Part 2: Power
Because MIYO is USB bus-powered, the power supply design is critical to ensuring the best possible audio integrity. USB power can be noisy, so we have a broadband LC Pi-filter right at the USB jack, as well as over voltage, over current, and static protection.
Digital logic and LEDs can create switching noise, so we have a separate broadband LC double-Pi-filter for the level meter LEDs. And the digital logic has its own separate LDO voltage regulator, giving it steady voltage and isolating it as a noise source. The master clock oscillators have their own separate broadband LC-filtered power following an LDO regulator.
We found some super low-noise LDO voltage regulators and use separate ones for the A/D converter and input stage, and the D/A converter, each optimized to the voltage required by the converter. And the headphone drivers have their own high voltage power supplies, which also have broadband LC Pi-filters.
Another important aspect of power is ground. Any current drawn from a power supply has to return via a ground. Great care was taken in the PCB layout such that the noisy circuitry had separate ground planes - isolating any circulating noise current away from the analog circuitry. The analog input and output grounds are also isolated from the digital and USB grounds using a proprietary approach to eliminate ground loops between the computer and any connected audio gear.
Achieving MIYO's level of performance via the power supplied by USB was quite a challenge, but completely necessary to ensure MIYO could be truly portable and provide world-class sound quality. In the final design, we're very pleased with what we hear.
In the next part of Let's Get Nerdy, we'll discuss the analog audio path in detail.