Identifying circuits like these are more difficult, since they are very complex. On the Grace m902, you can start from the AC inlet on the lower right part of the chassis. AC power is then routed to the travo (black circular piece), and out the travo is many colored wires that supply power to the different parts of the unit. The PCB on the left, mounted on the front panel, like on the DacMagic, is for the digital user interface section of the Grace m902. I really won’t go into much detail with the Grace, but if you can read the writing off the chips, entering the chip model number on Google would reveal what the circuit does, and that way you can identify what that section of the PCB is dedicated for. Since the input panel is on the right side, and the headphone output connectors are on the left side, it’s safe to say that the input signal flows from the right side of the board to the left side. The Grace m902 is a very versatile tool that accepts both digital and analog inputs. The digital input are in the form of S/PDIF connectors (Coaxial, Toslink, and AES/EBU) and USB — so there probably is a USB receiver chip close to the USB digital input. Close to the S/PDIF digital inputs should be the DAC circuit, and the top section is probably the headphone amplifier section, close to the headphone out jack.
Here is a close up view of the DAC section. The D/A chip is a PCM1730 (the one big rectangular chip on the right side of the picture), and on the left side is the analog stage of the DAC section. Not visible in the picture is the power supply for the DAC section, the internal DAC clock circuitry, the S/PDIF and USB receivers for the digital signal.
Some manufacturers decide to scrape off the markings on the chip, making it very difficult to study the circuit, like on the Ibasso PB1 circuit below. But generally, having access to the chip codes is essential to determining how a certain circuit functions, since the chip’s datasheet will give a fairly thorough information on the function of the chips, as well as a typical implementation circuit schematic. After some time, after much exposure to the chips, you can quickly identify different chips without having to consult Google (remember how looking at headphone pictures used to be so confusing, remembering which model is which?).
While circuit complexities don’t necessarily determine sound quality, they do have quite a strong correlation. Having auditioned a very large number of DACs and Amps, bigger units usually sound better than smaller units. But remember that there is always exceptions to the rule. While this has been quite a rough overview, I hope the article can help you to be more familiar with amplifier circuits.
For more reading, you can check out the discussion thread titledPCB layout, where we have some inputs from David and Pyschaudio regarding amplifier design.