Banzai's KM84 DIY Body & PCB kit build thread

[corgan4321]

I just measured the two remaining Fairchild 3N3919s that aren't soldered into my mic pcbs. They both at VGS off below -3V but they both had IDss of 9.4ma and 9.5ma. Would someone be willing to explain how that would affect the performance of the circuit given that the drain resistor is balancing this?

 

[Matador]

The gm of a JFET is maximized when operated near it's IDSS. IDSS and VGSoff are closely tied together, and vary widely between JFET to JFET even of the same type.

http://www.musicalsparks.com/images/curve_tracer/jfet_bias.php
Compare a JFET with IDSS of 2mA, and VGSoff of -0.5V:

RD = 46.666666666667 kΩ
VGSQ = -0.36921655743057 V
RS = 1.6409624774692 kΩ
gm = 2414.9534156998 µmho
AV (gain) = 112.69782606599 (41.038310772271 dB)

With another extreme of 10mA IDSS and VGSoff of -3V:

RD = 46.666666666667 kΩ
VGSQ = -2.8333333333333 V
RS = 12.592592592593 kΩ
gm = 900 µmho
AV (gain) = 42 (32.464985807958 dB)

The higher the IDSS, the more negative the VGSoff, which means higher source resistance, lower gain, yet more input headroom on the gate before clipping.

In other words, use high IDSS if you want more signal swing before distortion (e.g. more signal from the capsule), however less gain (you see 9dB gain drop between the two extremes). If you are close miking loud sources frequently, then use high IDSS. If you are distance miking quieter sources then use low IDSS.

 

[craigmorris74]

I made a post about buying a Warm Audio WA-84 omni capsule:
Warm Omni
I was pursuaded not to take it apart, but I cut the pin off flush so I would worm with my Banzai build. I'm attaching some acoustic recorded with the capsule included in the kit and the Warm.

 

[doqmemory]

In other news, I found a video demonstrating nickel plating in a very clear way, so I'm going to nickel plate the bodies myself. [Nickel Plating Guide]

I also ordered some Ferric chloride to etch some copper badges. I haven't finished the badge design yet, but I'm planning to use a purple enamel/nickel color scheme like the original. Here's the guide to acid etching I'm using, if anyone is interested [Acid Etching Guide]

Does anyone know the thickness of the original badges or a color code for the original badge color?

If I survive these experiments I'll post my results!

Awesome! I ordered some nickel bars and made the solution myself in an old pickle jar, following the guide. I have plated one of my pair of bodies, for comparison.

 

[Mworthin]

Awesome! I ordered some nickel bars and made the solution myself in an old pickle jar, following the guide. I have plated one of my pair of bodies, for comparison.View attachment 84199

Looks amazing! How does one dispose of the plating solution when you are done? Is it considered a toxic waste?

 

[doqmemory]

Looks amazing! How does one dispose of the plating solution when you are done? Is it considered a toxic waste?

The nickel acetate is poisonous, so it certainly should be disposed of just as you would any other toxic waste. I'll be keeping mine for reuse, marked "*** DANGER - TOXIC ***" or something like that.

 

[McIrish]

I've been looking around for platers in the Chicago area to do the nickel plating. Yours looks great! I had watched those videos on DIY plating. Looks like a fun project but I'm super slammed with studio work and can't take on another project for a while. I do like how they look as just bare brass, but I figure I would need to lacquer them to keep them from tarnishing. Also, I have to deal with removing the screen without completely wrecking them before I can plate or spray the bodies.

 

[stokesmusic]

Awesome! I ordered some nickel bars and made the solution myself in an old pickle jar, following the guide. I have plated one of my pair of bodies, for comparison.View attachment 84199

It looks so good! Congrats Doqmemory!

The mason jar I made my first batch of nickel acetate in broke while I was heating it in a water bath, so I had to make a second batch. Hoping to plate mine this week

@Mworthin Since I spoiled my first batch, I did some research on disposal. I'm not a chemist, but I read that you can safely evaporate the nickel acetate, removing the nickel from solution. What you have left over is just nickel, and can be safely thrown away.

 

[corgan4321]

I made a post about buying a Warm Audio WA-84 omni capsule:
Warm Omni
I was pursuaded not to take it apart, but I cut the pin off flush so I would worm with my Banzai build. I'm attaching some acoustic recorded with the capsule included in the kit and the Warm.

Hmm, these sound like different takes so it's a little hard to tell. I'm hearing way more bass with the banzai capsule which doesn't really follow the physics of the polar pattern, unless you were pretty close up to the guitar and we are just hearing proximity effect? The high end is

 

[doqmemory]

Also, I have to deal with removing the screen without completely wrecking them before I can plate or spray the bodies

I just plated the screens, too, since it's a very small amount of nickel added. It made for a fun weekend.

 

[doqmemory]

I'm not a chemist, but I read that you can safely evaporate the nickel acetate, removing the nickel from solution. What you have left over is just nickel, and can be safely thrown away.

Thanks, that's good to know.

 

[craigmorris74]

Hmm, these sound like different takes so it's a little hard to tell. I'm hearing way more bass with the banzai capsule which doesn't really follow the physics of the polar pattern, unless you were pretty close up to the guitar and we are just hearing proximity effect? The high end is

Yes, they're different takes because the capsules are on the same mix body. Distance is exactly the same.

I'm hearing the Banzai capsule as being the source of extra bass. I built it with the larger C3 and C4 values that were recommended.

 

[Mworthin]

It looks so good! Congrats Doqmemory!

The mason jar I made my first batch of nickel acetate in broke while I was heating it in a water bath, so I had to make a second batch. Hoping to plate mine this week

@Mworthin Since I spoiled my first batch, I did some research on disposal. I'm not a chemist, but I read that you can safely evaporate the nickel acetate, removing the nickel from solution. What you have left over is just nickel, and can be safely thrown away.

Thanks. I am looking forward to trying this.

 

[corgan4321]

I can post some comparison recordings shortly of my KM86s vs these clones with a few different variations on C3 & C4 values.
I landed at C3=2uF and C4=4.7uF. I feel this is giving me plenty of bass but is controlling it and getting it closer to the response of my Neumanns.
However, the high end is not right on these capsules. There is a large peak around 10k, not at all smooth sounding with brighter instruments.
I wonder about increasing the body vent holes by removing the radial vent parts that go around the mic. You’d be left with 4 posts essentially that connect the top of the mic where the threads are and the main body of the mic.
I also wonder about swapping the thick plastic(?) grate in front of the capsule. Is it safe to unscrew the retaining ring in the back of the capsule to take it apart to get at this grate for replacement with something different?

 

[mrgrooves666]

Sounds like the usual symptoms I've read about with these capsules. I found this article from a guy who tried this mod you're thinking about, and got good results with taming the HF response. What tool would you use for this, dremmel? I'm interested too, altough I'm still yet to build mine..
http://www.audioimprov.com/AudioImprov/Mics/Entries/2012/12/5_Modifying_the_MXL_991.html

 

[kato]

I also wonder about swapping the thick plastic(?) grate in front of the capsule. Is it safe to unscrew the retaining ring in the back of the capsule to take it apart to get at this grate for replacement with something different?

Perhaps it would help to mill out the backplate with the same crosshatch pattern of tunnels like the original.

Supposedly this allows better off-axis frequency response.



Picture from here:
http://recordinghacks.com/microphones/Neumann/KM-84

 

[corgan4321]

Sounds like the usual symptoms I've read about with these capsules. I found this article from a guy who tried this mod you're thinking about, and got good results with taming the HF response. What tool would you use for this, dremmel? I'm interested too, altough I'm still yet to build mine..
http://www.audioimprov.com/AudioImprov/Mics/Entries/2012/12/5_Modifying_the_MXL_991.html

I think I’ll do one with a dremel. Brass is quite soft and easy to machine so it should not a problem. Is it has decent results I would bring my remaining bodies to my machinist to chuck up on his metal lathe.

 

[corgan4321]

Perhaps it would help to mill out the backplate with the same crosshatch pattern of tunnels like the original.

Supposedly this allows better off-axis frequency response.

View attachment 84289

Picture from here:
http://recordinghacks.com/microphones/Neumann/KM-84

Yes, my understanding is that this is a crucial element to the function and design of a KK84… do these capsules not have this?
Any thoughts on removing the capsule parts from the capsule body? Will I be able to put it back together?

 

[craigmorris74]

Yes, my understanding is that this is a crucial element to the function and design of a KK84… do these capsules not have this?
Any thoughts on removing the capsule parts from the capsule body? Will I be able to put it back together?

I'm currently trying to make a CAD drawing of the KK84 backplate. I want someone to make or machine one for me. I don't do CAD though, so I'm going to get a friend's help to make it happen.

Also, it looks like there is a honeycomb like piece of brass in front of the diaphragm. In Kingkorg's thread about the KK84 capsule guts, I don't see this piece.
KK84 Capsule Guts

 

[corgan4321]

I'm currently trying to make a CAD drawing of the KK84 backplate. I want someone to make or machine one for me. I don't do CAD though, so I'm going to get a friend's help to make it happen.

Also, it looks like there is a honeycomb like piece of brass in front of the diaphragm. In Kingkorg's thread about the KK84 capsule guts, I don't see this piece.
KK84 Capsule Guts

I'm happy to make a CAD drawing, I work in Rhino. I could produce 2D and 3D drawings.