Neumann U67 Clone : D-U67 Tube Microphone Build Thread.

[TLRT]

Congrats!

Arguably the best mic design ever.

Sound samples?

Yes I will prepare them later. We have a pair of original U67's in good condition,so I can make proper A/B tests.

 

[micaddict]

Better yet. 

 

[loubli]

I made DU67 PCB.but zener diode and R2 became very hot.R2 resistor became red and having smoke one time.
I checked parts again and again with tester.and changed another parts'.each of parts seems no problem.and microphone get correct voltage.there is same case or anybody know where is problem?

 

[poctop]

I made DU67 PCB.but zener diode and R2 became very hot.R2 resistor became red and having smoke one time.
I checked parts again and again with tester.and changed another parts'.each of parts seems no problem.and microphone get correct voltage.there is same case or anybody know where is problem?

A) Wrong Input voltage
B) Wrong Value of Resistor
C) Wrong Power ( Watt) component ,
D) Pulling too much current
E) Component mistake
F) Short ?

would the be thing i am looking at first ,
i would not plug the mic until i can dummy load test the  power supply first and assess the heat dissipation and proper voltage and voltage drop acrros the filters ,

BEst,
dAN

 

[loubli]

Thanks Dan!

I checked everything again.but seems no problem.
R2 place has about 5~6V,0.22A.so that place needs 1W.then schematic says 2W.
actually I tried 22ohm 5W and 15W on R2.5W became very hot,15W became hot.
your R2 doesn't have any hot at all?

 

[mrerdat]

I had time to investigate the grounding issue I mentioned a few pages ago.  It was still bothering me because at some studios the mic had loud hum issues and the only surefire way to get rid of that hum was to use a cheater plug.... which is not safe.  So in attempting to find a more permanent solution I made the discovery that Poctop's grounding scheme in his PSU PCB layout is different than what's laid out in the Neumann schematic:

In his grounding scheme, the earth ground is connected to the chassis first, then to 0V which is connected to pin 1 of the 3 pin XLR with the option to tie pin 3 of the 7-pin XLR (cable shield) via SHLF jumper.  When I tried using the SHLF jumper in various situations I found that it didn't help at all. 

In the Neumann NU67 schematic, if I'm not mistaken it looks like pin 3 of the 7-pin XLR is connected to the chassis via a switch, and from that connection to 0V via another switch.  This is the only point in the schematic where 0V connects to the chassis, so the ability to lift 0V from chassis ground was part of the original design. 

If you wire the PSU exactly as written in Dany's guide, you'll find that there's no way to lift 0V from the chassis if you are in a place with dirty power/bad grounding.  It's tied in there no matter what, doesn't matter if SHLF jumper is in or not.  I think that this might be the cause for a lot of the grounding issues some people were talking about before:

Hi, quick question for anyone in the know. I have built two of these microphones and they sound great. However I'm finding that in certain places (My Studio) i am picking up noise from the mains power (Lights/LED). The noise is in the form of fizzles and hum, fairly low but audible if your doing voice overs. At my house there is no noise at all. has anyone else encountered this? i have other valve mice in my studio that don't have noise on them, have i just got my grounding wrong?

Pictured below is my workaround to bypass the permanent 0V to chassis ground connection via a switch that I had.  Note that the chassis connection to the 0V terminal block (not pictured) has been disconnected.  The safety ground is still connected to the chassis at one of the mounting bolts for the 20V transformer.  The 3-pin XLR PCB connections are bypassed entirely because pad 1 depends on 0V being tied to ground.  If connected to the PCB, pad 3 for the 7-pin  XLR will not connect to pad 1 for the 3-pin XLR unless the SHLF jumper is in.

[image removed to avoid confusion- see post #1130 for updated wiring]

In the future if I need to I might buy a triple throw switch so I could wire the positions:

1: pin 3 and 0V tied to chassis ground
2: pin 3  connected to chassis ground, 0V lifted
3: pin 3 and 0V lifted

I'll report back when I test my current configuration out at studios that have bad hum issues.

 

[poctop]

I had time to investigate the grounding issue I mentioned a few pages ago.  It was still bothering me because at some studios the mic had loud hum issues and the only surefire way to get rid of that hum was to use a cheater plug.... which is not safe.  So in attempting to find a more permanent solution I made the discovery that Poctop's grounding scheme in his PSU PCB layout is different than what's laid out in the Neumann schematic:

In his grounding scheme, the earth ground is connected to the chassis first, then to 0V which is connected to pin 1 of the 3 pin XLR with the option to tie pin 3 of the 7-pin XLR (cable shield) via SHLF jumper.  When I tried using the SHLF jumper in various situations I found that it didn't help at all. 

In the Neumann NU67 schematic, if I'm not mistaken it looks like pin 3 of the 7-pin XLR is connected to the chassis via a switch, and from that connection to 0V via another switch.  This is the only point in the schematic where 0V connects to the chassis, so the ability to lift 0V from chassis ground was part of the original design. 

If you wire the PSU exactly as written in Dany's guide, you'll find that there's no way to lift 0V from the chassis if you are in a place with dirty power/bad grounding.  It's tied in there no matter what, doesn't matter if SHLF jumper is in or not.  I think that this might be the cause for a lot of the grounding issues some people were talking about before:

Hi, quick question for anyone in the know. I have built two of these microphones and they sound great. However I'm finding that in certain places (My Studio) i am picking up noise from the mains power (Lights/LED). The noise is in the form of fizzles and hum, fairly low but audible if your doing voice overs. At my house there is no noise at all. has anyone else encountered this? i have other valve mice in my studio that don't have noise on them, have i just got my grounding wrong?

Pictured below is my workaround to bypass the permanent 0V to chassis ground connection via a switch that I had.  Note that the chassis connection to the 0V terminal block (not pictured) has been disconnected.  The safety ground is still connected to the chassis at one of the mounting bolts for the 20V transformer.  The 3-pin XLR PCB connections are bypassed entirely because pad 1 depends on 0V being tied to ground.  If connected to the PCB, pad 3 for the 7-pin  XLR will not connect to pad 1 for the 3-pin XLR unless the SHLF jumper is in.

In the future if I need to I might buy a triple throw switch so I could wire the positions:

1: pin 3 and 0V tied to chassis ground
2: pin 3  connected to chassis ground, 0V lifted
3: pin 3 and 0V lifted

I'll report back when I test my current configuration out at studios that have bad hum issues.

Very Good Catch , Thanks for taking the time to this superb analysis and upcoming solution ,
indeed your analysis make all sense,  this switching system should have been dual pole indeed to isolate the chassis from 0V,
i am glad it has been identified and i am glad it is also a simple fix,  please report back on your findings but i have an impression it might get rid of those special situation.
Let me know as i will be putting it in the first page notice as well for future builders,
Best,
Dan, <

 

[mrerdat]

Very Good Catch , Thanks for taking the time to this superb analysis and upcoming solution ,
indeed your analysis make all sense,  this switching system should have been dual pole indeed to isolate the chassis from 0V,
i am glad it has been identified and i am glad it is also a simple fix,  please report back on your findings but i have an impression it might get rid of those special situation.
Let me know as i will be putting it in the first page notice as well for future builders,
Best,
Dan, <

Thanks Dan.  Slightly off topic,  but I'm also building a pair of M49's and your PSU schematic for that has the same grounding scheme as the 67 PSU.  Cross-referencing it to the N52a/t schematics, there doesn't look like there's a connection from 0V or audio ground to chassis ground in the original PSU, so I'll probably leave both floating by snipping the wire that goes to the 0V connection from the chassis, leaving only the chassis connected to earth ground.

 

[poctop]

Very Good Catch , Thanks for taking the time to this superb analysis and upcoming solution ,
indeed your analysis make all sense,  this switching system should have been dual pole indeed to isolate the chassis from 0V,
i am glad it has been identified and i am glad it is also a simple fix,  please report back on your findings but i have an impression it might get rid of those special situation.
Let me know as i will be putting it in the first page notice as well for future builders,
Best,
Dan, <

Thanks Dan.  Slightly off topic,  but I'm also building a pair of M49's and your PSU schematic for that has the same grounding scheme as the 67 PSU.  Cross-referencing it to the N52a/t schematics, there doesn't look like there's a connection from 0V or audio ground to chassis ground in the original PSU, so I'll probably leave both floating by snipping the wire that goes to the 0V connection from the chassis, leaving only the chassis connected to earth ground.

i have made the  jumper to actually to only isolate the cable shield from 0V , in this scheme, but it can be connected to Chassis or 0V easily. all the possibility are worth trying when you have issue with hum or bad power,  Actually ground Scheme tends to be personnal and discutable and much more an habit , Since i never had any issue with star grounding each part of the psu to the box i guess i did not question my way of doing.  i made it a jumper also because so many people gorund scheme their mic their own way.......... until generally it gets silent , avoiding basic ground loop is key for the rest is praticalilty and esthetics effieciency ,  i remember reading a document called the Pin 1 Problem in regard to driving different equipement on realted ground without having issue and the need of a ground lift very interesting article , i am really glad you made this point ,

Please let me know if it does resolve your issue as i would make this page a sticky on page 1 ,
Best,
Dan,

 

[mrerdat]

all the possibility are worth trying when you have issue with hum or bad power

Definitely.  I would imagine that some people who build these mics only use them in 1 or 2 places where there are no issues with power where star grounding might work best.  But for other engineers who work out of different places all the time like I do - wiring a switch to a combination of different grounding configurations makes the most sense. 

Please let me know if it does resolve your issue as i would make this page a sticky on page 1 ,
Best,
Dan,

I don't have a session booked at any of the more problematic spaces this week , but when I do my main goal for the short term is to figure out if it's more useful on a double throw switch to have it toggle between star ground or:

a) lift only 0V from the chassis (pictured above)
or
b) lift both 0V and cable shield from chassis

 

[mrerdat]

I got to test out the ground switch at two different places, including the place where the ground hum was most intolerable.  I found that lifting 0V but not cable shield did not get rid of hum like a cheater plug did, so I rewired the switch to toggle between a star grounding position and another position that lifts BOTH 0V and cable shield from each other and the chassis.  That solved all of the grounding problems I was having! 

For my purposes I've decided to keep the switch simple and not wire any of the in-between positions (lift 0V but not cable shield, connect 0V to cable shield but lift from chassis). 

Here's how my updated wiring looks:

EDIT:  VERY IMPORTANT! Safety ground pin of IEC is still connected to Chassis!

 

[Pip]

Is your mic still grounded?! If not you could kill someone!

 

[micaddict]

FWIW I'll add that Brauner tube mic PSUs hav a three-position switch.
The options are hard ground, ground lift and soft ground.



From the manual:

The positions are as follows:

H (Hard-Ground)
The ground pin (Pin 1) of the XLR connector is
conductively connected to the ground center of
the internal circuit.

L (Ground-Lift)
The ground pin is completely disconnected
from the ground.

S (Soft-Ground)
The ground pin is connected to the ground via
a safety capacitor that decouples the AC portion
of the hum to ground.

From SoundOnSound:

The ‘H’ position is a hard ground, connecting the XLR’s pin one to the chassis earth. The ‘L’ position is a conventional earth-lift mode, allowing pin one to float completely isolated from the chassis. The third position, marked ‘S’, is a ‘soft earth lift’ which couples pin one to the chassis earth through a 0.1µF capacitor, providing a low-impedance path for audio, but very high DC resistance.

Carry on. 

 

[mrerdat]

Is your mic still grounded?! If not you could kill someone!

Yes, the chassis ground is still connected to the safety ground pin on the IEC.  I should have made that clear.  Safety was one of the main reasons for wiring the switch in the first place.  Unless I'm misunderstanding something and not connecting 0V to chassis ground is somehow a safety hazard?  I don't think Neumann would have a lift switch in the schematic if it was.

 

[Christopher L]

Hello I am almost finished with this build and was rechecking my work and came across high resistance on R7 in the power supply.  What should the resistance be across this part unpowered? I am getting very high Mohm resistance across this part but if I hold the multimeter leads across the resistor for about a minute the value drops down to 140-160k. I haven't powered anything up yet.
thanks

 

[Christopher L]

Please help these are my unloaded psu voltages.

Input 272 AC
Input.  20 volts AC
H-  5.64 volts DC
B+ 242 volts DC

Any ideas why the H- is so low? R7?

 

[Christopher L]

I believe power supply to fine unloaded.  I used the wrong test points.  Loaded I now have a prob.  Heater voltage will not drop below 16 volts. 

I read the whole thread. I don't see any others having this problem.

What  should  I check for high heater voltage  loaded? Thanks

 

[mrerdat]

I believe power supply to fine unloaded.  I used the wrong test points.  Loaded I now have a prob.  Heater voltage will not drop below 16 volts. 

I read the whole thread. I don't see any others having this problem.

What  should  I check for high heater voltage  loaded? Thanks

Is the heater at the same voltage when unloaded?  -16 Volts is roughly around what the mic should measure unloaded.  My guess is that the heater is somehow not connected with the PSU.  Recheck continuity at all possible heater connection points (PSU, mic cable, mic XLR, etc) while the mic is powered off.

 

[Christopher L]

That might be the prob since tube does not glow.  Earlier today before your reply I checked connections.  I will check again.  Could a bad tube cause these issues?  Or transformer?

Thanks

 

[Pip]

That might be the prob since tube does not glow.  Earlier today before your reply I checked connections.  I will check again.  Could a bad tube cause these issues?  Or transformer?

Thanks

Check R10 and R11