so I selected ADAU1452 with ASRC for cost efficiency. ADAU1466 (pin-compatible with larger memory) can be used if the FIR filter design requires using tools like RePhase. The power amplifier was selected with digital connection capability as a requirement. So I adopted TAS6424, a 4-channel version of TAS6422 with 2.1 MHz high-speed switching. The control system is handled by a PIC microcontroller. A 28-pin package was used, and a 4-digit 7-segment LED display allows gain display, including decimal values.
The combination of key components shows the tree of model variation as follows…
Originally, the design assumed a single-board configuration embedded in a 2-way speaker. However, considering usability as a standalone device, an external front panel PCB will be required
PCB Construction
- The FreeDSP INTEGRA MainBoard
- Front Panel Control Board