Collectors certainly wanted more than Decca could provide of this year’s centenarian Birgit Nilsson, an undoubted recording star of the great age of the stereo LP. Indeed, all of the 17 performances (12 of them complete) collected here have appeared before in some form, or condition, as ‘private’ or ‘pirate’ issue recordings. The sonic improvements now made by Sony to these original releases are often considerable and their selection is both logical and satisfying.

The calling card of a live (and open-air) 1973 Tristan und Isolde is the chance to hear Nilsson in her favourite role alongside Jon Vickers. At the start here audience chatter and picnic rustling seriously rival Karl Böhm’s gentle first placing of the famous chord. But after that it’s all gain as both his conducting and the cleaned-up recording spread themselves to accommodate the Orange acoustic in a performance worth acquiring now despite local difficulties. You can hear now, alongside the heroics, how sensitive, romantic and pained Vickers’s Tristan is, how strong and committed are the supporting cast – not least Walter Berry’s blindly faithful servant Kurwenal. And how Nilsson’s ‘loving Isolde’ – Wieland Wagner’s sobriquet – now informs her whole reading of the part, still vibrant top notes included. (Incidentally, Sony, the Steersman’s name is Paul Taillefer.)

The 1965 Metropolitan Opera Salome is hot. If you thought the (still remarkable) Decca studio version of some years before was a classic example of Glenn Gould’s ‘constructive cheating’ theory of recording, you’ll be surprised. Nilsson can do it all live, including some of the teenage-girl voice – as heard too on the Decca – that made Maria Cebotari’s assumption of the role so ghoulish. The Herods (Karl Liebl/Irene Dalis) and Narraboth (George Shirley no less) are both strong and unguyed, while Böhm has the orchestra at a thrilling pitch of both ensemble and wildness of colour.

The Elektras, two of them, are hardly less essential. The Vienna State Opera’s guest appearance at the 1967 Montreal EXPO starts distantly in what sounds like a studio with the reverberation left on, a strange cavern of an acoustic. But, apart from the timpani, it is soon tamed effectively by the ‘restoration’ engineers Othmar Eichinger and Harald Huber (their work is on most of this box-set) and the ear adjusts. This is an early performance of the role by Nilsson, excitable and vengeance-filled (as is her conductor’s), not unlike the Decca recording of less than a year before. Regina Resnik’s Klytemnestra is also present from that cast to give a portrait of the murderous queen to match her original. The Montreal crowd love her to the point of interjecting an ovation when they think she’s exiting. The remainder of the cast, Viennese company soloists all, give palpable special-occasion support. As recorded here, Böhm sounds more violent and loud than in his other traceable performances.

Nearly four years later the Met Elektra showcases another example of the most thorough orchestral preparation and delivery by the maestro, a more cumulative approach to the character by Nilsson and a peppy contribution by an obviously inspired Thomas Stewart. The smaller roles lack the familiar confidence of their Viennese equivalents in Montreal although an apparently sick Jean Madeira manages a gripping psychological portrait of Klytemnestra. It’s a more organised, better-sounding occasion than the EXPO tour and might be your choice if you want this conductor and soprano together in this work.

A Böhm/Nilsson Tristan from Vienna has Jess Thomas as the tenor lead, a 1967 pre-Christmas evening at the Staatsoper with a solidly experienced cast. Thomas’s passion and obvious intelligence with the text (but uneven tone production) does not always quite equal the achievements of his diva and conductor – he’s better at being neurotic than in love (Act 3 and the end of Act 1 are more in focus than Act 2). Nilsson and Böhm – who have by this time been working this opera for six successive Bayreuth Festival summers – are playing off each other beautifully: try their Liebestod, which is like a Lieder recital in its natural-sounding exchanges of colour and mood. And clearly an achievement post the one DG recorded at Bayreuth in 1966.

Turn immediately (if you can) to the Bayreuth performance under Wolfgang Sawallisch from a decade before. This is Nilsson’s real Bayreuth debut in a role suited to her more than the efficient but bloodless Elsa (here from 1954 under Jochum) and the plucky but overweighty Sieglinde (also here, from 1957 under Knappertsbusch), both the latter roles handed out when Wieland Wagner wrongly thought she was the next Maria Müller and insufficiently dramatic for Brünnhilde. The Isolde (which she had sung elsewhere) is already working well although it lacks the detail of later – reactions are more conventional princess-in-the-lead – and Sawallisch sounds to be accommodating her with skill rather than collaborating. Here, one year before the version released by Orfeo (A/18), this conductor is already cunningly moulding his minimalist Wagner sonorities to a music drama bigger than the early Romantic operas. Despite all the ifs and buts, this first joint outing for Nilsson and Wolfgang Windgassen (the Festival’s Liebespaar from now until 1970) is an unpredictably exciting evening.

The Karajan Die Walküre at the Met is professional but resolutely unthrilling – a show that has remained famous for Nilsson’s rehearsal joke in protesting stage director Karajan’s low-level lighting with a lit-up miner’s helmet. At least Karajan as repetiteur was in good form, securing a fine result from the orchestra and even getting his reluctantly chosen diva Nilsson (details in Thomas Voigt’s not otherwise over-informative booklet note) to sound younger and more girlish, like his first choice Régine Crespin, who’s here as Sieglinde. Everyone sings and plays at the right times but, for example, Vickers’s Siegmund – so alive in the DG recording and Salzburg tapes – projects everything very safely and Theo Adam sounds like he’s watching the maestro the whole time.

A live 1976 Die Frau ohne Schatten makes a companion for and contrast to the existing DG Böhm recording. The Dyer’s Wife was the last new role that Nilsson took on at a late career stage as well as the nearest to a character role. This very live performance finds her in spiky and lively partnership with Fischer-Dieskau’s Barak, most attentive to text and admirably wary of clichéd off-the-voice dramatics. And it’s a real collector’s item to have three great Nordic Wagnerians together – Nilsson, Ingrid Bjoner (Empress) and Astrid Varnay (Nurse). Sawallisch’s conducting is ever fluent and pacy but his preference for lighter sonorities can sound a little penny plain in this score.

A collection disc of three Wagner concert items is memorable for Sergiu Celibidache’s expansive and detailed handling of the Tristan Verklärung and Stig Rybrant’s of Nilsson in a very early (1953), very straight Götterdämmerung Immolation scene in Swedish, which sounds in this context almost like a Nordic original version of Wagner’s text.

The three non-Wagner/Strauss items are all superb, due in no small part to their conductors. The Swedish premiere of Bartók’s Duke Bluebeard’s Castle (sung in German) teamed Nilsson with Bernhard Sönnerstedt under the suitably dark and moody baton of Ferenc Fricsay. She sings strongly, straightforwardly and truly (and the climactic reaction to the Fifth Door, which we’re all waiting for, is predictably electrifying). Bernstein’s Fidelio for Rome Radio – his first try-out of the whole score – is grippingly held together and follows much the same dynamic path as his DG recording, ie a chamber opera until Pizarro comes, then baleful music drama. Nilsson is much freer and more at one with the text than in her famous early recording with Erich Kleiber.

Lastly, and very not leastly, the epochal Stokowski Met Turandot finally gets official mass release blessing. Aside from the colossal vocal battle between Nilsson and Franco Corelli (both on great form if a little careful of the conducting), there’s Anna Moffo’s absolute nonpareil of a sensual Liù (just her first ‘mi hai sorriso’ is overwhelming) and the maestro conducting the work like a key early 20th-century score, the orchestra bravely following his many tempo adjustments. A great triumph whose fearless vocalism and imaginative conducting even redeems Alfano’s blatant ending.

The complete set’s booklet has full performance tracking details and synopses but not nearly enough on the actual performances. The release makes for a courageous complement and alternative to (but not a substitute for) this year’s earlier release of the soprano’s commercial studio recordings.