Neumann Vintage U87 Clone : Build Thread.

[poctop]

Thanks for all the help I've received so far.  I want to try to bias the FET using the Room Capture software to measure THD.  Do I measure from the drain of the FET for this method?  Also, does the transformer have to be disconnected? 

Thanks,
Craig

been a while since i did not used it , but from what i remember you set the pot very low resistance and then with the room capture you basically inject a tone in the mic, (without capsule only) and then you route the output of the mic trough a pre to Room capture and then once the FET start to open up while trimming the pot you will hear it and then you monitor the THD , just before it opens up the THD will be very high and then slowly trimm until you reach a minimum Value in the THD , it will be somthing like 0.3% IIRC and then you stop there , this method was very sucessfull and gave the exact same results as with a scope.
Just a tad more fiddlings 
best,
Dan,

 

[craigmorris74]

Dan,

Thanks for the reply.  I attempted to bias by ear, and really had a hard time distinguishing distortion from the clean signal.  Not sure if I was feeding enough signal.  I'm going to try the room capture method before I finish the mic, just before my capsule arrives.

The mic works, though.  I soldered the capsule included in the Aurycle kit and got a good healthy signal.

Craig

 

[beatnik]

I got an Oktava MK-219 very cheap because it's not working. In fact, someone tried to mod it and completely ***** the pcb board. So it needs a complete re-build of the circuit board.

So, I'm thinking to keep the parts and put an U87 circuit in there.

Please can you help me to know if I could keep the capsule and the transformer, and maybe adapt them to the U87 circuit?

here is the MK-219 schematic, a little hard to read but I think it's ok if you zoom it.

http://www.sdiy.org/oid/mics/Oktava-MK219.gif

Let me know if you need more info and maybe I have to do measurements to the capsule and the transformer

 

[burglar]

Hi Folks,

I've just fired up my first U87 for testing.  It's passing sound but there is lots of feedback / interference.  I have only got the Behringer B2 capsule hooked up in cardioid at the moment.  I'm waiting on my BeesNeez K-87. 

The interference is responding to appliances in the room such as my solder light.  I had to drill out the mounting holes to fit the body so I have connected CGND to one of the rails for grounding. 

The interference changes frequently with movement but it sounds like a square wave.  Any thoughts?

Cheers

B

 

[poctop]

Hi Folks,

I've just fired up my first U87 for testing.  It's passing sound but there is lots of feedback / interference.  I have only got the Behringer B2 capsule hooked up in cardioid at the moment.  I'm waiting on my BeesNeez K-87. 

The interference is responding to appliances in the room such as my solder light.  I had to drill out the mounting holes to fit the body so I have connected CGND to one of the rails for grounding. 

The interference changes frequently with movement but it sounds like a square wave.  Any thoughts?

Cheers

verify that all the grouding is proper and remind that the mic body shell must be installed for the mic to be protected in regards to RF pickups,

hope this helps,
Dan,


B

 

[kante1603]

Hi Ed,

I'm 100% sure grounding isn't proper enough.
Running a wire to the mounting rails is correct,but maybe not enough.
Measure the resistance from pin 1 on the xlr to all metall parts of the mic housing and the head basket.
You should get very close to zero ohms here.Also measure this pin to the mounting rails and the pcb connection while you're on it.
I'm pretty sure you'll find an issue there.

Best regards,

Udo.

 

[burglar]

Thanks for your quick responses guys.  I don't have the mic body on.  The interference is very loud.  Louder than my voice is coming through. 

I have tested for grounding and It all appears to be fine including rails, head basket etc.  Any other thoughts? 

 

[kante1603]

Ed!
You must have the body mounted completely!!!
This is imperative or you'll pick up all possible (and invisible ) radio noise that is surrounding you!

Please try and report back,

Udo.

 

[burglar]

Hi Guys,

well, I have now put the body on and I have NO SOUND!  The interference has gone but there is no sound, just hiss.  If I remove the body, sound is back but with interference.  I can't see any short anywhere.  I have sanded the paint away at either end so as to make a connection with the screw on bass and the top.  It must be something simple???

 

[kante1603]

Hi Ed,

must be something simple.If the mic is working without the body then it is a very good sign.
Looks like something gets shorted or open condition.This can be wires,pads,maybe the transformer shorting out the xlr while fastening.Check for good clearance everywhere while assembling the body.
Maybe you can monitor the mic during assembly,this could give a hint (e.g. Does it happen when the metall tube goes on or later when screwing on the bottom ring etc.).Do this carefully!

Good luck,

Udo.

 

[burglar]

Hey Udo,

I tried the things you suggested and it lives!

The mic is a little on the bright side with the 797 capsule in it.  Looking forward to swapping it out.  I'm cutting a tricky vocalist on the 17th so I'll put up some results then.  Thanks again for all your help Dan, Udo & Dave. 

Chaaa

 

[kante1603]

Hey Udo,

I tried the things you suggested and it lives!

The mic is a little on the bright side with the 797 capsule in it.  Looking forward to swapping it out.  I'm cutting a tricky vocalist on the 17th so I'll put up some results then.  Thanks again for all your help Dan, Udo & Dave. 

Chaaa

Congrats Ed!!!

About the stock capsule:Throw it away as far as you can,this piece is just crap!
You'll be overwhelmed once you have a "real" capsule in-promised!

Have fun,

Udo

 

[burglar]

awesome,

Is there any reason that I shouldn't leave the trimmer pot in there instead of replacing it with a resistor?  Will I notice any difference sonically? 

 

[burglar]

Having left the mic running for a while I am noticing that it randomly makes a short scratching or even "farting" noise?  It doesn't happen often, just once in a while?  It only seems to happen when I am holding it, and yes, I'm sure it's not me.lol.

 

[kante1603]

awesome,

Is there any reason that I shouldn't leave the trimmer pot in there instead of replacing it with a resistor?  Will I notice any difference sonically?

If you have it biased correctly then yes,if you find the correct value you can easily exchange the pot to a fixed resistor.Simply take the pot out after bias and measure the value out of circuit,exchange it,done.
Btw.,was that  your last issue (pot touching the mic body or getting bent)?

Udo.

 

[kante1603]

Having left the mic running for a while I am noticing that it randomly makes a short scratching or even "farting" noise?  It doesn't happen often, just once in a while?  It only seems to happen when I am holding it, and yes, I'm sure it's not me.lol.

It can be you!
Not joking,it might be electrostatic discharge from your body.
This mic is definetely not built to be held in your hands,so mount it on a stand and monitor it again.
If it doesn"t change then you might have an issue with a faulty cap discharging randomly.
Make sure the mic is biased propperly.
If everything is o.k. after mounting correctly you're done.
Might be a not-so-clean phantom power supply too.

Hope to have helped,

Udo.

 

[burglar]

Thanks Udo.  I will install the BeesNeez K8 and run some tests.  I just set the mic to between 11 & 11.5v as per the video on YouTube.  I borrowed a scope off a mate yesterday though so I'll have a bash at biasing that way.

 

[kante1603]

Biasing by scope is the best way to do- what you see is what you get

Best,

Udo.

 

[burglar]

From the thread I see that the transformer and capsule need not be connected for scope bias.  Do I HAVE to disconnect them or can I bias the complete mic?

 

[kante1603]

You can leave the transformer connected,but must lift the capsule connection.No biggie,just lift one wire per capsule side.This way the capsule is out of circuit and can be attached back easily.

Best,

Udo.