Noise performance was our primary focus when designing the input and output audio circuitry of StarLab. To achieve pristine noise performance, we decided to group the INs and OUTs next to each other to optimize the circuit layout and grounding.
StarLab is a mixed signal system where extremely sensitive audio circuits coexist with potentially noise-inducing digital circuits (SHARC DSP, MCU, memory, etc.). The analog audio signal paths of StarLab have a signal to noise ratio of over 120dB. In other words, intended signals are 1,000,000 times larger than background noise. To ensure this level of performance, we had to isolate the time-varying current demanded by the digital circuitry away from the audio signal grounds. Even small amounts of unintended current on signal grounds induce noise through a process called common impedance coupling.
We grouped the audio INs and OUTs so that there would only be a single grounding connection between the audio and digital circuitry. This star grounding scheme keeps noisy digital currents from the audio circuitry. Additionally, by grouping the audio INs and OUTs, we were able to keep audio layout tight, prevent ground loops, and minimize susceptibility to environmental noise. To maintain this star grounding scheme, we also use a flyback converter to generate our own local power supplies that are isolated from the Eurorack power supply ground (ribbon cable).