An UPS won't solve your issue. You can buy those band-aids all day long and get no place. Best to have a proper AC line installed. Fix the problem at the root of the issue.
I'd put your large current draw items on a separate circuit. Like your HVAC and refrigerator for sure. Sounds like your stereo is ganged with the Air Con. Which would reflect your voltage drops.
You don't want anything connected to any line that has a dynamic voltage on it. Either plug in a table lamp into the circuit if you don't have a volt meter or use a volt meter to check voltage sags on larger drain item start ups. That lamp would show a temp bulb dimming effect on start up. These sagging circuit lines will kill your attached devices. Plus your looking at a possible fire.
Sagging voltage indicates some form of circuit overloading. Either excessive devices on the circuit, long runs, to small of breaker or small wire gauge feeding it. Old houses.....
I'd run a 40 amp breaker from your panel to the listening room. Use some 10-3 wire. Make sure each and every item has a built in fuse or breaker protection. Buy a 5 gang outlet box and fill with hospital grade outlets. Trim the box with some nice hardwood like Mahogany. Plug all equipment into this one dedicated line ONLY. Your problem solved. Shouldn't cost very much either.
BTW that 15 volt drop you see, it's a lot more in real time. Safe to say if you had an O-scope you'd see a ton more than 15 volts. I'd get my equipment off that circuit ASAP!
cityjim
