Neumann Vintage U87 Clone : Build Thread.

[poctop]

Hi. Thanks matador. Yeah the L1 and L2 voltages are ok. So polarization is good. The only problem I get is that I'm supposed to get 40V at C5/FBK/RBK but at C5/FBK I only get 17V and RBK +/- 0.06V or so. The capsule and switch aren't installed yet. I calibrate drain of the fet to 11.5V,  so what I think is maybe the problem should be around R13/15/16. That part of the circuit which inject voltages C6/R8/AA. I got 29V there. Maybe it's where the problem resid?

what transformer are you using ?, did you verify it if it is a T13. did you try the mic also ?.  there is a lot of subtility regarding polarisation voltage measurment like Matador points out.
D.

 

[Deepdark]

It's a Cinemag. I triple check my connections against the colors, etc. and it's ok. I found somewhere in the forum that I'm suppose to get +/- 43V at R13, which means that my 29V is way too low, and I can't get my 40V at C5. So there is something sucking out too much voltages here. I didn't tried the microphone yet, since I wanted to correct the 40V issue before trying it

 

[mica]

you are probably confusing adjusting the voltage with a sine tone ,  the sine tone is for biasing with a scope ,

so lets do a initial adjusment first ,  make sure the capsule is out of circuit or removed,  then set your DMM to dc Volt and stick the red probe on the drain pin of the fet and the black probe to the mic chassis,  and then adjust the pot R11 to read approximately 11.5V DC on the drain pin of the FET, make sure you removed any connection to the middle pin of the fET wich is the gate ,

once you have done this adjusments  try the mic out and see wassup ,  then if you have a scope you can tweak this adjusment by injecting a 1K sine tone trough r6 leg, do not add another probe to the fet gate.  then with the red scope probe on the drain pin of the fet and the black on the mic chassis , set the scope so you can see the sinus output from the scope and then increase the amplitude of the signal until the sine wave flat out,  the idea of this adjusment is to have the sine flat out equally on both phase at the same time so in short both top and bottom should flat out at the same time as you increase the amplitude of the signal . to see that happen you need to cycle the amplitude of the sine tone up and down while tweaking that drain value until this happens and then you have the optimized bias point.

hope this helps,
Best,
dAn,

Two questions:
-I need to connect the mic to a pre with phantompower on before this adjustment right?
-The middle leg of the FET is connected to GG and not to the PCB. The middle leg is drain right? Is this correct?

/M

 

[Deepdark]

The drain is the leg that is at the far left (when you look at the front of the fet where the writting are) and the pin at the far right is the source 

 

[poctop]

-I need to connect the mic to a pre with phantompower on before this adjustment right?

yes or the circuit wont work,

look for a datasheet on google for 2N3819


http://www.ee.buffalo.edu/courses/elab/2N3819.pdf

Best,
Dan,

 

[mica]

Thank you guys! 

 

[Deepdark]

When I take my voltages on the xlr, Hot and Cold I get 45.6V (not 48V). Now I measure before R19 and R18 and it's the exact same voltage. So I guess it's alright? what is the utility of L1 and L2? Between R18 and R19, I get 45.3V. So with those values, is it ok to say that the xformer seems ok and not the problem here? 

 

[poctop]

It's a Cinemag. I triple check my connections against the colors, etc. and it's ok. I found somewhere in the forum that I'm suppose to get +/- 43V at R13, which means that my 29V is way too low, and I can't get my 40V at C5. So there is something sucking out too much voltages here. I didn't tried the microphone yet, since I wanted to correct the 40V issue before trying it

you should be able indeed to verify the 40V on the front backplate connection not the front capsule pad when no capsule is in, since you are measuring it before the 60M HZ resistor  you should be fine with a decent meter, check R8 C6 node and C5 and lower look for cold solder and double check component you could also measure voltage drops and isolate that node from the front end to split the problem in half, I published a little table once in this thread with different voltage and the pattern
So you dont have any swithing happening and you are in cardiod right , hope this helps,.  you can thank Udo and Matador also for their precious help on this thread.
Dan,

 

[kante1603]

Wow ;D

 

[Deepdark]

Thank you all. Really appreciated.  ;D

I'll take my voltages from the nodes and check component and get back if there is anything.

 

[Deepdark]

For those who searches for a smaller pot, I found one from Murata that is damn small, 25K, 1/4w. Murata #PV37Z253C01B00. Found it on Digi-key by the way.

 

[Deepdark]

Just to let you know that Everything works just fine  ;D I decided to just don't care of that backplate voltages, I connect the capsule and Voilà! It sound warm, reaaaalllly silent in term of noise, no hum. No as hot as other modern condenser mic, but who cares? So I would like to thanks Matador, Dany and Udo. Thanks a lot guys for your help. 

 

[poctop]

Just to let you know that Everything works just fine  ;D I decided to just don't care of that backplate voltages, I connect the capsule and Voilà! It sound warm, reaaaalllly silent in term of noise, no hum. No as hot as other modern condenser mic, but who cares? So I would like to thanks Matador, Dany and Udo. Thanks a lot guys for your help. 

Glad you made it,  hopefully this is just a measuring issue ,  the U87A is about 10db colder than the 87AI or modern microphone so it is completely normal, and this is what it gives it his special sound 

Best,
Dan,

 

[Deepdark]

I compared it to my cheap AKG P220, and it's a completly different world haha. P220 sound thin, too harsh and the U87 so warm, more "proximity effect", colder in term of gain but I tested it really quickly and just put my pre at the middle and have enough gain while I was speaking so while singing, it will be just fine. For the backplate voltages, I re-tested with my mastercraft (dump) DMM and can't got the 40V. I thought about what Matador says about High impedance section of the circuit and all components that could influence the values I read and decided to just don't care and connect the capsule. Out of curiosity, if I've really got 17V at my backp;ate, what would be the consequences in term of microphone fonctionality? I saw couple of folks across the thread who had the same problem, but never came back to say if everything works, if it was just some false reading values, etc.

 

[poctop]

I compared it to my cheap AKG P220, and it's a completly different world haha. P220 sound thin, too harsh and the U87 so warm, more "proximity effect", colder in term of gain but I tested it really quickly and just put my pre at the middle and have enough gain while I was speaking so while singing, it will be just fine. For the backplate voltages, I re-tested with my mastercraft (dump) DMM and can't got the 40V. I thought about what Matador says about High impedance section of the circuit and all components that could influence the values I read and decided to just don't care and connect the capsule. Out of curiosity, if I've really got 17V at my backp;ate, what would be the consequences in term of microphone fonctionality? I saw couple of folks across the thread who had the same problem, but never came back to say if everything works, if it was just some false reading values, etc.

not sure where you are from but if you come in the montreal area somtimes,  I'd be happy to have a look at it and bias it for you,
PM me in any case. 

Edit: i trust no meter other than my Fluke 175 true RMS. and my old tektronix analog scope 

 

[Deepdark]

Haha cool. I'm from Quebec city. For now, I just calibrate it with my dmm and adjust the drain to 11.5V. I'll take a look with an oscilloscope when my father will be back from hunting trip. He has some great stuff at job. Yeah I cannot really trust my dmm. One strange issue is when I put it in Ohmeter, it read a fix value of 330ohm while I'm touching nothing with the probe hahahaha. Really cheap dump from canadian tire lol. You know what I mean  8) I just read post #831 and the guy had the same issue there. It finally ends up that he just don't give a f**k of his dmm and according to Matador some reply later, we can't really had trustable value with a non High Z dmm, according to this part of the circuit. An other one has the same issue on post #1833 but never came back with any result, so nobody know if everything is allright. It seems to be a common issue, hope it will help someone who may had the same issue than us

 

[fripholm]

Hi there,
I'm just finishing my first U87 clone (the kit from micandmod.com) - actually my first DIY mic. Everything is soldered together and today I biased the FET using the software scope from post #1 which worked fine. I've managed to find the spot on the trim pot where the THD value is at its lowest, which I think is desirable.

The only thing left is installing the capsule. But before I do that I need to confirm something. When hooked up to a preamp (+48V enabled, no capsule installed) this thing hums like crazy. As soon as I connect the probes from the scope, the hum vanishes completely and all I'm left with is the clean test signal on the mic's XLR output. As this is my first mic build I'm wondering if this is normal and goes away as soon as the capsule is installed - or am I having grounding issues which I should look into?

 

[kante1603]

Hello and welcome,

when you have an open condition on the input of the circuit you will get everything like rf sounds and hum.Once closed with the probes you load the input and have a lower impedance that depends on your dmm,so behaviour will be better.Be aware that you might get hum and rf again once the capsule is attached in the open microphone.This is normal depending on the environment (induced noise by cell phones,machines,lights etc.)
After full assembly of the mic body and good grounding among the metal parts it should disappear close to completely.
Looking good so far,

best,

Udo.

 

[fripholm]

Hello and welcome,

when you have an open condition on the input of the circuit you will get everything like rf sounds and hum.Once closed with the probes you load the input and have a lower impedance that depends on your dmm,so behaviour will be better.Be aware that you might get hum and rf again once the capsule is attached in the open microphone.This is normal depending on the environment (induced noise by cell phones,machines,lights etc.)
After full assembly of the mic body and good grounding among the metal parts it should disappear close to completely.

Vielen Dank Udo!

You're right, closing the mic body gets rid of the hum.

I'm still hesitating to install the capsule because I think I've also (as some others here) received a 67-type from micandmod.com instead of the 87 with the isolated backplate. The packaging clearly says MK-87, though. I'll be waiting for an answer from them.

 

[kante1603]

Bitteschön!

Yes,seems like waiting for the isolated version might be the best as long as you want a figure eight.
Mic & Mod says it will be available in around a month.
Member RuudNL created his own solution by redesigning the circuit and making his own pcbs,cool!

Have a nice sunday,

Udo.