Banzai's KM84 DIY Body & PCB kit build thread

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Hi everyone,

I finally got a scope so I opened up my pair of mics, and attempted to bias them correctly.
Signal generator > fet input (adding a 1000pF cap make it super radiophonics), mic out > preamp with 48V > oscilloscope

One of the mic is louder than the other, and I can't seem to bias them so that clipping is symetrical. It always starts distorting on the bottom of the waveform, and the top stays pretty clean. I've adjusted them so they output approx the same, but they sound low in volume and a bit lifeless / dull...

Any pointer, or similar experience ?
 
The thing is back then I set them up around 18K like you mentioned because I was going on a remote recording and was in hurry, needed them a few days later for a month or so. Back then they sounded OK but slightly unmatched. They ended up as overhead on drums. The two first singles of that album have been released, aired and nobody complained about the approx biasing haha.
But I always thought I should bias them properly. Which I did try yesterday with my new scope :)
I started at 200mV with my signal generator, 1K sine, (tested with the 400hz sine too and also with triangles and squares, out of curiosity) then I tried with louder signals, no real change.
I have no idea what value I ended it up with, to me they are still not properly biased so I didn't even bother measuring :)
 
The bias is going to be completely dependent on the FET that you use. I tried a few different variants and had to use extremely different resistor settings to get them to bias correctly. Here are notes I wrote to myself when I did the build last year:

The Central FET seems to need a very different bias resistor, R3. Of my two mics, one required 18.5K and the other 21.5K to get the most gain and symmetrical clipping.

C4 value per bansai is 4.7uF but Neumann shows it as 1uF. I changed it to 1uF film. Had to but it in sideways to fit.

Changed the FET to a LS846. New value of R3 is 3.9k. What a huge difference the FET makes to the bias resistor.
 
Thanks for your help ! I am a bit surprised because I thought I had received a matched pair of FETs ... maybe I'll unsolder them and mesure them, which I didn't do back then.

What was the reason for changing the provided FET for a LS846 ?
 
I can't remember my exact reasoning but it was most likely lower noise. The Central FETs, though marked as a 2N3819, they sure didn't bias like one. Lately, it seems there are so many Chinese knock-offs of hard to get components but they don't have the same specs at all.
 
One of the 3 tiny screws that hold the XLR output in place is missing in my mic.
Just to make sure: does anyone know if these screws have a regular M2 thread?
 
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Originally did a rough bias when I made two, and they were working fine, but it seemed like they distorted a bit so I finally biased with a REW. Not sure WHAT happened, but just did a session with them as overheads and they were making really unsymmetrical waves on my scope, and distorting BAD on snare hits. I've read quite a bit on scoping on here. Did I miss a step? Advice?

Ryan
 

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If you can't get equal distortion on both sides of the wave form, then the trimmer you're using doesn't have enough resistance, or the FET needs too high a bias resistor.

When I biased mine, I wasn't happy with the resolution on my scope. i.e. it seemed like too much of a variance on the trimmer where the wave forms were symmetrically clipping. I ended up getting it close on the scope, and then biasing by ear like described somewhere in this thread. That may seem bass ackwards, but once you hear it happening, it's pretty easy to dial in. I'm sure other people's scopes are more accurate.

I also recommend the LS846. Had the same issue as everybody else in that the original transistors need to be biased with around 20K resistor. I didn't listen too close to the outcome, so maybe that's fine, but the LS846 ends up requiring 3-4K bias resistor.
 
JW said:
If you can't get equal distortion on both sides of the wave form, then the trimmer you're using doesn't have enough resistance, or the FET needs too high a bias resistor.

When I biased mine, I wasn't happy with the resolution on my scope. i.e. it seemed like too much of a variance on the trimmer where the wave forms were symmetrically clipping. I ended up getting it close on the scope, and then biasing by ear like described somewhere in this thread. That may seem bass ackwards, but once you hear it happening, it's pretty easy to dial in. I'm sure other people's scopes are more accurate.

I also recommend the LS846. Had the same issue as everybody else in that the original transistors need to be biased with around 20K resistor. I didn't listen too close to the outcome, so maybe that's fine, but the LS846 ends up requiring 3-4K bias resistor.
Click to expand...
I don't really know what happened. I got the wave form symmetrical on the REW scope but it clearly wasn't? I had plenty of resistance left going either direction on the trimmer. Going to try again later. Thanks for the tips.
 
oongbac said:
Ordered new 3u audio capsules for slotted body. After those are arrived I'll leave some comparison with Maiku one.
Click to expand...

That comparison would be great to see and listen, a solution to the cheap/crappy capsule supplied in this Kit is needed for sure, it doesn't provide a frequency response similar or close to the Neumann KM84 which was what was advertised by Banzai when he sold the kits.
 
Whoops said:
That comparison would be great to see and listen, a solution to the cheap/crappy capsule supplied in this Kit is needed for sure, it doesn't provide a frequency response similar or close to the Neumann KM84 which was what was advertised by Banzai when he sold the kits.
Click to expand...
That's a funny comment on a couple of levels--the first of which being that, as I auditioned the half-dozen well-maintained 84s to find the 2 I picked, I heard at least 3 distinct flavors--not all were good. Cutting to the chase, I own 2 pretty well-matched, middle serial number KM84s and, although I like my 84s a lot, I prefer my banzai kits (which have a mod or two). The only thing that I like less is that the banzai capsule's version of cardioid is a bit wider than my 84s. One thing I can say for sure is that the banzai kit that I made specifically for snare--with George Massenburg's mod--definitely beats the KM84s (3 so far) it's gone up against and I'm not the only one who thinks so.
 
Well, I don’t want to lit some controversy about this kit again. I just glad to find capsules that compatible with this mic. Now I have some other options to choose.
 
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maarvold said:
my banzai kits (which have a mod or two).
Click to expand...
maarvold said:
the banzai kit that I made specifically for snare--with George Massenburg's mod
Click to expand...

I have no idea what mods you performed.
But in stock form, and I also had access to 3x KM84 to compare (that I used in a lot of recordings over the years) these mics were not even close to the Neumann KM84. The voicing was completely off.
Compared to the KM84 they were bass heavy (and low mids) and with a lack of detail in the high end.
I didn’t find them useful at all and tossed them immediately.

The worst link in the chain is the cheap generic Chinese capsule that has no resemblance whatsoever with the KM84, being it the construction, the frequency response or the Polar pattern.

maarvold said:
I'm not the only one who thinks so.
Click to expand...

One thing is for sure, I’m not the only one that found that these kits don’t provide a similar or even close performance to the Neumann KM84, this has been talked by many people in this thread in the past, and there’s actually members that shared they regretted ever buying this.

Although I’m positive that the 3Uaudio capsule that oongbac is getting might provide a big improvement. I hope it could be a solution for members to be able to save their kits and actually build a proper microphone
 
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I REALLY want to know about the 3U capsules. The Maiku capsules sound nothing like a KM84. I had multiples. Also, of the two Maiku capsules I got, they are a few dB off from each other. A different capsule might help to save these mics. The PCB and components seem to be correct. But, it all comes down to the capsule. Which exact capsule is it that you bought? The site is a bit confusing for 3U.

oongbac -please report back. Do they need any modification to fit?​

 
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McIrish said:
I REALLY want to know about the 3U capsules. The Maiku capsules sound nothing like a KM84. I had multiples. Also, of the two Maiku capsules I got, they are a few dB off from each other. A different capsule might help to save these mics. The PCB and components seem to be correct. But, it all comes down to the capsule.

oongbac -please report back. Do they need any modification to fit?​

Click to expand...

It’s on the way, so I’m not sure about that, but I think(and hope) it will fit. I ordered capsules those have 0.5mm post from end of the capsule.

And… yes. I have same problem. I have no idea about sound of real KM84, but mines were not matched at level. ;(

I promise I’ll report back. It will be arrived during next week.
 

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