Can a european amp version 240 be changed to us 120?

Might be best to simply ask the manufacturer of the amp in question, if and how

more power = better sound

Actually, he/she needs a step up transformer. -120v to 240v, unless there is an internal switch.
B

Don't think too hard. 
Bad things might happen.

The answer is sometimes.. 

is it a switch inside the unit? Likely not in most cases. Takes some re wiring IME, I have tried with several pieces. Ended up just getting a transformer. 

You need to be very specific about your amplifier make/model.

A lot of units are designed with transformers that will do either, but need to be wired correctly.  I've not seen a power amplifier that could switch by a mere switch, but I suppose it's possible.  These are usually high current connections so screw terminals are the preferred way to do it.

Otherwise, a step-up transformer will work, but since amps take a lot of power the right one is more expensive.

How bout converting a preamp between us and europe?

@OP, 
Once again, consult the manufacturer.
B

Often times the power transformers will have both 110 volt and 220 volt windings.

A long time ago my tech converted a Micro Mega Classic Solo CD player from 220 to 110 for me. He didn’t consult MM because he was a good tech and figured it out on his own. It’s usually an easy swap if you know what you are doing.

That MM Classic Solo was an excellent CDP and I enjoyed my for many years.


Often amps do have 220-110 switches which are usually on the back but I have seen them inside the unit too. It’s fairly common on Chi-Fi gear.

Good luck and good listening!