The Wunder Audio CM7 Microphone

What makes this mic sound so good?

The original M7 capsule "the Berlin M7" is actually very different from the Gefell M7, which is machined differently, especially at the isolator rims. The Berlin M7 was only used in U47, M49 mics, and the CMV3a, the so-called Hitler mic.

We purchased 150 U47's over a six-year period as a vintage mic dealer. We became heavily into collecting them for recordings and held back 8 or 9 of the best sounding examples. Being very familiar with the changes over the years, we noticed that the M7 capsule was better than the K47 capsule.

The M7 capsule is a piece of art in itself. It is the only 1" large diaphragm capsule ever made with a diaphragm that is glued on with precise tension, similar to the way a snare head is stretched on a rim. The contact point is where the diaphragm is only touching a very thin rim. This rim is precision milled into a flat piece of brass, which contains 90 holes per side, located with exact precision, and then super polished "lapped" for smoothness.

The diaphragm is made of a Mylar substrate that has the original thickness of 6 microns and placed in a vacuum chamber where a 250 angstroms (1/40th micron) layer of gold is evaporated onto its surface in a 1" circular pattern. The gold becomes a conductive surface on the Mylar on which a voltage can be applied, creating one of the plates of a capacitor. On our M7 capsule, these diaphragms are stretched to a specified tension with a 9oz. tensioning weight and glued to a ring by the original Neumann capsule expert in Germany.

All other capsules use a flat washer shaped spacer to determine the space between the diaphragm and the backplane thus saving machining cost. Finally a bezel is screwed down over the whole assembly to hold it in place. This is the brass or plastic ring with all the screws we all see on all other large diaphragm capsules. This O-ring spacer is very wide compared to the thin rim of the M7, which ultimately dampens the sound.

M7 is much harder to manufacture because of these reasons but it is worth it. We actually offer different biases of the M7, one that's higher end biased and one that's more low end. 17/10000 of an inch is the normal distance from the diaphragm to the back plate. If it's 15/10000 there's a variance, as there is at 19/10000. Depth of the back plate and the tension of the skin all play a huge role in the sound of the capsule.

It is the capsule, transformer and tube, in that order that is responsible for the sound of the U47. The most unique thing we're doing is a remake of the very earliest U47 transformer that came out between 1949 and 1952. During these first three years of of production the mic had the larger transformer with 25 percent wider windings. This, plus the earlier Mu-Metal made the transformer saturate nicer than the later smaller U47 transformer. When you get close to the mic, the higher vocal level plus proximity effect causes saturation to take place. One really needs to sing close to the grille because the way it handles pressure-gradient proximity effect is the key to the U47.

The Grille of the U47 is very unique in the way it allows a 1.5 db boost at 5k to take place with astonishing presence and then cuts 8K about 1dB to take the sibilance away. Then there is a 1 dB bump at 10 k, then rolling off at 11k. This natural EQ is unbeatable. This unique grille has a fine gauge mesh wire sandwiched between an inner and outer heavy gauge grille wire. We use the identical grille setup as the original U47. If the grille were changed to something simpler the frequency response would be much flatter and polite sounding taking away its cutting edge. The exact shape of the grille is also critical to achieve the proper resonances. The grille and the hand built M7 is the beauty of the design.

The transformer is specially manufactured in the US for us by an ex-Telefunken Germany engineer. These are wound the old way on vintage machines. Large transformer U47s are sought after sonically and achieve a little more saturation and low end. German IRT broadcasters asked George Neumann in the early 1950's to make the U47 with less bottom end for Radio Broadcast specifications, so Neumann changed the transformer. Subsequently they made the body shorter by 7/10 inch.

The Tube is an EF14. We can also use a VF14 at a higher price point but it is not worth it. Basically, the VF14 and EF14 are tooled identically inside. Telefunken made some of the Stahlroehren (Steel Valves) before WWII; these are the less desirable versions. Telefunken moved the tube manufacturing to West Berlin. There is a difference between prewar (and later east German) and postwar tubes. The prewar tubes have a bulky ring around the Bakelite base; the postwar tubes are more streamlined.

The VF14 is a dinosaur, it's extinct. The EF14 is the savior of the mic. The only difference between the VF and EF is the filament voltage requirement, but they sound the same if set up correctly for a U47. These tubes have the same interior tooling; the only difference is how the filament wire is attached.
The VF14 is under heated. If you use an EF14 and run a separate filament at 4.95 volts, the EF14 acts identical to the VF14. We add a capacitor and resistor on the back of the tube socket called a "dummy load" to make the EF14 behave like a VF14. Without that, the EF14 will not perform properly.

We don't use PVC, which was used extensively in the U47 in the diaphragm; the point-to-point hookup wires within the mic and more critically, the insulating wires that come out of the transformer were sheathed in PVC. That's the "Achilles Heel" of the transformer. As these wires deteriorate, there can be a loss of high end or low end. The biggest culprit was the PVC, which is constantly deterioration and cracking. Every year with the change of seasons the PVC will expand and then shrink and bubble up and crack. A quick test for a capsule is to make a breath like you're fogging up a window, and if you hear a rush of white noise, that means the capsule's shot. PVC, Poly Vinyl Chloride was invented in 1912. It hardens and has a relatively short shelf life. Neumann knew this as early as the 1950s, that's why he eventually changed the skin of the capsule in the U47 from PVC to Mylar. As the PVC hardens, it becomes less pliable.

We also use very smooth high-end Metalized Polypropylene capacitors that weren't available back then as filtering and corner frequency caps i.e. low frequency cap. We doubled the value of the corner frequency cap that enables more low end to come through. The caps Neumann used had a tendency to dry or crack over the years. They also used a ceramic capacitor that would crumble in the old U47's.

The power supply has nothing to do with the circuit at all; it should only supply the tube with the proper voltage. The problem with the old Neumann NG supply is that there was no voltage regulator, so it was unstable. The voltage for the tube should be exactly 105 VDC. I have discovered old PUS's yielding as low as 50 VDC and as high as 160 VDC, which can irreparably damage the tube. Also the 50-year-old cable from the mic to the PSU should be discarded, as it will be crumbling due to the PVC insulation.

back to CM7