DC: Polyakov (III) Audio Again
2020/4/9
Motor Boating
Polyakov DC
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.
New Final Audio Stage
I actually produce several small audio module, and left that PCB conner for long time. I decide it's good time to build one and make it work for Polyakov DC receiver.
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.
key wire to fix motor boating wire
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.