Neumann Vintage U87 Clone : Build Thread.

[ianhu]

Hi all.

I built my first clone U87 tonight. Well actually I have been building it over the course of the last two weeks, but only because the mailing of some of the parts took some time.

Thanks to Poctop for the PCB's and Chunger for the STY 5 donor body.

I ordered all the BOM parts from mouser as indicated. And I bought a AMI T 13 Transformer locally.

I dont have access to a scope until tomorrow since I have to borrow that from work, but working from all the threads and an excellent blog build from chunger (http://studio939.blogspot.co.uk/2012/12/vintage-u87-circuit-clone-microphone.html) I was able to put it all together without a hitch.

I did some reference voltage checking from the schematic and they all are pretty much near as. I have the pots still in the PCB for bias calibration and I was expecting to see voltage differ across the zenner when adjusting pot R14 but the zenner remained at 22.6 volts. The drain on the Fet was 10.6 and that does move when the other pot is adjusted. So not sure why the zenner voltage remains static when R14 is adjusted. But either way I checked the Mic for fuctionality when I connected the capsule and the transformer.

Have to say it sounds a world away from the ROdE mic I was using. And possibly I could make it sound even better if scoped for bias accurately??

Have to say thanks to all you guys who have posted in this thread. I have read and read a lot of them prior to and during my build. It's all good info.

I wondered what tests I could conduct or what others have conducted to ascertain the best settings on the mic - and I am not really sure that sticking a scope on with make that much difference to my ears as it does sound good already with lots of body.

I did wonder - although I have stuck the AMI transformer T13 in the build - what the role of this tranformer plays and how much it effects the overall sound - perhap someone can enlighten me.

Regards

Ian

 

[poctop]

I have the pots still in the PCB for bias calibration and I was expecting to see voltage differ across the zenner when adjusting pot R14 but the zenner remained at 22.6 volts. The drain on the Fet was 10.6 and that does move when the other pot is adjusted. So not sure why the zenner voltage remains static when R14 is adjusted.

the Pot in R14 is not necessary and confusing , the stock 10K value is absolutely fine ,  the voltage over the Zener is made in a sense that i will not vary , and  will sit at about 2 times the average  bias supply ,  thanks for the nice word , Rock on,  Setting the Bias will make the mic headroom optimal and set the lowest THD point at the same time.
Best,
Dan,

 

[ianhu]

Thanks for that Dan, I shall re-insert the 10K resistor.

Just saw your family Mic build - nice I have to say. You have been very busy. Which is your favorite?

Ian

 

[poctop]

I have no favorite these are all my babies  8),

yes you can put the 10K back in ,
Best,
dan,

 

[ianhu]

A Tube based Mic for vocals does somewhat get my thoughts running I must say!!

Ian

 

[poctop]

A Tube based Mic for vocals does somewhat get my thoughts running I must say!!

Ian

The Choice is Yours......

 

[Toure14]

Are there any subs for the zener diode? Mouser is on back order...

 

[poctop]

512-1N4749ATR

Best,
Dan,

 

[jnTracks]

just threw this together tonight and it fired right up.

but the output is very low.  i don't hear any distortion in it but i have to crank my pre way higher than a condenser should be.

i'm using the AMI transformer. i got a body and capsule package from Matt at Microphone-parts.com and the capsule i got has a shared backplate. so i connected the backplate wire to both backplate pads on the PCB.  it means the figure 8 position sounds just like the cardioid position.  is this correct?
any ideas what i should check to figure out why it's so low output?

 

[Hank Dussen]

just threw this together tonight and it fired right up.

but the output is very low.  i don't hear any distortion in it but i have to crank my pre way higher than a condenser should be.

i'm using the AMI transformer. i got a body and capsule package from Matt at Microphone-parts.com and the capsule i got has a shared backplate. so i connected the backplate wire to both backplate pads on the PCB.  it means the figure 8 position sounds just like the cardioid position.  is this correct?
any ideas what i should check to figure out why it's so low output?

Did you reverse the transformer? The data sheet on the AMI-website was/is wrong. The transformer should be installed black wires on the capsule side. See post 1338: http://www.groupdiy.com/index.php?topic=48030.1338

Also did you bias the FET correctly?

 

[jnTracks]

The FET seemed to bias OK using the drain voltage method. I haven't used a scope yet, but that should be close enough to not cause something like a huge level drop, right?
As for the transformer I just measured my transformer and found that the yellow wires were 22.5 and the black wires were 395 or so.  so that means i had them reversed, i guess.  so i flipped the inside and outside pairs, not the black are at the capsule and the yellow are at the output.
could it matter which way the wires go between the black and the yellow pairs?  now that i flipped them, the output level seems maybe slightly better but it's still many dB quieter than the RODE NT2 i'm checking it against. about 25dB according to my preamps.

Just to be sure : the RT and SW pads that are open, the bigger ones, not the transformer's little ones, are for the scope test signal, right?

I also just noticed the pads marked glink and cgnd are open on my mic. Schematic looks like those should be grounded? What's the best pot for that?

 

[poctop]

Just to be sure : the RT and SW pads that are open, the bigger ones, not the transformer's little ones, are for the scope test signal, right?

I also just noticed the pads marked glink and cgnd are open on my mic. Schematic looks like those should be grounded? What's the best pot for that?

you can't actually compare the output level of this vintage circuit mic in regards to a modern mic , they are not gonna output the same it should be about 10db quieter that modern mic ,  it actually allows when at the proper level to get more color out of your pre... but you need a good pre for that  Just make sure you dont have a bad wiring scheme and the Pad function is not shorted that will kill a lot of dB as well,

The RT and SW big pad are for checking the headamp not for the Bias,  the injection point for the Bias is at the R6 install backward on pcb Point ,
the ending Voltage should nearby 10-13V in general , 

I also just noticed the pads marked glink and cgnd are open on my mic.

this are grounding point for the headbasket and transfomer core if you dont use them that is fine if you have no grouding issue , those point on the pcb are to the mic chassis ,

Best,
dan,

 

[HellfireStudios]

I would try to hook up the capsule as cardioid-only (just the front diaphragm and front backplate hooked to their respective FC PCB connections). Then, I would get one of the approriate capsules for the job because the isolated backplates are integral in the switching of polar patterns.

-James-

 

[ianhu]

Made my clone U87 and seems to be fine by ear.

However wanted to bias the FET using a scope.

Sine wave of 1Khz being injected into R6.

With the sine directly attached to the scope and therefore not in the mic circuit, as you would expect it is showing correctly. 


Image 2 shows the output from the FET as being Square? which has me baffled.

I wondered if and of you have seen this before and have an idea what I need to check.


Regards

Ian

 

[ianhu]

my FET is 2N3819 if that helps

 

[poctop]

Made my clone U87 and seems to be fine by ear.

However wanted to bias the FET using a scope.

Sine wave of 1Khz being injected into R6.

With the sine directly attached to the scope and therefore not in the mic circuit, as you would expect it is showing correctly. 


Image 2 shows the output from the FET as being Square? which has me baffled.

I wondered if and of you have seen this before and have an idea what I need to check.


Regards

Ian

did you install R6 backward according to the build instruction and did you put the scope on the FET drain Pin ,
if so you have past the bias point or somthing else if going funny ,
hope this helps,
Best,
Dan,

 

[HellfireStudios]

Try lowering the sine wave output. You may just be clipping the FET.

-James-

 

[ianhu]

Yes I installed R6 backwards like in chungers article, and yes I am probing from the drain pin.

Hmm I will try to lower the sine output as suggested and report back.

Ian

 

[poctop]

Yes I installed R6 backwards like in chungers article, and yes I am probing from the drain pin.

Hmm I will try to lower the sine output as suggested and report back.

Ian

HellfireStudios  has a very good point here,  the idea is to increase slowly the amplitude from the sine generator and find the point (adjusting The pot) where it will start clipping equally (from top and bottom simultaneoulsy)

Let us know,
Best,

 

[ianhu]

Thanks to Hellfire's tip. Yes I was overloading and the Fet was clipping.

I have managed to get it right now.

Thanks again.