I am a firm believer in power conditioners. I learned the hard way trying to keep our computer network at the office running without "glitches."
However, I only use "Double Conversion" power conditioners. A Double Conversion (aka Dual Conversion) unit takes the A/C from the wall outlet, converts that to D/C and then charges the battery array. Then, it takes the D/C from the battery array and converts that to regulated pure sine wave power.
Most power conditioners (unless they specifically say Double Conversion or Dual Conversion) are "Line Interactive" units. A line interactive unit runs its output from the A/C from the wall outlet until it senses a problem with the incoming A/C power. Then it switches to the backup battery to provide power. There is always some time delay in this switch over process (perhaps microseconds, or milliseconds, but still a delay). Switching power supplies (commonly found in computers, or computer based units) can be VERY sensitive to ANY switch over time, no matter how short.
Most high end audio units do not use switching power supplies, but use the "old fashioned" power supplies with big transformers and big capacitors. These are not typically sensitive to switch over time delays. But, a lot of digital components are computer based, and use switching power supplies.
Even though my McIntosh power amps all have big (read that HEAVY) power transformers, and capacitors big enough to electrocute someone, I still plug my power amps (and all components) into dual conversion power conditioners. With the power conditioner making highly regulated sine wave power, I know the power going into the amp is cleaner than the local utility power.
There are power conditioners that treat the a/c before they send it to the components. But, many of these do not have battery backup. Even the ones with battery backup have to switch to the battery in case of power failure (thus creating the switch over timing problem). On the Gulf Coast we have a lot of storms coming in, especially in the summer. Even a good windstorm can blow a tree down over power lines, causing power interruption. Some components loose their settings if power is lost. So, then one must go back through the setup process on these units. Battery sourced power is the only way I have found to avoid this process.
The only downside to Double Conversion power conditioners is cost. A 2,000VA unit will be about $2,000 new. A 3,000VA unit will be about $3,000. Used, refurbished, units can be found for about half these amounts.
The biggest problem with used/refurbished units, is that they are typically older technology, many of them only 70% efficient. So, a 2,000VA unit may only deliver 1,400 usable watts. Even newer units typically do not exceed 90% efficiency.
Another problem is cooling fans. Since Dual Conversion power supplies are really industrial units, they do not worry about fan noise. At home, I have the power conditioner for the Audio System in the Attic. The power conditioner for the Video system is in a cabinet with closed doors. At the office, it is in a closet. When I tried to mount the power conditioner in the same rack as the audio units, the fan noise got in the way.
I try to get units with enough power to everything can plug into the unit. Otherwise, different components may have a different path to ground - raising ground loop hum difficulties. If the problem is with digital source units, the problem can be solved with toslink connections. But otherwise different paths to ground (at least with unbalanced connections) can create ground loop problems.
But - at the risk of repeating myself - my real reason for using double conversion power conditioners is that it generates highly regulated pure sine wave A/C.
As someone said earlier in this thread - the best components deserve to be fed the best power.
Or as I read 30 years ago (when everything was analog) "when you listen to your power amp, remember you are not listening to the original signal - you are listening to what comes out of your power supply as modified by that original signal."
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