It may be a good idea to define "M7 capsule", considering that more than one manufacturer currently uses the term which originated as a Neumann product almost 100 years ago.
The original M7 capsule design was first developed in the early 1930s by Georg Neumann with the help of Erich Kühnast (his son later took the helm of Neumann/Gefell/Microtech Gefell, now retired), though the term was not used at the time. It was a dual-diaphragm, single backplate construction capable of three patterns (cardioid, omni, figure-eight) when correctly polarized. It had cast PVC diaphragms and a characteristic hole pattern to the backplate which to a large degree is still present in the polyester-skinned K47 which succeeded the M7 in the late 1950s and which is still being produced by Neumann today.
After Neumann/Berlin switched to Mylar K47, Neumann/Gefell, later renamed to Microtech Gefell, continued producing its version of the PVC M7, to install it in all of its large Diaphragm mics. Around 2000, the German government clamped down on environmental toxins, including the PVC solvent Microtech Gefell used to make these capsules, forcing MG to reformulate the PVC mix.
MG also added a PE- (polyester) skinned version of the M7 to its lineup, using it in some of its LD mics.
AFAIK, only the flagship model, 92.1S, still uses the PVC version M7.
Gefell's PVC M7 made prior to approximately the year 2000 were every bit as sexy and emotionally attractive-sounding as the original Neumann/Berlin version, yet with a slightly different timbre, which changed (at least to my ears) dramatically when MG was forced to reformulate the PVC mix and their PVC capsules never sounded the same again. If you are ever in doubt which version you have before you: the Gefell version used and still use an M1.2 center lead-out screw, while Neumann always has used M1.4.
PVC M7 diaphragms were recreated by Siegfried Thiersch in Möschitz, East Germany, in the 1990s (Thiersch had resigned from MG, where he was in charged of nickel diaphragms). He first offered reskinning of original Berlin and Gefell M7 capsules, later he also started offering complete M7 capsules with PVC ("Blue Line" and PE ("Red Line") skins. STM (Thiersch's monogram) no longer offers the PVC version. As Thiersch also could no longer use the original toxic PVC material after 2000, its capsules too have the mid-forward emphasis of the post-2000 MG M7 capsules.)
All current copy versions of the "M7" (which Neumann neglected to trademark) currently manufactured use polyester/Mylar® material for its membranes.
Aside of one manufacturer who will remain nameless, to my knowledge, the original M7 PVC backplate dimensions and acoustic network were never modified by any of the copy companies to accommodate a polyester skin of roughly half the thickness of the originally 10µ PVC membranes.
In sum, there are many versions of "M7" in circulation today, and to my ears they all sound different from each other, with none sounding identical to the original M7 PVC Berlin or Gefell version.
Please add any corrections or other relevant M7 facts.
