Banzai's KM84 DIY Body & PCB kit build thread

[Potato Cakes]

I have no idea. I had permanently installed the trimmers so after I saw symmetrical clipping I reassembled the body and capsule and called it good. It's too much hassle for me to do the temporary trimmer install then try to find the closest resistor to what makes the circuit biased. It's mostly because I'm lazy, but for this particular build I didn't want to solder anything more than once.

If you're getting 6dB down you may not be getting a good connection to either pin 2 or 3 inside the mic at between the transformer output and the XLR. Or it could be because it is not biased.

For self resonance, I would send some pink noise at the transformer input and see if it's coming from that. I'm not familiar with the 3U GZT-84, but I don't know where else it could be coming from unless there was something off with the capsule.

Thanks!

Paul

 

[TillM]

I clipped my signal generator to the mic. Positive to the spring loaded contact touching capsule(without capsule). Negative to ground/chassis/pin 1 of the mic. I connected an XLR "y"(f/m/m) to the mic. I connected one side of the "y" to a pre-amp providing +48V. I then connected my oscilloscope(positive to pin 2 and ground/chassis to pin 1) to the other side of the "y". I understand that there is DC voltage at this point, but it did not SEEM to affect my biasing process. I can say that I fried two 2N3819s by sending way too much voltage at them. Shouldn't need more than 1V @ 1Khz, maximum, to determine bias at max input. Not sure when they "give way" but 10V  is definitely enough. Was for me, twice. Check for continuity/resistance at your primary and secondary of your output trafo before starting. I received some faulty ones(2 at once) from a particular company during this endeavor. That combined with my fried transistors made for some real GDIY troubleshooting.

Thanks ! That makes it much clearer !

 

[Moby]

It is completely pointless. Doesn't add any cost to use different colours, and save everyone the hassle. Same issue on all their wired transformers.

I'll talk to Moby and see if he can do a run of BV107's for us

Hey Banzai, thx for contacting me. I wrote you some message by PM but no response. Can you lease drop me e-mail regarding this?

 

[Banzai]

Hey Banzai, thx for contacting me. I wrote you some message by PM but no response. Can you lease drop me e-mail regarding this?

Hey Moby, thanks for your pm. Will reply asap!

 

[jrmintz]

Does anybody know the thread size of the little set screws that hold the xlr into the mic body?

 

[craigmorris74]

Hey folks,

My buddy who isn't a forum member got in on the group buy and bought a couple of kits.  He successfully biased one of his mics and was working on biasing the second one when it stopped working.  I'm guessing he shorted something out with his screwdriver while biasing.  He says his JFET has continuity between gate and source and gate and drain, so I guess it's shot.  Could he have fried anything else?  This is the second project he's ever worked on, so he's a bit of a DIY newbie.  Any help would be appreciated.

Thanks,
Craig

 

[Potato Cakes]

To verify if anything else was damaged he would have to meter all of the resistors and capacitors whilst following the schematic. He also needs to check the transformer. There's no way of knowing what he did, so after he changes out the FET and it still doesn't work he'll have to check all the other components.

Thanks!

Paul

 

[craigmorris74]

Thanks for the tips, Paul!

 

[Banzai]

Does anybody know the thread size of the little set screws that hold the xlr into the mic body?

Three screws holding the body sleeve should be M2.

Will check tomorrow.

 

[Potato Cakes]

I just finished building numbers 5 and 6 with the AMI transformers and I also used 10pF caps instead of 4pF just I wanted to pad it a little and I also didn't have any 4pF caps. When I went to bias them I noticed the bottom of the wave form wouldn't square off like the top but was more slanted. I don't remember the Cinemag version I built behaving like this, but still was able to finish the procedure. My scope is also an old Tektronix, so maybe it's just tired.

On initial testing I had one that had mostly constant static noise which was not remedied by swapping capsules. I reflowed a number of solder joints and I feel that the culprit was the lead from the capsule to it's isolated pin as it move a little closer to the pin when I reheated the connection indicating a possible half cold solder joint. Now it's perfectly fine like and ready to be put to use. Once again, as with a vast majority of the problems when building anything here, it's either a bad solder connection or component placement.

For the bias trimmer, I find it less accessible with the adjustment screw facing towards the connector then to the side like the first version (green PCB) and I'm sure there was a reason when the layout was updated. I wrapped my screw driver in electrical tape up to the blade to avoid shorting and destroying anything as it slipped several times when turning it. I guess I need to get one of those fancy types with the shroud covering the blade. Still don't know why trimmers don't use Phillips screw heads.

A little note on the Cinemag vs. AMI side debate that seems to have taken place during this process. I lent my other DIY KM84s with Cinemags to a fellow recording friend to track drums and he could not have been happier with how they sounded on overheads and hi hat. The drummer also liked them. Maybe the AMI version would not have been as preferred with extra top end on cymbals. Unless there is a side by side comparison I don't think anyone's going to know. It seems that with either transformer you are going to end up with a great sounding microphone as also reported by others who have built them with both of these transformers. It just comes down to whether or not you want to spend $75 and wait a couple of weeks (Cinemags are usually made to order) or $120.

Many thanks to Banzai and Graeme for making this available to the rest of use. These fantastic sounding mics that I hope to put to frequent use once I put my soldering iron down.

Thanks!

Paul

 

[Nele]

Just a small heads-up to say I'm organizing a group-buy for cinemag transformers:

European Cinemag GB

 

[jrmintz]

Does anybody know the size of the screws that hold the white thing onto the board? I didn't seem to get any and I want to pick some up in the morning. Thanks.

 

[Banzai]

Does anybody know the thread size of the little set screws that hold the xlr into the mic body?

Sorry for the slow reply, but just to expand on this for everyone:

These tiny set screws are a menace – the heads tend to strip at the slightest bit of force. I would advise to replace with M2 x 3mm stainless steel slotted set screws. These will have to be fully removed for the body to come off, but they're also much stronger.

Another option is to use a single normal M2 screw to hold the body together whilst testing and modding. Then when the mic is ready and won't need to be opened and closed a lot, use the set screws to finish the assembly.

Also found it best when tightening the XLR connector screw into the XLR shell, to remove these set screws first. I had one or two where as soon as I removed the set screws, a previously tight XLR ended up being very loose (that was obviously happening everytime the set screws were extended into the body as well).

 

[pH]

Enjoyed the easy build and really liking the sound of this microphone.
However I'm noticing that it's picking up some RFI or computer hash noise in my small recording area, none of my other microphones have this problem.
When people say that the KM 84 is a bit noisy is this what they mean?

 

[Potato Cakes]

All of the ones I've built from the first batch and this one have been dead quiet, and I have EMI issues with certain things in my space.

Thanks!

Paul

 

[jplebre]

Hey guys, any tips to install/secure the GZT84 trafos?
I'm afraid of pushing it in, but seems that it won't slot as the cables make the wrapper a bit wider than the rest of the trafo's body

 

[craigmorris74]

Hey guys, any tips to install/secure the GZT84 trafos?
I'm afraid of pushing it in, but seems that it won't slot as the cables make the wrapper a bit wider than the rest of the trafo's body

I had to file away a bit of the PCB to get mine to fit.  Be careful not to cut any traces if you do that.  I added a couple of drops of glue to hold them in place as well.

 

[GraemeWoller]

Enjoyed the easy build and really liking the sound of this microphone.
However I'm noticing that it's picking up some RFI or computer hash noise in my small recording area, none of my other microphones have this problem.
When people say that the KM 84 is a bit noisy is this what they mean?

They're fun little buggers to build alright, I'm pleased you're digging it!

Did you make sure you'd cleaned the flux up well? It's easily forgotten about, but it makes a hell of a difference to stray hums, buzzes, and general shenanigans.

Is the RFI definitely on that mic and being recorded, or is it just on the outputs to your monitors? My Apogee Duet 2 can be a bit of a pain with digital hash from time to time. Not saying that's your issue, but worth asking.

 

[GraemeWoller]

Hey guys, any tips to install/secure the GZT84 trafos?
I'm afraid of pushing it in, but seems that it won't slot as the cables make the wrapper a bit wider than the rest of the trafo's body

It can be the same with the Cinemags too, the wrap dimensions vary a little, or so I've noticed. As Craig said, give it a little file on the sides of the cutout, just enough to get the trafo in but leaving a little friction to keep in place. A dab of hot glue will keep it from moving if you need a little more reassurance.

 

[Potato Cakes]

+1 for hot glue.