KM84 project build

[Leslie 123]

Hi,my first post here and condenser mic build.
Since i winding transformers i took challenge to wind one from 60s german mu metal iron. It was EI transformer lam which i cut at half to form U transformer pattern. After cutting it was bathed to form again Fe oxide to minimize mr. Eddy loss. Home made paper bobbin.
Fet side of problems, i tried 2n3819 and very similar sony 2sk126 but all them seems to fail as i was trying to reach 10V at drain, leaving failed drain to source which both then read about 3,6V  So i moved to another candidate which seems to do it well, BF245
With variety of BF245 and target  drain 10V, source reads from 1-2 Volts and that seems okay, no noticeable
sound difference. Stock values 470pf, feedback 4pf. If you have mod suggestions just shoot!
Sound clip here, i'm using new Oktava mk012 for comparison.With guitar both mikes sound more similar so i used mandolin to hear high spectra better. 
Dry, no eq, no compression.
https://www.youtube.com/watch?v=JGWR1dSl9BA

 

[Leslie 123]

Mic, donor MXL 606s
Primary 1400 Ohms, turns ratio 6.9:1

 

[kingkorg]

At risk of sounding like a broken record, there is no km84 without km84 backplate behind capsule membrane.

The key with this mic is even off axis (and 180°) response and flat on axis response. Which is achieved with unique backplate and front honneycomb grille.

Going so deep in other details is pointles, it's like sticking a Kia engine in a Ferrari. Your build being a Ferrari, you are lacking just the capsule to nail it! Great work!

http://cdn.recordinghacks.com/images//mic_extras/neumann/km84_capsule1.jpg

 

[craigmorris74]

Great work and cool build!

Could we hypothetically have KM 84 style backplates built to swap out in these capsules?

 

[kingkorg]

Absolutelly. Mxl603 backplates are quite simmilar, the grove pattern is an issue. The other plastic backplate on mxl is right, but should be built in metal. Røde makes those backplates in metal. Nt5 for example. But groves have wrong pattern on those as well. Røde has honneycomb pattern front grille, which is great. Not crazy expensive either.

https://www.sweetwater.com/store/detail/NT45card--rode-nt45-c-interchangeable-cardioid-capsule


And here they are side by side.

http://cdn.recordinghacks.com/images/mic_extras/mxl/MXL-603-capsule.jpg

Here's why it's important.
https://patents.google.com/patent/US3418436?oq=neumann+georg

 

[Leslie 123]

At risk of sounding like a broken record, there is no km84 without km84 backplate behind capsule membrane.

The key with this mic is even off axis (and 180°) response and flat on axis response. Which is achieved with unique backplate and front honneycomb grille.

Going so deep in other details is pointles, it's like sticking a Kia engine in a Ferrari. Your build being a Ferrari, you are lacking just the capsule to nail it! Great work!

http://cdn.recordinghacks.com/images//mic_extras/neumann/km84_capsule1.jpg

Hey KingKorg, so you basically say i need a better capsule and larger mic body vents. Any suggestions where i can find a nice capsule?

 

[kingkorg]

Not larger vents, just capsule. However i am not aware of any faithful replicas of that capsule. All of them have wrong type of backplate. Even the most expensive ones i won't name here 

According to measurements i have taken so far Røde nt5 cardioid capsule comes closest.

 

[Leslie 123]

Not larger vents, just capsule. However i am not aware of any faithful replicas of that capsule. All of them have wrong type of backplate. Even the most expensive ones i won't name here 

According to measurements i have taken so far Røde nt5 cardioid capsule comes closest.

Oh now i see,
just to make it clear By measurements you mean machining/design of the bacpklate or frequency graph/sound experience? Thank you !

 

[kingkorg]

Frequency measurements i took at my place. These of mxl type have annoying 8k deviations that just won't go away.  It's either peak at 8k or creates enormous notch dip if backplate is adjusted for slightly more hypercardioid response in attempt to even it out.

Due to lacking honeycomb plate and wrong type backplate off axis sound doesn't hit the diaphragm at right angles and you don't get finely difused/distributed soundwaves.  That part is explained in detail in patent files. 

However a great plus for both Neumann and MXL type capsules is that they have excellent 180° rejection, much better than most large condensers i've seen. So i use them regularly when rear rejection is priority regardless of their on/off axis responses.

 

[Leslie 123]

Frequency measurements i took at my place. These of mxl type have annoying 8k deviations that just won't go away.  It's either peak at 8k or creates enormous notch dip if backplate is adjusted for slightly more hypercardioid response in attempt to even it out.

Due to lacking honeycomb plate and wrong type backplate off axis sound doesn't hit the diaphragm at right angles and you don't get finely difused/distributed soundwaves.  That part is explained in detail in patent files. 

However a great plus for both Neumann and MXL type capsules is that they have excellent 180° rejection, much better than most large condensers i've seen. So i use them regularly when rear rejection is priority regardless of their on/off axis responses.

So neumann has backplate which is machined with slit grooves for improved linearity and mxl followed similar hole pattern but without slit. Ok understood. So what's solution with mxl, does the rode capsule fit mxl body or should i machine new slit style brass backplate ? Have you tried mic parts or 3U SDC capsules if they have more even frequency response 4-10 kHz?

 

[kingkorg]

Mic parts and 3U follow the same principle. 3U being closer to Røde.

I am not sure if Røde would fit MXL body.

The point is that without that backplate no matter what you do with the electronics you won't be getting what km84 is all about.

Km84 is all about that capsule which is described in the patent. And backplate is the key. Electronics part was designed just to be flatest possible at the time, and stay out of the way of the capsule.

If you find a way to machine that backplate, you can fit it into Røde without having to modify anything else. The good part is that capsule has simple construction, and can be assembled and disassembled easily.

 

[Leslie 123]

Mic parts and 3U follow the same principle. 3U being closer to Røde.

That's nice information, thanks!

I am not sure if Røde would fit MXL body.

Probably not, rode nt 5 is listed on mic parts site to not accept mxl or their upgrade capsule which i suppose is the same as 3U capsule.

If you find a way to machine that backplate, you can fit it into Røde without having to modify anything else. The good part is that capsule has simple construction, and can be assembled and disassembled easily.

maybe i could do something about it, 4 flute end mill cutter/bits make wonders. Do you have any info about membrane tension or its just screw it on with plastic washer thing ?
Here is better look at rode backplate, like you said it seems like easy disassembly

 

[kingkorg]

Tension is fixed, not related to the screw. You would need to re skin the diaphragm to change the tension.

The tensioning ring affects somewhat sensitivity, if you disassemble it try to remember the feeling of how much pressure you needed to apply to losen it. I know it's not exact science, but apply so much tension it stays in place, without overtightening it.

Some kind of acoustic measurement setup would be required.

You can measure capacitance of the capsule before you start modding it but if you modify the backplate that will change anyways.

 

[craigmorris74]

The Warm capsule looks close from the front and rear.  I don't know who makes it, and haven't seen one dissected.

Since the patent is expired, it's a shame no one makes a version.

 

[JW]

I'm curious to know if the 3U capsules will fit on anything besides the 3U bodies . . .
They seem to have a good reputation.

Yes, please, someone start manufacturing backplates! (Aputis?) kidding

 

[Delta Sigma]

What about the Beesneez Lulu capsule?

 

[kingkorg]

Nope, the same generic capsule. Doesn't measure flat either. The circuit is done great though, and the transformer is impressive for it's size.

 

[Delta Sigma]

Nope, the same generic capsule. Doesn't measure flat either. The circuit is done great though, and the transformer is impressive for it's size.

I have a pair (not on hand at the moment, so I couldn't peek at the capsule). They're great sounding mics though. I wonder if Ben would be interested in doing a different version of the capsule.

 

[rockinrob86]

I built four km84 clones in mxl bodies with 3u capsules.  I found the performance to be basically indistinguishable from my real km84.  The below video is my comparison -
https://youtu.be/nVJxx7YM-6I

 

[JW]

So, 3U capsules fit on MXL (603?) bodies? What transformer is that?