This project documents the design and construction of a high-quality sound card featuring USB and optical (TOSLINK) inputs. The USB interface works in device mode, receiving audio from a computer, while the optical input carries SPDIF signals.
A USB isolator (ADUM4160) provides galvanic separation to prevent noise from the computer. Incoming digital audio streams are converted to PCM via USB-to-I2S (PCM2707) and SPDIF-to-I2S (CS8416) converters. A multiplexer selects the audio source, and an asynchronous sample rate converter (SRC4193) handles resampling and bit-depth conversion before sending the data to the PCM1792 DAC.
The DAC outputs a current signal, which is converted to voltage and fed to a TPA6120 headphone amplifier. A precise clock system (FOX924B + PLL1708) ensures low-jitter operation.
Design Notes:
- USB and TOSLINK inputs for versatility
- Ground isolation via USB isolator
- Multiplexer required due to converters lacking tri-state outputs
- SRC4193 handles both resampling and clock synchronization
- TPA6120 provides high-current headphone drive
References / Source:
This project is based on my MSc thesis: https://sylwekkominek.github.io/SoundCard/
In some high-end audio equipment, separate transformers are often used for the digital and analog sections. Since transformers are relatively costly components, it is worth evaluating whether a dual-transformer design is justified.
Tables 9 and 10 present S/N and THD+N measurements under different resampler operating modes. Table 9 shows data with two transformers powering the card, while Table 10 shows data with a single transformer.
With dual transformers, the S/N ratio at a 64 kHz resampler frequency improved by only 1 dB, and the THD+N at 96 kHz decreased by 1 dB. These results indicate that using two transformers has a negligible effect on the sound card’s performance and is practically inaudible.
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