Power conditioners come in a number of forms and price, and address slightly different needs.
Slight variations in mains voltage are nothing to worry about at all. There is always a bit of variation. But the supply is no more than about 10% out you won't see much in the way of ill effects - certainly not in a head amp. It can be a bit critical in tube based designs - they may start to wear out tubes significantly faster with higher voltages.
Where audio likes power conditioning is not so much in regulating the voltage, but in keeping higher frequency crud out of the feed.
Cheap computer power conditioners are essentially the same as uninterruptible power supplies. In fact most UPS designs are conditioners as well. The cheap ones are connected in parallel with the supply, and actually buck any change in voltage (all the way to total loss of supply, when they are acting as a UPS.) These designs do not provide much isolation from lone noise, and are pretty useless for audio.
The most expensive dual conversion designs take the mains, take it down to a lower DC voltage (which is the same as the battery voltage) and then up convert to AC again. This has very high quality isolation and provides very clean power.
Alternate conditioners that are not based upon UPS designs are either auto-transformers or ferroresonant conditioners. Auto transformers only provide a constant voltage and use a servo mechanism to select the appropriate tap on the transformer to maintain the right voltage. They do provide limited cleaning of the power due to high frequency losses in the transformer core.
Ferroresonant conditioners use a mixture of capacitors and inductors in a very clever system in which the saturation of the inductor core is used to regulate the output voltage. These can provide quite high quality power and voltage regulation within reasonable bounds. They are expensive and really heavy! But a favourite with some audio buffs.
OK, but a schematic? No. There is no chance at all that you could ever compete on price with the cheap UPS systems that are available, even proper dual conversion ones. You can get dual conversion conditioners - it is simply the UPS without buying the batteries. Which can be a big saving. Building a UPS is not a DIY project. You are both using, and generating lethal voltages. Worse, the up converted voltages are just as lethal, and your house's residual current detector (or ELCB) won't detect electrocution with that supply. Auto-transformer and ferroresonant conditioners cannot be built form parts. The only source of parts will be manufacturers of conditioners.
