Ah - here is a rare series which very few remember.
When Sony was just about to go retro-style with the surprising rocker switches of its 1st gen' ES-II looks in 1973 (clearly lifted out of a Jaguar 420), Philips came up with something just as retro as an old Scott console.
The really retro part of the RH521 is its roundy meters which could hold a pair of ECC83 or any other glowing tubes - and that's what is remembered.
So much so that in the 1990s several manufacturers took on that cutie trick to coat their hybrid tube/digital processors with a vintage gloss - Behringer for instance.
Of course, the strange part is that "retro" in 1972 was just around the (back) corner and leaning closer to the "has been" area...
However, Stereo Sound made a beautiful summer 1973 cover with the RH521 - retro already had an edge in 1972 !
The rest of the RH521's design leans on the more recent 1960s - or the less retro if you like : long straight lines, large nude surfaces of finely brushed aluminium and a wooden enclosure.
The sides are however made of reinforced polystyrene - a futuristic touch in 1972 which to us is now... retro :)
Anyway - the RH521 is polarized in Class A/B (pre-drivers in Class A, power stage in Class B) and has all the (many) controls you can see above plus a strange one named "Stereo 4" : it is a way to spread a stereo signal to the "remote" loudspeaker pair - nothing more than that :)
Siblings with the same cutie windows :
RH520 : a 521, smaller and without the multi-filters and Stereo 4
RH720 : a 521 receiver shown here to the right, feeding a couple's nostalgia.
The 720 has preset feather-touch buttons added on the top strip : something Harman/Kardon would re-use this for its Citation Receiver.
Sibling of a different kind :
RH832 all-in-one with three cutie windows in slanted style - soon on TVK !
And of course, all of these saw Radiola versions depending on markets : RA5520 (= RH520), RA5521 (= RH521), RA5720 (= RH720) and RA832 (=RH832).