U47FET in BM800 microphone body

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mihi_fuchs

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I started DIY three years ago right before all the lock down action started and as it provided quite some nice distraction I had quite some projects going. In the dropbox link below you can find my attempt on transferring a U47FET based layout to the BM-800 donor body.
The donor body is particularly cheap and does the job well in my opinion.

In the link below you can find the BOM, a detailed building guide and some sound samples recording kick and snare with the mic. The sound samples are unprocessed. Both instruments were recorded by the U47FET-based clone in the BM800 body.

Here is the link:

IMG_9969.jpeg IMG_9966.jpeg IMG_0092.jpeg
I had to split the curcuit part into two because of the limited space. I used a MAIKU U47 capsule, a 3u audio GZT-47 transformer (ratio 6.5 : 1) and a 3D printed capsule holder. Most parts were obtained from mouser, the styroflex capacitors were purchased from reichelt.at and budgetronics (for details see the BOM in the dropbox folder).

Last but not least the project has been highly influenced and some solutions were simply copied from members of this forum (e.g. matador, khron, poctop, chunger, and others) - thanks for all of their posts, ideas and solutions that they have shared over the times.

As there has been already a discussion about the BM-800 donor body maybe the sound samples prove helpful to some forum members.

Thanks again for all the nice hours I spent browsing through this forum,
all the best
mike
 
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mihi_fuchs

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Looks nice! Are you planning on selling the cb's as well?
Thanks. I still have left a few spare ones and the same is true for the U87 (I am currently writing up the building guide too). I can put the spare ones on the black market post if you like.

I'm based in AUT so delivery costs to Belgium should be ok ...
 

wlinart

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Thanks. I still have left a few spare ones and the same is true for the U87 (I am currently writing up the building guide too). I can put the spare ones on the black market post if you like.

I'm based in AUT so delivery costs to Belgium should be ok ...
I'm definitely interested, i've got a spare bm800, transformer and capsule at the moment
 

mihi_fuchs

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I'm definitely interested, i've got a spare bm800, transformer and capsule at the moment
I posted the PCBs on the black market. Here's the link to the thread: PCBs for U47FET in BM800 donor bodies

Feel free to contact me at any times via the platform for selling - that's the easiest way.

Thanks again for your encouragement and your interest. If you have PCBs of other projects around I would be also interested in exchange maybe ;-) ...

Cheers
Michael
 

mihi_fuchs

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As one of the new members pointed out, I forgot to describe the PAD and Low Cut function. These can be simply jumpered on the board. Additionally one can use the pins for jumpering to wire the PCB to a real sitch. More details can be found in the new Building Guide in the dropbox folder (Version 1_1).

Apologies as I forgot to include it right away.

Cheers Michael
 

mihi_fuchs

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R22 and R23 are missing in the BOM. They are:

56R, 1%, 1/4 Watt.

I will update the BOM ASAP - BOM V3 has these missing parts already included.

Best regards
Michael
 

mihi_fuchs

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here is another link to leftover Styros for the built - maybe somebody is interested:

Cheers
Michael
 

jimmykennedy

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Feb 7, 2022
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Hi Micheal, I'm interested in an making a neumann U47 clone but I would like cardiiod and omni patterns if possible and even better figure of 8 as well I know that may be easier said than done. I would like a Neumann capsule if not a Thiersch or a Heiserman or any other you could recommend. I have limited experience with electronics but I'm handy with a soldering iron. I have a Tube Tech Mic Pre so thats why I think fet with output transformer would suit my needs. Cheers Jimmy.
 

mihi_fuchs

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Hi Micheal, I'm interested in an making a neumann U47 clone but I would like cardiiod and omni patterns if possible and even better figure of 8 as well I know that may be easier said than done. I would like a Neumann capsule if not a Thiersch or a Heiserman or any other you could recommend. I have limited experience with electronics but I'm handy with a soldering iron. I have a Tube Tech Mic Pre so thats why I think fet with output transformer would suit my needs. Cheers Jimmy.
I think to get the mic to omni would just be the connection of the two diaphragms. If you include a switch from the back diaphragm you can switch between omni and card. Fig. 8 would be definitely a bit more trickier. I attached the schematics with this post, so you can have a look and maybe you find a good solution ...

Nevertheless, I would be very careful to compare the U47FET to the U47 tube mic, even if you have a tube preamp in the chain ... My clones sound very, very different ...

I would definitely try the U47 for vocals any time, for the U47FET the vocals should be loud and my to go application for the FET is more on drums ... maybe bass guitar ... maybe guitars ...

But of course that's my opinion and that may be completely different to your taste ...

If you'd like a good transformer mic for vocals maybe have a look into the KM84-circuit for LDC mics that is around here in the forum ... I heard a lot of people saying that this one of their fav. mics for vocals when its coming to non-tube mics ... The circuit is based on the KM84 and has been adopted to for LDCs ... So best results according to opinions were observed by pairing it with a linear capsule like C12 based or K47 based clones ...

But as I said I do not know your go to application and all I said may be very much aiming the wrong way ...

Hope at least that could help ... if you need anymore infos or anything else, just let me know ...

Best regards
Michael
 

kafuhaus

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Jun 9, 2005
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Austria
Now that I've built two of these babies, both sound very good in front of a kick drum, but they do sound a bit different. One seems to be a couple of dBs louder and brighter.

Part of the problem might be the 220MOhm resisitor (R2) that I didn't have. Just had two 100MOhm. Is this critical?

Then I didn't match the FETs I used, as I only had like 5pcs. I really have now idea how to correctly set the trimpot. At what value (Point EE I guess) do you girls/guys set it?

I used the maiku capsules on both.

Cheers
Max
 

mihi_fuchs

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Hi Max,

first of - and pls do not take it offensively - you checked that the jumper's setup is the same (low cut and pad). I'm just asking because it happened to me once that I forgot that I had engaged the pad on one and it cost me almost an hour to think of it ...

Other than that, the U47FET is actually selfbiased - the pot is for vds fine tuning. See here for the original idea by Dan: Neumann U47FET Clone: D-U47 FET Microphone Project PCB Build thread

So I guess that this should be little of your problem - to make sure you can just bridge R6 and the potentiometer with a 6k8 resistor as in the original circuit.

So my first guess - if jumpered right - would be the 220M resistor. Did you use the 2 x 100M variation for both builts?

If that is not the problem I attached some information on JFET matching in the pdf below. I really do not know the origin so my apologies if I'm not able to give the credit to the author ... but the procedure looks quite doable.

But honestly, in my case I did not match JFETs either and they sound very similar (I used them for stereo recordings). So either I've been lucky or it does not do so much of the magic.

Another thing could be the transformer - make sure you did not invert phases on one - that could also lead to the effect in the right environment.

Last but not least - one of the capsules could have been damaged during transport or installation - but I really hope that's not the case.

Those would be the first things I would check - hope you're finding the solution quick.

Best regards and thanks again for the work on the capsule holder :)!!!
Michael
 

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kafuhaus

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Austria
Hi Michael!

Thank you for your answer!

Actually the jumpers made me think for a while, as the markings on the PCB confused me a bit first.

For one mic I use 2x100Mohm, and for the other, I have to admid, I used a 200MOhm resisitor from a tbone mic I am converting (thanks to your elam251 project). Of course I didn't measure it out of pure laziness.

I should be receiving 220Mohm resistors from a friend today, I will report!

Cheers
Max
 

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