DC: Polyakov (III) Audio Again

2020/4/9

Motor Boating

Ever since it been made last time, I had performed several little change, I mean, teared it down and remove some stuff. There is a simple RC filter, between gain and Audio PA, I remove that, and forgot I had remove it. 

Year/years late, I find it began severe motorboating. 

Fix motorboating

New module installed, but motoboating not stop. I try to realign the overall topology to avoid output running near to input, and etc.  Nothing works, not at all. 

I finally  find out what will ending the motorboating. from Gain stage to Audio PA, there is long wire running to volume POT, and back. This is key wire.  A 0.1uF/0.01uF capacitor connected very near to the Audio input terminal.

Why this work: Just guss,  output of gain stage contain much high frequency audio,  this capacitor attenuation much of that --- and the high frequency is much easier causing cross-couple motorboating.