The Heart of Manley Labs
What I found particularly interesting is that the Heart’s co-designer was Paul Frindle, a pro-audio engineer with whom I had become acquainted back in the days of Usenet. At very low signal levels, delta-sigma DACs suffer from what are called “limit cycle” spuriaethere is insufficient data in the delta-sigma loop and this leads to instability and the appearance of low-level enharmonic tones in the decoded analog signal. (You can see these in some of the spectra I publish in Stereophile’s DAC reviews.) Frindle’s patented topology adds low-level noise at the input of the loop to keep it busy and eliminate limit cycles. However, as the overall DAC circuit is differential but the noise is common-mode, it is canceled out in the reconstructed analog signal. Neat.