Hi this is my first post here,
i no this is a little old but i saw this searching about my amp. I baught an SA-620 not too long ago and im loving it, i think its the 1981 model, with the blue bars on the front like the 520. Visually it is very similar to the 520 but looking inside the componants are completally different. Im no electritian but it doesn't take a genious to tell the 620 is allot better internally plus sound quality is better by a large margin. Overall this is a lovely amp, i only got it because i blew up my 520 if im to be onest, but thats a project for after my uni course 😀