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Old 21st November 2015
Gear Maniac

CM67se tube microphone No 12AX7!!!

Hi Ragan, not sure what you mean by OEM???

Just trying to keep you guys in the loop about what some of the professionals are using in major studios around the world LOL!!!

BTW, I am repairing a lovely old KM86i for a client.

The KM86i is a side address 3-pattern fet microphone.

It has what looks like two KM84 type capsule configured on a single mount.

The switch selects:-

OMNI, both capsules with their diaphragms pulled to ground.
Both backplates are connected to 48v

CARDIOD, capsule 1, connection
capsule 2 diaphragm is connected to +48v
Both backplates are connected to 48v

FIG 8, capsule 1, diaphragm connected to 48v and back-plate connected ground.
capsule 2 diaphragm connected to ground.
Back-plate 2 connected to 48v

The backplate of capsule 1 is connected into the fet via a 470pf capacitor.

The KM86 capsules measure about 33pf each.

In an LDC that measures about 80pf then you need a larger 1000pf.

From the FET on the circuit is identical to a KM84.

The distance between the diaphragms of the two capsules is 28mm
while the distance between diaphragms in an LDC capsule is roughly 6mm for a AK47 type and 9mm for an AK12 type.

It will be interesting to listen and plot the difference in response in Fig 8 with the diaphragms being 4-5 times larger.

In a ribbon microphone of which 90% are FIG 8 only the distance between the front and rear "diaphragm" is the 2-5 micron width of the ribbon material.

So, in theory the ribbon will be more phase "accurate" in FIG 8

Hence, my interest in hearing the KM86 in FIG 8.

I will post some pictures on Gear Slutz to see the inner workings of a classic microphone and if I can get my plotter software to load into the new laptop with Window 10. Then I will try and run some comparison curves.

I love working on microphone and analogue circuits but my head wants to explode somedays trying to load software and get a simple laptop to run.

In haste, back to the workbench.

Cheers, Dave