Like all singers of real character, Janowitz’s voice is instantly recognisable – pure, flexible, almost instrumental, and warmed and lifted by a very fast vibrato that gives it enough body to ride an orchestra of Richard Straussian proportions. Hugh Canning, in his affectionate booklet-note, talks of her ‘lunar beauty of tone’ and it is exactly that luminous quality of her voice that must have attracted her greatest musical patron, Herbert von Karajan. He engaged her throughout the 1960s and ’70s when her voice was in its peak condition, often to give a ‘halo’ to concerted numbers (the celebrated 1962 Beethoven Ninth is a perfect example).

For die-hard Janowitzians, the works here that are being made generally available on CD for the first time are the 1965 recording of Telemann’s dramatic cantata Ino, excerpts from Handel’s Messiah (sung in German) from 1964, a 1968 live Holland Festival Four Last Songs under Haitink and, appearing for the time in any format, two arias from Idomeneo and Così fan tutte with the Vienna Symphony Orchestra under John Pritchard (who conducted her Glyndebourne debut, alongside Pavarotti, in Idomeneo back in 1964). The Telemann received a bit of a rough ride in these pages on its initial release and of course we’ve had period versions since, but despite some slightly heavy-handed orchestral playing, Janowitz characterises with real feeling and a wide palette of vocal colours.

Janowitz recorded a lot of Baroque repertoire, usually with Karajan or Richter, but I find the colour of the voice becomes a little monochrome in this music; it’s in Mozart and later repertoire that colour and style really mesh. Her Figaro Countess is one of her finest creations, slightly world-weary but with mettle enough when her husband tries her patience just once too often: her ‘Porgi, amor’ is exquisitely done. Similarly, she characterises Ilia’s ‘Padre, germani, addio’ and Fiordiligi’s ‘Per pietà, ben mio, perdona’ on the two new items here with an evident sympathy for those women’s very different plight. Mozart concert arias (from 1966 under Boettcher) fill out the first disc and again, she displays considerably more fire and variety than she is often given credit for – ‘Ah, lo previdi’, for example, really is the mini-drama it should be.

Her partnership with Karajan is represented by two excerpts from Beethoven’s Egmont music and the soprano solo from Brahms’s German Requiem (we might have had the Missa solemnis or The Creation but they’re pretty easily acquired separately). Perhaps their most celebrated collaboration was Richard Strauss’s Four Last Songs, still the version I cherish. Well, for this set we’ve an earlier live version under Haitink but it sounds extraordinarily similar to the Karajan interpretation in phrasing and tempi and, apart from some hiss and one of two of the lower notes sketched in rather than sung full out as studio conditions would allow, I’d have been hard-pressed to tell the versions apart (and I must have listened to the Karajan performance hundreds of times).

Excerpts from Weber’s Der Freischütz come not from the celebrated Carlos Kleiber set but from a 1967 recital under Ferdinand Leitner, and the voice is in even better condition: from the same programme come ‘Ozean, du Ungeheuer’ and ‘Traure, mein Herz’ from Oberon and excerpts from Tannhäuser, Rienzi and (again eschewing the complete recording) Lohengrin. It’s a shame she didn’t record more complete Wagner roles (her Eva, for example, would have been a real treat).

I once asked Janowitz what were her favourite roles: ‘Ah, the three As,’ she replied, meaning Donna Anna, Agathe and Arabella. The Anna was never recorded commercially (though pirate versions are plentiful) – a proposed complete Don Giovanni for EMI was scuppered by the death of its conductor George Szell, the Agathe (Freischütz) we have in the Kleiber version already mentioned and her Arabella is enshrined on the Unitel video version conducted by Solti – in my view one of her finest performances (along with that other ‘A’, Ariadne, which she recorded for EMI). And the single most exquisite singing in this box? I’d suggest the two little numbers from Orff’s Carmina Burana (under Jochum in 1967): the composer, who was present at the sessions, must have been enchanted.

Janowitz never achieved quite the recognition outside the German-speaking world she deserved, but her legacy on disc was rich and wide-ranging and this appealing five-CD set is an overdue tribute to one of the finest Mozart and Strauss singers of the last four decades of the 20th century.