I would say Busa is correct about this Integrated amp.
I could not find any info this particular model at the Yamaha site but the blue book does list the A-420 at 50wpc.The A-520 does show at 75wpc so that makes sense in the Yamaha scheme of things.
If anyone owns a Yamaha product most manuals can be downloaded at the above web page.
Complete Yamaha 2000 Series Components
MX-2000 Class A Power Amplifier
CDX-2020 CD Player
KX-1200u Cassette Deck
Acoustic Research Pro Series Interconnects and Speaker Wire
Atlantis Reference 5 Audio Rack
1994 KB-WO Klipschorns