allanhurst:
Are you proposing to build a device which will make a loud noise, or a proper HiFi amp? And at what power level?There's an awful lot of difference....
Understood. Per my subject, I am building "a simple 8-ohm speaker amp" at novice level driven by an arduino and a 5v powersupply.. nothing HiFi or high powered!
Thanks to the many respondents with suggestions I buy a different amp, or build a different amp, and special thanks to MarkT and SouthPark whose comments helped me understand the two specific questions I was asking about the amp I'm trying to build -- where the "breakthrough" comes from and why my switch appeared useless. I've rewired my volume pot/switch to eliminate breakthrough (bingo, MarkT!), keeping the rest of the circuit design since at least (SouthPark!) I now understand it. Next I'll build something incrementally fancier even if I'm still a ways from making sense of some of the other suggestions.