Banzai's KM84 DIY Body & PCB kit build thread

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I've a quick question about biasing these mics - specifically, while 10V drain voltage is noted on the Neumann schematic and referenced several times throughout this thread, is it always the right point? I'm asking as my testing suggests looking at a slightly higher voltage, and I'd be grateful for a sense check.

Test conditions
  • I've injected a 1kHz sine at around 200mV direct to the sprung contact with the capsule removed.
  • I'm monitoring drain voltage at the drain/R4 junction using the multimeter; while observing the both the waveform shape on my Picoscope, and the harmonic distortion profile using Voxengo's SPAN plugin.
  • JFET is a Motorola 2N3819, IDSS is 5.4mA and Vgs(off) is -1.81V.
Observations
  • I've taken measurements at 10V drain (R3 = 5.3k) - however, increasing source resistance to 6.7k (12.2V drain) led to a dramatic decrease in 3rd harmonic distortion, by almost 40dB. 2nd harmonic increased by about 2dB, and the 1kHz tone dropped by about 1dB. 4th, 6th and 9th also dropped (though 7th and 8th increased).
  • When increasing the test tone amplitude, I was similarly able to dial in a significant reduction in 3rd harmonic by further increasing the source resistance.
  • I've not been able to get a 100% symmetrical waveform at any value on the trimmer, or symmetrical clipping.
  • The same pattern is apparent across all the KM84 kits I'm building.
I attach screenshots of the distortion profile and scoped waveform at 10V and 12V drain voltage. In light of the above, should I be looking at a bias point that gives lowest 3rd harmonic (40dB reduction seems worthwhile on the face of it!), or just go for 10V and leave it at that? Are there trade-offs with headroom if I do go above 10V?

Thanks for any help and insight!
Andy


10V drain:
KM84 clone - 10V drain voltage.png

12.2V drain:
KM84 clone - 12V drain voltage (lowest 3H).png
 
TwentyTrees said:
I attach screenshots of the distortion profile and scoped waveform at 10V and 12V drain voltage. In light of the above, should I be looking at a bias point that gives lowest 3rd harmonic (40dB reduction seems worthwhile on the face of it!), or just go for 10V and leave it at that? Are there trade-offs with headroom if I do go above 10V?
Click to expand...
I've seen people who prefer to bias it either way, with the most symmetrical clipping or with the lowest 3rd order harmonic.

I am sure they are both fine. An old school mic tech friend of mine biases til the top and bottom look perfectly symmetrical, and then backs the gain off a bit and them tweaks it so the bottom clips a TINY TINY bit first.

So that's what I do.
 
Great, thank you - that's really helpful. I'm not planning to record jet engines or Motorhead cover bands with these, so biasing for lowest 3rd harmonic makes sense to me.

Much appreciated.
 
Doesn't this circuit have inductors on the outputs? I'm not seeing them in the BOM, and no results with searching this thread.
 
Unless I'm missing some turn in the thread, the Bansai KM84 has a transformer in the output. I didn't like the capsules it came with. I ordered some 3U capsules. I'm looking forward to them showing up in the next few weeks.
 
Got the 3U capsules in yesterday. I'm looking forward to doing some shootouts with these again. I didn't like the Maiku capsules. They had a very odd low end off-axis. I'm hoping the 3U capsules make these mics usable. I spent way too much on all the upgrades.
 
McIrish said:
I didn't like the Maiku capsules.
Click to expand...

Just to make it clear for everyone that doesn’t know this, those are not “Maiku” capsules, Maiku didn’t manufactured or produced them.

Those are just very cheap generic OEM capsules imported from China.
There was no quality control made by Banzai or Maiku and that’s pretty well documented by the inconsistencies between capsules reported here.

Also Maiku is no-one on the microphone Capsule bussiness it’s just an importer of cheap capsules from China. It can be just a guy with a website that lives in a room in his mothers house.

3U Audio is a real company and manufacturer and produces very good quality capsules.
I wish you enjoy and do a lot of good recordings with your 3U Audio capsules.
 
Last edited:
How did you finish your brass bodies? I've been thinking about what to do. I'm a bit nervous about trying to remove the inner screen without mangling it
 
I was a bit nervous as well, but in the end it wasn't too bad. I soaked them in acetone (well, nail polish remover) for a while, that dissolved the glue and the screens then popped straight out. I reattached with conductive glue once the bodies were finished, using a toothpick.

The bodies themselves are cerakoted - it's a tough ceramic finish, often used on bikes and firearms as far as I can tell! I'm really happy with how it turned out.
 
Chrisfromthepast said:
@TwentyTrees How close were you able to match the preamp output levels and distortion profiles?
Making mine match has been more difficult than I first expected.
Click to expand...
I built three pairs, matching all components as closely as possible and then testing all six bodies with all six 3U capsules(!). Preamp output levels and distortion profiles were almost identical, the variance came chiefly with the capsules. In the end, my "A" pair is <0.3dB different across the whole frequency spectrum, and <0.1dB between 300Hz and 3k; the "B" pair is about 0.7dB different in level but almost identical in frequency response; and the "C" pair is just under 1dB across most of the spectrum but diverges below 375Hz.

It's worth noting that all six mics were closer than either of my Oktava MK012 or Lewitt LCT340 "matched" pairs. I think having a few capsules to pick from was the key for me to get these results.

Of course, will it matter in the field to have them matched so close? Probably not, but at least this way I can only blame myself and not the equipment!
 
Last edited:
TwentyTrees said:
Of cours, will it matter in the field to have them matched so close?
Click to expand...
Omg, fair enough. Last time I complained to my ENT about my ears’ asymmetry, he pretty much told me to consider myself lucky with “only” 20db down at 250hz on the right side.

I pretty much gave up on the dream of them all having the same output level.
Frequency response matching I can’t do better than pairs. I built more km84 clones in different bodies, always with a very similar bom. The SMD fets have been more consistent but the swap is no fun. I kind of see why charge amps have so many detractors.
 
McIrish said:
I used the 3U capsules in a session two days ago. Huge improvement. If anyone has these kits and are unhappy with the sound, replace the capsules. They were unusable with the Maiku capsules. The 3U casules worked well. They still don't sound like a KM84 but at least they sound good.
http://www.3uaudio.com/cp.asp?id=700Get the mm5 version
Click to expand...
I have some test files with Graeme's KM84 clones (which I built with the 3U capsules) next to real KM84's and the recordings are indistinguishable from one another. I can't speak for the Banzai clones however.
 
When I get a chance, I will do some comparisons. We were in a hurry and just wanted to have a listen to the new capsules.
 

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