Need Help with Voltage for Integrated

Changing the fuse value will not change the voltage capability of the unit.  I believe the manual is stating that because the required fuse amperage is half the value at 240V when compared to 120V.  For example, if a unit requires 10A fuse for 120V, it would require 5A at 240V. 

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If you have a 240V equipment and plug it into a 120V outlet, you are basically under-powering the unit.  The FMJ 18 is is mostly an pure analog integrated amp.  The amp circuit is most likely an "un-regulated" power supply and will run at half voltage (just like op amps can run fine within a voltage range such as +/- 5V to +/- 15V).   The front panel is probably running off of 9V or 12V DC - which is most likely regulated down from the main amp power supply which is at least 35V DC.  If running at half voltage, it will see about 17V DC, which is still fine to be regulated down to 9/12V. 

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So, ultimately, if the amp seems to be running fine, it will have no problem.  However, you won't get all the power output capability.  FMJ 18 appears to be a 50 watt per channel amp, so you might get 20-25 watt per channel when plugged into 120V outlet.

Changing the fuse value will not change the voltage capability of the unit.

No one said otherwise.
The issue is that the unit has multiple internal locations for the fuse, depending on wall voltage. Each location probably requires a different amperage.

Erik, you're reading skills strike again! lol.  If you scroll up to reubent's comment above, the first thing he says is this:

Per the downloadable users manual Version at the Arcam site,the voltage is changeable via a fuse internal to the amp.

http://www.hifishock.org/galleries/electronics/arcam/integrated-amplifier/fmj-a18-arcam-1.JPG

Look like there's a voltage conversion switch next to the IEC socket.