AD797 is an excellent opamp, still one of the best ones. However, it may be prone to oscillations. Looking at the noise floor modulation in your plot, I would say it was the case. You know, it is hard to find anything with lower noise than AD797, even nowadays. Kudos to Scott Wurcer.Your audio buffer looks nice! Before purchasing Cosmos APU, I made a simple imput buffer using an OP AMP (Analog Devices AD797). I have adopted the noninverting configuration of the OP AMP, and its impedance is enough high for the DAC. However, I could not lower its noise level as Cosmos APU's. Unfortunately, I am a software engineer, not a professional circuit designer.