As presented in my previous log, I've tried audio amplifier implementation, and after prototyping I moved to "tiny production" for tindie. HOWEVER, tight soldering to PCB (prototyping stage, it was tentative) leads to un-solvable ultra strong microphonic making ultra loud howling through speaker-tube feedback. I've tried several material attachment to VFD to reduce feedback, but sometimes it is succeeded but another day it goes super howling. Eventually I gave up this game. The 7-seg VFD does not have any responsibility because they are made for "digit display". Fortunately I have another VFD display, DM160. It is not segment one but its "audio application" is far stable and reliable, and also characteristic variation is very small between devices. So I changed tube from 7-seg to DM160.

Actual operation is presented in the following movie. One additional function of this new amplifier is "power-on speaker pop canceling". You will see "zero pop" for power on in the following movie..