Neumann U47FET Clone: D-U47 FET Microphone Project PCB Build thread

Hi guys,

don't know if this is the right place to ask but..... my fet 47 has a low output. 20db down from a tlm103 ( which is a very sensitive mic naturally).

i have  just put together a fet 47 using the schematic and what i can make out from the layout provided here and.........

1. a RK7 capsule,
2. a peluso BV8PS xformer with the 11:1 and 5.5:1 option. ( which i bought of Sir POCTOP ). I'm using the 11:1 option of course.


The output seems kinda low. I'm sure the 10db pad is not engaged. The spec sheet for the fet 47 original says it has a sensitivity of 8mV/PA which should make it 10db quieter the a TLM 103 at 23mV/PA ( this mic i have for comparision ). However the output is 20db quieter than the TLM 103.

Is this correct? or is the problem likely to be.......

1) the xformer which at 11:1 is not quite right?

2) the 2n3819 is biased wrong or something. Should i add the 25k trimmer that you guys have and play with than in series with a 3.6k resistor?

3) the use of a polystyrene 2200pf cap? what effect would changing this have?


The sound of the microphone is ok. It's full range. Sounds good on a bass drum but sounds very 4-5K on the vocal and lacks a little bit of bass, maybe believe it or not. ( maybe thats the xformer. It's also very directional.


Is everything ok with this microphone?

Many thanks in advance for any help or pearls of real DIY MIC wisdom

Michael
oxford
england

My mic sounded a bit thin and also had a fairly low output at first. In my case the problem was that I had wired up the rear diaphragm to the backplate connection on the pcb. I hadn't realized that the backplate is actually the middle part of the capsule. After I disconnected that and ran a wire from one of the screw holes on the side of the capsule to the "back" connection on the pcb the mic came to life.
I can say that the mic seems to be working great with the 2200pf film cap and with the jumper installed in place of the trim pot.

Are you getting signal on + / - sides of the XLR?

That would account for 6db if it's unbalanced.

Maybe check the transformer wiring/output.

Regards,
Mark

Make sue your Transistor are properly oriented so easy to make a mistake and with the part number as well double check that ,
the pot + the 3.6K resistor permits to set VDS conditions but as explained it does not make a difference cause it feeding back to self bias.
you can jump it plus the 6.8K resistance if you are not sure . if you have it installed then you can check that you can have a VDS of about 15V ,

double check  the voltage in the schemo as well, might help to find the spot,

Hope this helps,
dan,

hey guys,

thanks for getting back.

here are some voltages. do these look good?

on the 2n3819 T1 i have

drain=  40.3v
source =  25.1v

on T2 i have

E= 25.1
B= 23.7
C = 0.62

on T3 i have

E=44.5v
B=30.2v
C=40.3

T4 i have

E= 20.7v
b=21.4 v
c= 24v

T5 i have

E=43v
B=41.8v
C=21.5v


on T6 i have


E=0v
B=0.6v
C=21.6v



T7 i have

E=44.3v
B=44.6v
C=46.4v


and 15.8v from the 10m resistor to the junction with the 1g resistor.


how does it look to you guys ?


the capsule is connected properly. one diaphragm connected to 1g and the polystyene 2200pf cap. and the middle bit to earth.

not sure how to test the ac across xlr 2 and 3 cos of the 56r resistors. do i test before these?

man thanks in advance



michael

At a quick glance it seems fine from here ,  can you check capsule polarisation volatge and xlr and also traffo config  ,  check the capsule orientation also and switching config if used

hope this helps ,
Dan,

Hey Dan,

i switched to using the  other side of the capsule and nothing has changed. my 100m resistor is hard wired to point BB on the schematic.

The voltage on the gate of the 2n3809 is 4V. That doesn't say much. The Drain is 40V


where do i measure capsule polarization voltage? is it where the 1g resistor meets the 2200pf cap or is it at the gate of the fet? Also, how do i measure the xlr. IS it AC across pins 2 and 3 ? or is it AC between ground and PIN 1 and PIN 2 then PIN 1 and PIN 3?

Many thanks again

Michael

muffy1975 said:

Hey Dan,

i switched to using the  other side of the capsule and nothing has changed. my 100m resistor is hard wired to point BB on the schematic.

The voltage on the gate of the 2n3809 is 4V. That doesn't say much. The Drain is 40V


where do i measure capsule polarization voltage? is it where the 1g resistor meets the 2200pf cap or is it at the gate of the fet? Also, how do i measure the xlr. IS it AC across pins 2 and 3 ? or is it AC between ground and PIN 1 and PIN 2 then PIN 1 and PIN 3?

Many thanks again

Michael

Click to expand...

you can measure the polarisation voltage on the back side of R1 ,  the microphone output is suppossed to be in the range of the u87 maybe a litle hotter but not like 20db thats make no sense ,  i would also verify the board wire hookup also , what is your capsule like ,
hope this helps
Best,
Dan,

Hey dan you alright?? Sent you an email few days ago but no answer.

hey dan,

the voltage between r1 and the R8 = 15.2v. relative to earth is that correct?

if not here, the what should it be in volts?

what seems to be happening is a mix up on the original schemo with the voltages to R16. Should that not be 45v to  R16  and there be 46.5 going to T7?

michael

muffy1975 said:

hey dan,

the voltage between r1 and the R8 = 15.2v. relative to earth is that correct?

if not here, the what should it be in volts?

what seems to be happening is a mix up on the original schemo with the voltages to R16. Should that not be 45v to  R16  and there be 46.5 going to T7?

michael

Click to expand...

I believe it should read the polarisation voltage there 45V DC , I had to to put aside my 47FET circuit away for the M49 coming , i will try to hook it back and take some reading when i have a chance ,
I will keep you posted,

Best,
Dan,

The Store is back online,

Happy New year All,
Best,
Dan,

does anyone know exactly where on the circuit you would measure the polarization voltage for  the capsule?

it can't be after R8. That's a 10m resistor. Surely. How can 40v+ get there?

Many thanks for any help

Michael

You mentioned the transformer can be wired 11:1 or 5.5:1, is there four leads/or pins on the primary? do you have a wiring diagram for it?

Here's some photos of my recently completed Fet47 and an audio clip.


On acoustic guitar:
https://www.dropbox.com/sh/hfq2uaj7230vdgj/3uzuNH4c-o

I tried to go high-end on this. I found a re-skinned Neumann K47 on ebay (from seller alphaville777, who usually has one or two listed). I also got the direct replacement Fet 47 transformer from AMI, who had it in stock. All the electronic components are from the Mouser cart and the JustRadios capacitor cart. Total cost was about $1,350. Much cheaper than a vintage Neumann, or a modern Fet47 reproduction...

I found that the MXL 4000 body works great for this project. I got one on ebay for $140. Everything fit very well. A couple tips for using this mic body:

• I used AMI's K47 capsule holder. I enlarged the MXL's white plastic base that the capsule holder mounts to very slightly with a Dremel tool. I got the AMI capsule holder to fit with a very snug friction fit. A very small amount of water and dish soap helps it slide through.
  
  You can make room for the transformer by hacksawing off the two lower threaded screw holes on the mic rails.
  
  If you drill out the top two mounting holes on Dan's FET47 pcb with a 5/64 (.078") drill bit, you can use the pcb mounting screws that came with the MXL. Since you have to cut off the lower mounting holes to make room for the transformer, the pcb is only held in at the top. It seem very secure, though. Much easier than drilling and tapping the rails
  
• I didn't try to use toggle switches for the pad and low cut, but I will try this later I hope. I think if you cut a plastic sheet to the shape of the MXL's switch pcb you could fabricate a holder for the switches. Unfortunately the MXL's switches are spst so they don't work for the pad, and the stock circuit printed on the switch board keeps you from using one for the low cut. Not sure if a DPDT switch would fit in the space provided on the mic. It could probably be made to work with some creativity.

Spray paint to taste.

[/list]

I haven't had a chance to put the mic in front of a drum kit yet. The audio clip is a few seconds of acoustic guitar.

The mic isn't very hot. In the clip the mic is less than a foot from the guitar, with 65db of gain from a GML 8304 preamp. Maybe I've done something wrong here? I took my best guess at the transformer wiring, and went with the way that had the fullest (but not the loudest) sound.
Another thing I would like to learn about is picking the transistors. There are a lot of spares in the Mouser cart, which I assume are there so you can pick and match the best ones.

Great project, Dan! I think that putting my DIY energies into microphones is going to be the best way to improve the quality of my recordings. Very exciting.

thanks SkidmoreBay taking time to do this, The Clip just got that Vibe  8) awesome ,

The mic is not supposed to be very hot as overgained modern mic  ,  as a single stage FET driving circuit as in vintage U87 circuit.
this vintage circuit will give about 10-12db less than modern mic , this allow the preamp to give its full color range when using this type of vintage circuit and also more headroom on loud source as Bass Cab and front kick mic,  i would really like to hear a clip of your mic in front of a bass cabinet  ,
the transformer should wired 9:1 primary to secondary.

Best,
Dan,

Hey,

Brand new to this forum and pretty new to DIY projects in general but this looks like an awesome project.  Thank you Dan! 

The link to the Alctron GT-2B shows a body with a large "bulb" where the transformer would go at the base of the body.  Is this section removed when the mic is switched to a three pin XLR out?

poolshark said:

Hey,

Brand new to this forum and pretty new to DIY projects in general but this looks like an awesome project.  Thank you Dan! 

The link to the Alctron GT-2B shows a body with a large "bulb" where the transformer would go at the base of the body.  Is section removed when the mic is switched to a three pin XLR out?

Click to expand...

you just need the 3-pin xlr insert that comes with Chunger body kit.  you can swap it easy.
see here ,

http://www.groupdiy.com/index.php?topic=50015.0

Best,
DAn,

Yes, That's the transformer. It will be replaced with another one.

Awesome, thanks!