jon tiltman wrote:BTW I've used (still use in fact) mains conditioners and I've never had one that got hot or sapped much power. A couple of them have hummed due to being transformer based but their biggest problem IME is squashing the dynamics of power amplifiers. Not all of them do this, just the cheaper ones but they're the ones that give all conditioners a bad name!
Without knowing the design of the units you're talking about, it's a bit difficult to make any kind of meaningful comparison. Decent quality units of contemporary design tend to operate at efficiencies approaching 85-90% (can be significantly less at low loads), the difference is irradiated as heat so depending on the overall rating of the unit, this wasted energy can be quite considerable. Also, the noise (hum) produced by these devices is due to quite complex ferro-resonant interactions between inductive and capacitive components, not a transformer as such. They utilise what is called a "Tank Circuit" as a feature of their design.
Modern solid-state designs tend to head down the Switch Mode front-ended Sine Wave Inverter route these days as they are silent for all intents and purposes and when properly specified, can be very reliable and quite efficient from 30% load and up - Good one's are not cheap though...