Using a good quality isolation transformer or "less intrusive" PLC typically results in the following results. That is, so long as the transformers or filters are properly sized and do not restrict current.
1) Lower noise floor ( "blacker backgrounds" )
2) Less grain and glare ( digital sounds less digital )
3) Increased liquidity i.e. voices and instruments sound more natural. ( Many PLC's don't do well here as they they can tend to sound more "sterile". Many people are willing to live with this trade-off due to having some form of spike / surge suppression )
4) Increased separation of instruments and notes ( there is less noise to fill in the gaps that would normally be quiet )
5) Increased dynamic range ( due to lower noise floor )
6) More stable soundstage and imaging ( less noise "blurring" the actual spatial information / musical content of the recording )
I think that you get the idea. I must state that results will vary as AC varies from location to location quite a bit. Both the effectiveness and the results observed will vary with the types of filtration / isolation devices used, the quality of AC to begin with, the amount of filtration designed into each individual audio component, etc... My experience is that all systems will benefit from isolation transformers, so long as they capable of passing more than enough current than any individual component would need at any given time. As mentioned, it is best to isolate each component if possible or at least separate analogue from digital.
The most "bang for the buck" installation would make use of an isolation transformer for digital, one for analogue and one for the amp. Obviously, one need not use mega-watt devices for the digital and analogue gear unless those devices pull a lot of power. If all of your digital gear adds up to pulling 100 watts, shoot for an isolation transformer that can sustain 250 watts or .25 KVA. If all of your analogue gear pulls 400 watts, shoot for an isolation transformer of 1 - 1.2 KVA. In essence, you want to keep the load passing through the isolation transformer at around 40 - 60 percent of capacity. This pretty much guarantees that you will not saturate the core under any circumstances ( the one potential major drawback to this approach ) AND gives you some headroom in case you swap gear for something that is a little more "thirsty" for current in the future. As far as power amps go, especially big brutes, you can't have a big enough isolation transformer. Use the biggest one that you can find with the highest KVA and most massive iron core.
Someone that was shooting for the "ultimate" in noise reduction would use a properly sized isolation transformer for each individual component. This is obviously easier to do if you have a smaller system with less componentry.
I do not recommend using voltage stabilizers or regulators unless you have a problem with voltage in your area. Most of these devices produce / add distortion of their own. You may now have more stable voltage on the average, but you've substituted one problem for another. Obviously, there are a LOT of different products and ways to do this, so there are bound to be exceptions to what i've stated. Running dedicated heavy gauge lines directly back to the mains with clean connections and no breaks in the line will typically reduce voltage drops or "sagging" to a minimum. If you've got an over-voltage condition, that could be a problem that only voltage regulation could fix if the AC provider is unresponsive. To me, AC regulation would be a last resort but at that stage of the game, it might be a necessary one. Sean
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