KM84 DIY Body & PCB kit – CLOSED

[Colorblind]

My kits arrived in Canada today.  They look great.  Thanks again Banzai!

 

[duantro]

Arrived in middle of USA. Thanks!!

 

[mcrowe]

Picked mine up today from a stern lady in my local Chicago post office. Everything looks great.

 

[mickdundee63]

Received in Boston! Many thanks

 

[salz-elektronik]

They sound really good! count me in for another pair!

 

[BladeSG]

Arrived in Australia. Thank you

 

[Mrosso]

Arrived Thursday in California. Thank you!

 

[Leon]

My pair arrived last week too.  Just built one today. Everything went fine and I was able to bias up to 1v with nice looking wave form. However, when I put on the capsule and gave it a test. Nothing but a constant hum. Any idea what went wrong? Thanks in advance.

Leon

 

[Banzai]

My pair arrived last week too.  Just built one today. Everything went fine and I was able to bias up to 1v with nice looking wave form. However, when I put on the capsule and gave it a test. Nothing but a constant hum. Any idea what went wrong? Thanks in advance.

Leon

Hi Leon, did you try the other capsule to rule it out as a problem?

 

[Leon]

Thanks for your help. Yes. Tried both with same problems.

 

[Leon]

Here is a pic of the finished board:

 

[Leon]

and the back side:

 

[Leon]

I did have to enlarge the hole slightly  in order to fit the transformer. Could that cut off  some hidden circuit traces?

 

[Banzai]

I did have to enlarge the hole slightly  in order to fit the transformer. Could that cut off  some hidden circuit traces?

Possible if you enlarged sideways. 2 traces run on each side of the transformer.

Traces are visible on the boards if you look closely. You can use your second one as a guide to see where traces connect, and check if everything is still okay with your first. Just test for continuity between pads above and below where you enlarged.

I'd test the entire board anyway, starting from the spring contact all the way down to the connector. Could be something simple.

 

[Leon]

Thanks. Both sides of the hole are tested to be  continuous. I will do a more thorough test and will update if I found the cause.

 

[Leon]

I tested point to point but didn’t find anything wrong. Also cleaned the whole board. I gave it another 1kHz signal, but this time I didn’t get a clear tone like when I did the bias earlier. I suspect the FET is broken and will try a new FET tomorrow.

 

[Fuzz Face]

I tested point to point but didn’t find anything wrong. Also cleaned the whole board. I gave it another 1kHz signal, but this time I didn’t get a clear tone like when I did the bias earlier. I suspect the FET is broken and will try a new FET tomorrow.

When I first connected the mic circuit (without capsule) to my DAW for biasing it was picking up all kinds of noise. I recorded a track of it with no gain applied and could wave it around my desk and change the hum dramatically

I gave everything another thorough cleaning (make sure not to touch Hi Z section with fingers) and used my DMM to set the trimmer to 3.9K in order to get the FET at least in the ballpark for biasing. When I connected again to my DAW, everything was much more manageable

Since this uses a 20K trimmer, if you leave it at the center you have about a 10K source resistor, which in my experience is high enough to make this thing go wild

Good luck, these sound fantastic when they’re done and the supplied FETs should be totally working

 

[Leon]

When I first connected the mic circuit (without capsule) to my DAW for biasing it was picking up all kinds of noise. I recorded a track of it with no gain applied and could wave it around my desk and change the hum dramatically

I gave everything another thorough cleaning (make sure not to touch Hi Z section with fingers) and used my DMM to set the trimmer to 3.9K in order to get the FET at least in the ballpark for biasing. When I connected again to my DAW, everything was much more manageable

Since this uses a 20K trimmer, if you leave it at the center you have about a 10K source resistor, which in my experience is high enough to make this thing go wild

Good luck, these sound fantastic when they’re done and the supplied FETs should be totally working

Thanks a lot for the help. I have a bunch of J113 so I installed one. I now can get least get a clear 1kHz tone. The hum is also quieter but still there. A steady 60HZ hum. The hum exists even with 0v signal, but doesn’t become louder as I turn up the signal.  I settled on a trimmer resistance of 5.1k and installed a fixed resistor there.

I also tried with the capsule on, still only hum but nothing else. No response to any sound. Guess I still have to dig hard.

 

[Fuzz Face]

Would you care to continue this discussion at the build thread? I have a couple ideas but the build thread seems to be a better place for this

 

[Leon]

Would you care to continue this discussion at the build thread? I have a couple ideas but the build thread seems to be a better place for this

Thanks. I was not aware of a build thread. But anyway,  I found the problem. I was not aware the mic body needs to be grounded. A rookie mistake but it was not mentioned in the build guide. There is a small screw on the “4th” leg of the XLR that can be screwed up to make contact with the body. Once that was done, the mic worked beautifully. No hum so I guess the 60Hz hum I had previously was from the signal generator. Anyway, just throw it out here so newbies like me will not make the same mistake as me.

Thanks all!