KM84 DIY Body & PCB kit – CLOSED

[TillM]

The Ohmite 1gig resistors are expensive, and the TE ones are backordered till June.
Do you think these are usable? I have a few lying around from canibalised chinese mics.

Do you try Digikey ?
https://www.digikey.de/product-detail/de/te-connectivity-passive-product/RGP0207CHK1G0/A106006CT-ND/3477617
https://www.digikey.de/product-detail/de/stackpole-electronics-inc/HVA12JA1G00/HVA12JA1G00CT-ND/6195878
https://www.digikey.de/product-detail/de/vishay-dale/RNX0381G00FNEE/RNX1.00GCCT-ND/2712940
and so on...
maybe this help.
But I also think, that you can use the Chinese ones.

After troubleshot my second mic, I found out, that I've a cold solder joint at the trimmer.
If a second run ever happen (which would definitely really, really nice) my only wish is to make the solder pads a bit thicker, this would be awesome.
I get the Cinemags last week, cause I already ordered them before the group buy.

 

[Banzai]

Nice build!!

Because of size constraints, seems best to stick to 1/4W 1GΩ resistors (0207 package)
Will give more space to work with.

Have updated Mouser BOM.

 

[fsalter]

How about the use of C0G’s instead of the Polypropylenes. Other than this recent debate about them having better dialectric properties, the lead spacing and their size would make them fit better?
Is 100v really necessary?

 

[Banzai]

Hey Fred,

No harm in trying C0G's, doesn't cost much to get both. The boards are also good quality – they'll survive desoldering if you want to play around with options.

I've otherwise never used them, can't tell you if it would be an improvement. Maybe Graeme?

 

[fsalter]

Thanks for the reply Mike. It's just that the Wima's prescribed are backordered. I found some subs, but i'll try the C0G's too.

 

[Potato Cakes]

I've got my kits in and I have a couple of questions as I gather parts:

The schematic says 2n3891 for the FET but the Mouser cart has 2n3819. Does the schematic have a typo?

There is a screen in the bag with the white spring assembly. Is this used somewhere in the build that I can't find? I don't see it used in the couple of build pictures.

Thanks!

Paul

 

[Banzai]

I've got my kits in and I have a couple of questions as I gather parts:

The schematic says 2n3891 for the FET but the Mouser cart has 2n3819. Does the schematic have a typo?

There is a screen in the bag with the white spring assembly. Is this used somewhere in the build that I can't find? I don't see it used in the couple of build pictures.

Thanks!

Paul

Hi Paul,

The Mouser cart is correct (2n3819)

The screen is to cover the slots right underneath the capsule. It's not attached in case you wanted to plate or paint the body first.

To install, no need for anything special. You can just push it into place – glue is optional, it will hold without it.

 

[Ericbazaar]

Beautiful!!

Good picture of that mega spring pin too.
If you cut it shorter, you can gain more inside space.

More space between the capsule and the tube ist better to reduce the heat. I have experimented with underheat the tube in the last week and the 6s6b works perfect with 4,4v @ 105mA, it will reduce the heat in the mic strongly. I have reduce the capsule and circuit noise. This mic sound thicker and better in the bottom like the UM57 Microphone i had here for reair. Well balanced Capsule.

I hope this tube works long with this heavy underheating : btw i could underheat the tube up to 3,2 V without loosing performance. Stunning little russian tube...

Thanks Mike for these beautiful Mic Kits

 

[Potato Cakes]

Hi Paul,

The Mouser cart is correct (2n3819)

The screen is to cover the slots right underneath the capsule. It's not attached in case you wanted to plate or paint the body first.

To install, no need for anything special. You can just push it into place – glue is optional, it will hold without it.

Good to know.

I'm waiting on transformers then hopefully I can get these together and put them to use. I haven't decided on what color to paint them.

Thanks!

Paul

 

[Potato Cakes]

What threading is used for the spring assembly to attach it to the PCB?

Thanks!

Paul

 

[Banzai]

What threading is used for the spring assembly to attach it to the PCB?

Thanks!

Paul

M2

 

[Potato Cakes]

Fantastic.

Also of note, when using the Mouser cart, one should double check the lead spacing as there are a number of capacitors that should be 2.5mm pitch and are 5mm in the cart, and vise versa. But I do appreciate having the cart as there are couple of items on it that I wouldn't have know which category to search in to find the correct part.

Hoping to get these put together when I get home next week. Thanks again for putting this project together.

Thanks!

Paul

 

[Banzai]

Fantastic.

Also of note, when using the Mouser cart, one should double check the lead spacing as there are a number of capacitors that should be 2.5mm pitch and are 5mm in the cart, and vise versa. But I do appreciate having the cart as there are couple of items on it that I wouldn't have know which category to search in to find the correct part.

Hoping to get these put together when I get home next week. Thanks again for putting this project together.

Thanks!

Paul

Fixed!

C3-4-5-6 changed to 5mm spacing
C7-8 changed to 2.5mm spacing

 

[GraemeWoller]

Hi folks, sorry I've been afk for a bit.

These build pics look great, I'm super pleased that things are going well with the kit for everyone. I'm also really keen on that tube build. Awesome!

Poly or C0G. To be fair I haven't noticed much of a difference and there's always a lot of "discussion" about which is best. Like Mike said, the pads will take a bit of desoldering with no issues so it'd be really interesting to try various ones out and report back. Maybe we can start another war about the subjective sound qualities of each type!! LOL!

Sorry about the typo on the diagram for the FET. The BOM is correct... Might have to add components to my autocorrect!

I reckon we need to have a thread of people's recordings with these wee guys. It'd be wonderful to hear what everyone is doing with them when they're completed!

Graeme

 

[delaymix]

I wuld to have a valve circuit in the microphone, which scheme of interest to follow?

 

[TillM]

I would look for a Neumann KM54 schematic or any other small condenser tube mic.
But I would go for the KM54.

 

[Ericbazaar]

I would look for a Neumann KM54 schematic or any other small condenser tube mic.
But I would go for the KM54.

KM54 Schematic won't work with this Mic Kit. The Front Capsule here is on 0V Pontential. KK54 Capsule and Schematic are different.

 

[TillM]

You are right!
Next time I take a Bette look on the schematic.

 

[delaymix]

KM54 Schematic won't work with this Mic Kit. The Front Capsule here is on 0V Pontential. KK54 Capsule and Schematic are different.

So, what is an appropriate approach for a valve microphone?

 

[damianschwartz]

Will you make more available?? interested in two units and missed this  :-\