I've tried a number of the suggested changes to reduce output section noise. I have increased C6 to 33nF, use a battery pack for power, disabled Bluetooth and WiFi to only minimal improvement. Looking at the schematic and the Sallen-Key filter doc from TI, it would appear C6 and C7 are related. Should C7 also be changed when C6 is changed? Any other tips for noise reduction? Thanks.
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R2&C2 have a fc= 4.9Khz, if you double C2 (just placing a 6.8n Cap in parallel with C2) the new fc= 2.6KHz
With the S&K filter is the same, placing 6.8nF caps in parallel with C6, c7 and C8 will half the fc.