FET847 Microphone Project Circuit By Jonathan Burtner PCB Layout By Poctop

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Jul 15, 2009
The FET847 is an adaptation of a Vintage Neumann KM84 circuit designed by Jonathan Burtner  for LDC capsules in a small and economical donor body.


Items provided in this project:

● Circuit Schematic
● List of parts used in prototypes
● Mouser BOM
Here are some bullet points:
● KM84 circuit adapted for LDC use.
● This is a very simple circuit with minimal parts and a single FET for impedance conversion.
● The Mic can be assembled with one mouser order provided body, capsule, and transformer
are acquired via preferred 3rd party.
● As designed can be hardwired Cardioid or Omni.
● Cardioid wiring only uses one side of the capsule while omni uses both sides.
● Floating Hi-Z
jumper or switch may be added for switched card/omni operation or rear
diaphragm can be soldered to the FET gate for permanent omni operation.
● Designed for use with LDC k47, M7, or CK12 style flat large diaphragm capsules.
● Very similar to vintage U87 circuit without the de-emphasis
portion and different
● New Cinemag CM5722
transformer is 7:1 for original KM84 ratio.
● U87 uses the 10:1 with essentially the same FET driver circuit so those can also be used for
lower output and different needs.
● 2n3819 FET should be selected and biased using the same methods as a U87.
● Efforts were made to keep the PCB small and “feature light” to provide a simple high quality
Mic with flexibility regarding primary build options such as capsule and transformer
● Donor body selection may require mounting HW to fit the PCB but it is designed to fit a BM800 in this
● BM800 Donor Body Accept HT11a Sized head basket
● Transformer also fits in the BM800 and is secured using E6000 adhesive.

Schematic Poctop

Schematic JB

Schematic KM84

Circuit Description

BOM Mouser Link

BOM Description and Hints

Cinemag 5722 Diagram

Cinemag 5722 Footprints


Enjoy ,

PM me if interested

[list type=decimal]
[*]The Silk Screen contains 1 Errata the P+ , P- and S+ , S- are Inverted but the Picture shown in this thread shows the proper position
0dbfs said:
rockinrob86 said:
How does this compare to the 47Fet?

It's different. This is more like a KM84 with an M7/K47/CK12 capsule... Or sorta like a U87 with a flat impedance converter (no de-emphasis).

I haven't put one in front of a kick drum yet but I will :)


Right, but if a 47fet is a k47 capsule with a fet amp into a transformer, isnt a km84 with a 47 capsule very similar? 

I have an extra k47 hanging around and was about to order parts to build a D-47FET.  IT seems like the 84 will be a little cheaper due to the lower parts count, although I haven't seen a price on the new cinemag transformer.
...inexpensive sources for BM800 body/shockmount:




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I already have one at home and was going to build a Schoepps circuit on strip board. Maybe I should change my ming hehe 8)
AMI's T8 also fits the BM800 body (same size as the CM5722) if you want to think of it like a U47 with a single FET instead of a tube. However not a FET47 or a U47.... Nor is it really a KM84.. :)
I have only used the CM5722 though.

This build has transformer options as well as capsule options and isn't necessarily intended to be a KM84 clone since the capsule is different anyway... Consider that a U87 uses the same fet with the same operating points and the transformer in a U87 is different ratio as well...

Some additional notes:
- The bias trim pot has a jumper so that once you know the R3 value (FET Bias) you can order a more permanent precision resistor and jumper that resistor into circuit.
- As designed it's hardwired cardioid only or omni only. It might be possible to float a jumper or switch for pattern switching. I am working out a solution for a possible switch but didn't feel that it was really a design requirement.
- Since there is only one PCB there may be room to install a larger PIO (not too large mind you) for the output cap on the back-side. I've got some on order to see if I can make them fit and I'm hoping they do fit!
- Dany also made provisions on the PCB to fit styro's in the regular places if desired.
0dbfs said:
AMI's T8 also fits the BM800 body (same size as the CM5722) if you want to think of it like a U47 with a single FET instead of a tube. However not a FET47 or a U47.... Nor is it really a KM84.. :)
I have only used the CM5722 though.

I have the AMI T8 in a 251-inspired FET build and it sounds terrific...


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I would be curious to know the tone of the mic with a K47 type capsule. I was planing building a pair of U87 for Room miking but, that one make me think that I could build them instead.
I guess the SCM900 would work, too. Just more room Inside the body. I got 2 of them here, maybe they could work. Do you have the measurement of the board Dany?
Thanks. Once I got my SCM900, I'll take a look at the rail to confirm how it would fit. I think I'll go with a pair of FET847 and built them Inside the SCM900. Their will be enought room to put some bigger caps  8)
RuudNL said:
What is the function of C1 ("Tuned to taste")?
How do we "tune" this?
C1 as in the 4pF NFB cap? That cap certainly provides some NFB back into the gate portion which in my understanding stabilizes the amp, reduces gain, and I believe it also reduces output impedance... I'll read up in my RDH4 on NFB maths...
For tuning the specific value try some different values, sweep the DUT / graph the results for quantitative measurements, listen in context for qualitative measurements...
I don't know of another procedure used by the OEM but would like to hear about it if anyone knows or any additional discussion on the topic for that matter.


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