Mini Shotgun Mic

Today I repeat the test usign a the same circuit preamp and just switching capsules, the modded and the original.

I use this circuit:

At 3V. Curious thing, the modded capsule is less sensitive than the original one, also it picked alot of line noise (50hz), might be a shaving acting as an antenna or something.

The capsules were tested at 10cm of a speaker, at 0° (on axis) and at 90° (off axis) with a 500hz tone.
I also compared an Audiotechnica AT897 (Condenser hipercardioid) and a Shure 8800 (dinamic, cardioid) at 30cm of the speaker.

(Warning: ugly circuites ahead)

The results were measured usign Audacity, by duplicating the track and usign the tool "Contrast" on the Analize menu. This tool allows to compare two selections on different tracks and calculate the RMS.

In this test no filtering to isolate the 500hz tone were applied.

Now the Audiotechnica AT897

Shows -10.3dB off Axis

Shure 8800

Showed -1.4dB off axis

I'd like other people trying this mod and testing it, so we can compare results.

Also i will try to find bigger capsules to see how they behave, mostly because i think such a small capsule would't be apropiate for a shotgun mic.

Or maybe build my own capsule

Well, i wanted to try if i was right. So i drill several holes in a new capsule, to form a pressure-gradient capsule, effectively transforming it from an Onmidirecctional capsule, to a Figure 8/cardioid capsule.

Surprisingly, i didn't damage neither the FET nor the membrane. I cleaned all the metal shavings with compressed air and I wire the capsule to test it with my zoom h6.

I used a 500hz tone coming from a single speaker. I could hear a clear difference in the level of the sound when I rotated the capsule.

Sadly the high noise-floor of this configuration made difficult to measure the level in the recording.

Audacity shows about -5db off axis.

This could be improved with a better preamp and the interference tube.

Besides the unresolved issue with the 48v phantom, i discover that the microphone had very low off-axis attenuation. 

I research the capsule, and apparently it's pressure operated, which means it has a constant internal pressure, and it's the sound wave what changes the outside pressure and moves the membrane.

This is grat for an omni-directional mic, but not for a shotgun.

Shotgun Mics use pressure-gradient capsules. This capsules have vents on both sides (front and rear) to reject sound comming from the sides.

There is an exelent video about it from the EEV blog here:

I dissasemble the capsule to see if can be modified, by making vent holes on the body. The extremly small side of it make it very hard, but not impossible.

The fix plate is drilled so it can be converted to a pressure gradient ( I probably will destroy a few with the drill before i success)

My first try at making the preamp was this schematic, a classic push-pull configuration. But i notice that only works well at 12V phantom. With 24 and 48v phantom the noise floor become noticeably higher.