Neumann Vintage U87 Clone : Build Thread.

[xqtion]

Hmm my DMM always shows in mV instead of just V. Anyways I set it so that it measures 10.5mV at the drain and then when I measure the potentiometer resistance, it comes out to 7.8k.

Does all this sound right ?

Also I plan to swap C7 tantalum with mundorf 1uf polypropylene cap later. Do I have to re-measure the bias when I do the C7 swap?

Thanks always!!

 

[poctop]

Hmm my DMM always shows in mV instead of just V. Anyways I set it so that it measures 10.5mV at the drain and then when I measure the potentiometer resistance, it comes out to 7.8k.

Do you mean 10500 mv  that would make more sense , 7.8K is in the ballpark ok, 
if you change the cap it would not hurt to Re-Bias in the final configuration for sure,
Hope this helps,

Dan,

 

[xqtion]

Hmm I've connected the capsule and I'm not getting any sound...

Is this Cinemag connection correct ?

RT = red from cinemag 2480 Transfo
SW = brown from cinemag 2480 TRansfo
WS = yellow from cinemag 2480 Transfo
BL =  orange fromcinemag 2480 Transfo,

Maybe the orange should goto WS ?

Any ideas ?

Also when I listen thru headphones, I hear this wind sound...???

 

[poctop]

Here is the cross reference for the CM2480 so the connection above are correct.
,  did you make sure you check all the initial voltage first ,  there are mentionned above in the thread,

do you have your drain voltage to 11V DC ,

is your mic body shielded and closed , ?

verify your capsule connection ?

 

[xqtion]

Drain voltage is still 10.5mV.
I'ma goto sleep for now and do more trouble shooting tomorrow. Maybe XLR pin 1 and 2 should be swapped ?
Thanks guys I'll let you guys know how it goes...

 

[saxmonster]

I made my drain volt 10.5 should i make it 11volts would i notice a difference?  What about 10 volts, maybe then i would have a resistor that would work instead of the 15.6kohms i need now which no one has.  or any combination of resistors.  errrr.. would still like to get rid of the pot.

 

[poctop]

10.5-11V is the safe Zone, only a extensive Bias( room captulre THD or Matador Method )  you will give the optimum value,  that doesnt mean it might be very different  that much from that safe Zone anyway at the end the mic has to sound good to your hear ,  but still there is no counter indication to leave the pot inside if you can.

i will be building 2 extra mic very soon and i will try to setup a procedure with the Room capture software for evryone to see how it works,

Dan,

 

[poctop]

Drain voltage is still 10.5mV.
I'ma goto sleep for now and do more trouble shooting tomorrow. Maybe XLR pin 1 and 2 should be swapped ?
Thanks guys I'll let you guys know how it goes...

Are you measuring  DC or AC 10.5mv is not gonna work you need approximatively 10.5-11V DC on the drain ,

Dan

 

[Matador]

I made my drain volt 10.5 should i make it 11volts would i notice a difference?  What about 10 volts, maybe then i would have a resistor that would work instead of the 15.6kohms i need now which no one has.  or any combination of resistors.  errrr.. would still like to get rid of the pot.

10-11V DC on the drain should be reasonable for most FETs that are around nominal values for IDSS.  The sound won't change much around this point, but headroom might be slightly less than optimum.

Where I noticed it most:  the signal to the FET gate must be pretty hot.  I tried close mike'ing a drum head, and set the bias so the signal clipped after about 100mV on the gate (drain sat at about 15V).  On quiet hits it sounded fine but on a hard hit there was an audible SPLAT and burst of distortion.

All in all I wouldn't worry about it:  this mike wasn't designed for that kind of duty in any case, but if you like to sweat the details then you can squeeze a bit more headroom on loud transient sources by hand selecting the FET and optimizing the bias point.

 

[poctop]

I guess we could add to that that there is actually 2 ideal Bias point where 1 is optimized for THD and the other one is optimzed on headroom so it is up for us to decide, 

Dan,

 

[saxmonster]

I made my drain volt 10.5 should i make it 11volts would i notice a difference?  What about 10 volts, maybe then i would have a resistor that would work instead of the 15.6kohms i need now which no one has.  or any combination of resistors.  errrr.. would still like to get rid of the pot.

10-11V DC on the drain should be reasonable for most FETs that are around nominal values for IDSS.  The sound won't change much around this point, but headroom might be slightly less than optimum.

Where I noticed it most:  the signal to the FET gate must be pretty hot.  I tried close mike'ing a drum head, and set the bias so the signal clipped after about 100mV on the gate (drain sat at about 15V).  On quiet hits it sounded fine but on a hard hit there was an audible SPLAT and burst of distortion.

All in all I wouldn't worry about it:  this mike wasn't designed for that kind of duty in any case, but if you like to sweat the details then you can squeeze a bit more headroom on loud transient sources by hand selecting the FET and optimizing the bias point.

What is the "nominal values for IDSS"  I have two other fets hear to look at and or try.  Also The higher the voltage, say 11v compared to 10v would give you more headroom or is it the opposite?  in other words which gives you more head room 10v or 11v?  Louder or softer I guess right?  If I have a preamp with lots of gain I could go for the less headroom right?

-Scott

 

[wave]

What is the minimum power requirement for the biasing resistor?

Dave

 

[saxmonster]

I think when I was looking it was a 1/4 watt?  Don't quote me though it could be 1/2 watt anyone know for sure?

 

[Matador]

What is the minimum power requirement for the biasing resistor?

Dave

The source resistor has perhaps 1mA going through it and perhaps 2V dropped across it.  That's only 2mW....so essentially anything you can put in there will be adequate.

What is the "nominal values for IDSS"  I have two other fets hear to look at and or try.  Also The higher the voltage, say 11v compared to 10v would give you more headroom or is it the opposite?  in other words which gives you more head room 10v or 11v?  Louder or softer I guess right?  If I have a preamp with lots of gain I could go for the less headroom right?

-Scott

The nominal value is that given in the datasheet:  somewhere around 10mA perhaps.  My statement was generic and does not apply to an individual FET in an individual board, etc.  "Optimum" may be 9V on one board, and 11V on another....this is why the biasing procedure must be done on each and every board/FET combination.  This is not unique to Dany's boards:  it's just a fact with this circuit topology.

Staying generic and speaking from 10,000 feet up:  running at higher currents should provide more gain, however everything else being equal it will take less input signal before the stage clips, which means headroom would be less.  In reality, VGSoff, IDSS, and gm all vary from transistor to transistor:  biasing by scope or by THD method optimizes the actual circuit around all of these (to the extent needed by this circuit).

So don't worry too much about the drain voltage unless it's WAY different from about half of the supply voltage (which in this circuit is about 21V at the top of the drain resistor).

 

[wave]

I asked that because saxmonster couldn't find a 15.6K resistor to replace the trimmer.
I found this one on mouser

http://www.mouser.com/ProductDetail/Vishay-Dale/RN55C1562BRE6/?qs=NVJATC80C4%2fyLVR6unXY27Zon6%2fPsp8TbniFs21qaqY%3d

it's 100mW
Could it be used?

Dave

 

[poctop]

I asked that because saxmonster couldn't find a 15.6K resistor to replace the trimmer.
I found this one on mouser

http://www.mouser.com/ProductDetail/Vishay-Dale/RN55C1562BRE6/?qs=NVJATC80C4%2fyLVR6unXY27Zon6%2fPsp8TbniFs21qaqY%3d

it's 100mW
Could it be used?

Dave

Absolutely, Nice Gesture Dave ,

Dan,

 

[poctop]

I have heard good things about this software based oscillscope so if someone is interested in trying it ,

here it is:
https://cdn.groupbuilder.com/groupdiy/u/39511/58d0281993770

Cheers,
Dan,

 

[kante1603]

Hi,

I did some first trials to bias mine without a capsule  or trafo connected and ended up as follows:
Using a scope and a variable sine generator gave me a nice feel,I was able trimming the pot pretty well where-while playing with the generator level forth and back-I easily was able to get a symmetric clipping.Nice so far I thought.
After that I measured the drain voltage at 11,9vdc and the pot at 12,56kohms (in-circuit,pardon me).
I used a fet at an idss of 8,7 which was the lowest one from a shootout from arround 20 pieces I had handy.
My feeling says that I'm right in trusting my scope readings.
Boosting the generator level gives me a really nice identical clipping at the same time and amount on both sides of the sinewave.
Condition was inserting the signal at c4,not at the recommended insert point between r9 and r10.
Will repeat this at the weekend on the measuring inputs,don't have the time now.

But in general:Am I right trusting the scope or am I too far off with drain voltage and/or resistor value?

Thank you all,

Udo.

 

[poctop]

Hi,

I did some first trials to bias mine without a capsule  or trafo connected and ended up as follows:
Using a scope and a variable sine generator gave me a nice feel,I was able trimming the pot pretty well where-while playing with the generator level forth and back-I easily was able to get a symmetric clipping.Nice so far I thought.
After that I measured the drain voltage at 11,9vdc and the pot at 12,56kohms (in-circuit,pardon me).
I used a fet at an idss of 8,7 which was the lowest one from a shootout from arround 20 pieces I had handy.
My feeling says that I'm right in trusting my scope readings.
Boosting the generator level gives me a really nice identical clipping at the same time and amount on both sides of the sinewave.
Condition was inserting the signal at c4,not at the recommended insert point between r9 and r10.
Will repeat this at the weekend on the measuring inputs,don't have the time now.

But in general:Am I right trusting the scope or am I too far off with drain voltage and/or resistor value?

Thank you all,

Udo.

Trust the Scope  ;D
what you see is what you get,

What then would be your Final Source Voltage and Iddle Current trough R11 Then? I am really curious how the THD method and Scope method will closely relate, i know i used both and they were very  similar results,  i am building 2 more so i will be able to add to this Topic, the Tricky Part is to Stop as in ( dont go to far for nothing bias Point where you loose SPL), I have found with the THD Method that at certain point arround 0.39% that the increase in Drain Voltage reflect in very poor decrase in THD there is where you stop.
impressively earing works well, can also try  is very fun and challenging i have done it many times as a 3rd method,
you inject the 1K and and right after passing the opening Fiz you will hear a nano-hiss  and just pass this point if you continue just a little bit and concentrate on the nano hiss you will hear it go faint on a very precise spot.
For now it makes all sense, you are in the Zone  8)
Dan,

 

[kante1603]

Trust the Scope  ;D
What then would be your Source Voltage and Iddle Current trough R11 Then? I am really curious how the THD method and Scope method will closely relate, i know i used both and they were very  similar results,  i am building 2 more so i will be able to add to this Topic,
For now it makes all sense, remember what you see is what you get,
Dan,

Thanks Dan,

it was just a quick shot I made,and having bought the scope pretty cheap on ebay (Old 2-ch. HAMEG HM204-2) it had to remember the how to use it stuff in a hurry.Haven't used one for about 25 years,but all is coming back now.
"what you see is what you get":that means I only get some green curves and no microphone? ;D

Will work on it a bit more,maybe I have some nice ideas for it,but must check them first....

Thx Dan,

good night,

Udo.