Neumann Vintage U87 Clone : Build Thread.

[Pip]

The gate is what gets the signal. The Source is not the audio source.

I think if things are working in Omni you are really close.

Not to go Master of the Obvious again but to clarify. A signal level difference of 10db is big. The reduction in perceived loudness can be as high as half at levels above I believe 60db. The point being this mic is never going to have the bite of a U87Ai, they are truly very different animals. Instead this mic is going to have a, to my ears, more musical warmer quality. As an aside have you noticed the spike in prices of U87i's lately.

Double check your ground integrity throughout the build this could be part of your noise issue.

I would try to find an LCR meter or DMM that does capacitance and check that capsule.

Keep at it its right there.

 

[JessJackson]

Hello J,

your measuring pin connection looks correct (drain).
In the link you posted the drawing seems to show the bottom(!!!) view.That's done pretty often,sometimes stated somewhere close to the drawing or pic.All good here I think.
About lowish level "all" might have:The issue was solved a while ago and refered especially to the AMI T13 transformer meaning the colour coding was reversed on a batch a lot of us bought them from.
I have done four mics meanwhile,and they are working fantastic.Again:A u87 compared to a u87Ai does have a lower level around -10 to -11dB,not more.

Best regards,

Udo.

Edit:Here's a proof for that jfet-thing (taken from the fairchild description):
http://www.mouser.com/ds/2/149/2N3819-106215.pdf

Edit #2:It still seems you have your capsule miswired.Do you have a capacitance meter or a dmm capable measuring capacitors?
If so measure say the front diaphragm connection to one of the backplate wires,you'll get a reading on your meter.Write it down or remember it.Now while leaving the front dia. wire connected swap to the other backplate wire.You'll get a reading from here too!But it will be different from the first!
The pair with the higher value is the correct pair to hook up to the front connections on the pcb.
We are looking for roughly 50 pF.
Same on the other side.

Edit:About out-of-phase signals:In figure eight the back is always reversed compared to the front!
Search for "Blumlein" or "MS" stereo techniques on the web,it's very well explained there.

Your absolutely right.

The diagram is from the bottom. Thanks for clarifying.

My mic is good now. still low output and high noise floor (pre is on 48db for speaking) but fully working in omni and 8. had a bad connection to the front backplate screw.

I also changed my FET from to IDSS 10.2 to 4.2.

I have DMM capable of 30meg and capacitance, cant go as low as 50pf though.

is the front backplate is directly behind the front diaphragm then i am wired in correct.

Cheers

J

 

[Pip]

Hi
I've built 2 of these lovely mics, using the 'set-and-forget' method of setting 11.5V at the drain.

I'd like to go back and bias the fet using the listening method, but can't find much in these pages related to it.

Can anyone give any tips on how to do it, what I should be listening for?

These are not my words they belong to another and used to be available through a link that was part of the first part of this build thread that appears to be broken.


Biasing of single FETs in mic applications is done to minimize distortion and maximize output. As FETs vary, it needs to be done for each individual FET in each individual mic.

I set these FETs up the following way:
I feed a 1K sine wave signal to the gate and slowly increase the source resistance from 0-10k Ohm with a temporary trim pot (10k) until I reach the lowest distortion of the FET.

This will be a pretty precisely defined point right after the highest output of the mic, right after the first heavy distortion audible when the FET turns on, and right before the more gradual onset of the distortion as the FET's output decreases again.

The trick is to feed enough 1k into the gate that at this ideal bias point the distortion is indeed barely audible or gone altogether, and clearly audible to each side of the ideal bias point.

All of this is best done at very low listening levels over headphones in a quiet environment. If you don't want to do it aurally, you can use an oscilloscope.

I then copy the ideal value I found  with the trim pot and choose a fixed source resistor of the same value for final installation.

If others have found another or better way to bias source-biased FETs in condenser mics, please post them!
But please be conscious of the forum's rules: keep the explanations simple!

This is done of course without the capsule connected.

EDIT: It is not broken anymore if you want to read it from the source:

http://repforums.prosoundweb.com/index.php/topic,11630.0.html

 

[kante1603]

Instead this mic is going to have a, to my ears, more musical warmer quality.

Absolutely agreed Pip!
I really prefer it over my original U87Ai-Mics!

Cheers,

Udo

 

[kante1603]

I would try to find an LCR meter or DMM that does capacitance and check that capsule.

Keep at it its right there.

Yep!(see my post #1698 edit 2).

Best,

Udo

 

[sr1200]

I re-biased my mic yesterday using a scope.  Excuse the noobness of this question but, figured id ask since i cant find an answer using the mighty google (probably because i dont know what im looking for).  Seems I could squeeze about 10db more gain out of the fet if there were like a vertical offset of some kind.  My fet only distorts from the bottom for much of its "available" bandwidth.  By the time i get it to clip on both sides, ive lost a TON of gain. (mic sounds pristine, but need like 40-50dB of gain for decent levels which is not out of the question, but more for my own edification, is this possible at all or is this just a characteristic of FETs?

 

[Sredna]

Try to find a FET with lower IDSS, at least lower than 10.

 

[JessJackson]

Double check your ground integrity throughout the build this could be part of your noise issue.

Even if its just hiss?

 

[kante1603]

Double check your ground integrity throughout the build this could be part of your noise issue.

Even if its just hiss?

Is this hiss static or does it change when moving the mic around or touching the mic body/xlr plug etc.?
When not grounded properly you can pick up a lot of noise covering the complete audible frequency range.
If I were you I'd measure resistance/continuity from xlr pin 1 to all metall parts including the rails and the ground planes on the pcbs.Also the basket!
You should end up very close to zero ohms everywhere.

So best to check this first because it's easy to do and might give us a hint where else to look at.

Best,

Udo.

 

[poctop]

Awesome Guys, Many Thanks For this.


http://youtu.be/DtNH46jpwJo


Quote From Peterson Goodwyn At DIY Recording Equipment

Matthew knocked it out of the park, and feedback in the hour since I
posted it has already been great. It's on pace to be DIYRE's most
popular Facebook share ever with 46 Likes and 5 Shares in the last hour.

Best,
Dan,

 

[JessJackson]

Double check your ground integrity throughout the build this could be part of your noise issue.

Even if its just hiss?

Is this hiss static or does it change when moving the mic around or touching the mic body/xlr plug etc.?
When not grounded properly you can pick up a lot of noise covering the complete audible frequency range.
If I were you I'd measure resistance/continuity from xlr pin 1 to all metall parts including the rails and the ground planes on the pcbs.Also the basket!
You should end up very close to zero ohms everywhere.

So best to check this first because it's easy to do and might give us a hint where else to look at.

Best,

Udo.

Hiss static but when moving mic near other gear thats on, hum appears also hum appears when touching switches.

 

[poctop]

Hiss static but when moving mic near other gear thats on, hum appears also hum appears when touching switches.

Definitevely grounding issues,  have you double checked your xlr input config,  everything from the mic chassis pcb gnd and metal part of the switch must be short to this point , if you have the large body switch board , make sure you have soldered all the mouting pins cause only one of the pin is actually on the ground plane and the screw to the metal and gnd plane must also be short ie.. 0 ohm  trough this same board and everywhere else on the mic chassis and pcb gnd and xlr pi 1 are all the same thing after completion.

hope this helps, 

Dan,

 

[Pip]

I re-biased my mic yesterday using a scope.  Excuse the noobness of this question but, figured id ask since i cant find an answer using the mighty google (probably because i dont know what im looking for).  Seems I could squeeze about 10db more gain out of the fet if there were like a vertical offset of some kind.  My fet only distorts from the bottom for much of its "available" bandwidth.  By the time i get it to clip on both sides, ive lost a TON of gain. (mic sounds pristine, but need like 40-50dB of gain for decent levels which is not out of the question, but more for my own edification, is this possible at all or is this just a characteristic of FETs?

Not to go Master of the Obvious again but to clarify. A signal level difference of 10db is big. The reduction in perceived loudness can be as high as half at levels above I believe 60db. The point being this mic is never going to have the bite of a U87Ai, they are truly very different animals. Instead this mic is going to have a, to my ears, more musical warmer quality. As an aside have you noticed the spike in prices of U87i's lately!

I quoted myself here as it is just easier. I know it can probably be expressed better by others. As to the FET issue:

This is from Dale Ulan:
"If you are in a pinch or in a hurry, and don't mind a compromise, you can use the same trimpot method except measure the voltage at the drain of the mic and get it to about 10 to 11 volts - that'll be in the general ballpark. Then, speak very loudly, very close, but without popping air at the capsule, into the mic while measuring the drain voltage on a DC voltmeter. If the voltage drifts around a lot, then adjust the pot a bit one way or the other to try to keep it from doing that. This is a very crude way of doing what Klaus' method is doing. Obviously his is better and more accurate, but I've done this in a pinch.

If you stop at the first (rough) part of this test (just setting the voltage), you might find that the mic will cut out occasionally on plosives or even loud passages.

If you are using this voltmeter method, make sure you measure the voltage using either a digital or a vacuum-tube voltmeter or an oscilloscope. You can't use a normal 'analog' voltmeter as it will upset the circuit too much."

I would rough it in there.

More here http://repforums.prosoundweb.com/index.php/topic,11630.0.html Also do a search for just IDSS on Dans build thread.

More gain = lower value in the IDSS department. This leads to lower headroom though. This sort of speaks to your offset question.

Really scan the links at the top of Poctops (Dan) page especially the ones regarding FET biasing also:

http://www.groupdiy.com/microphones/newbie-builds-u87-clone/msg640898/?topicseen#msg640898

The Neumann U87 and U87A are truly different. The gain level you speak of is a bit high but so much can effect this. So go back at bias and try again.

 

[kante1603]

Hiss static but when moving mic near other gear thats on, hum appears also hum appears when touching switches.

Hello,

that's a 100% hint on a bad grounding.Audibly the mic should be pretty close to dead quiet.Meaning you won't notice it while listening,not technically of course.
You're very very close to get it working!

Best,

Udo.

 

[sr1200]

@pip, The mic is biased correctly, and sounds phenominal. I just noticed though that more gain could be obtained without distortion if there was a way to apply a positive DC offset to the AC signal hitting the FET with little to no space (its already pretty tight in there). 

 

[JessJackson]

Hiss static but when moving mic near other gear thats on, hum appears also hum appears when touching switches.

Hello,

that's a 100% hint on a bad grounding.Audibly the mic should be pretty close to dead quiet.Meaning you won't notice it while listening,not technically of course.
You're very very close to get it working!

Best,

Udo.

yeah interesting, I have 0 ohms everywhere on the mic body and pin one xlr. Outer metal shell of switches have 0 ohms but switch lever itself is not on ground.

I used the smaller switches reffered to somewhere in the middle of the thread.

Cheers

J

 

[Pip]

@pip, The mic is biased correctly, and sounds phenominal. I just noticed though that more gain could be obtained without distortion if there was a way to apply a positive DC offset to the AC signal hitting the FET with little to no space (its already pretty tight in there). 

\

Just out of curiosity how closely matched are your R18 and R19?

 

[sr1200]

I will have to check that... good call though.

 

[sr1200]

Pulled out the resistors and checked, theyre identical to 3 decimal places (my meter only goes to 3).  I decided to just put it at max gain (well, just before) regardless of the clipping on the bottom.  I then hooked the capsule back up, and played some sine waves through it.  (some pretty loud) and I see NO clipping on the recorded signal... hmmmmmm.  Problem solved? lol

 

[Mr.GrinchSD]

Finished. With T13 transformer and Tskguy's Capsule!!!!!!!!