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

[HeavyHand]

i'm ready to install the transformer but im not sure how to secure it in place. any ideas? I'm using the Cinemag. on page 53 of this thread Dan has some pics of the transformer sitting in the bottom bell of the mic but I can't figure out how to keep it in place.

 

[tamtamstudio]

Yes! my babies seems sounds correctly! what a sound, now i've got a  small noise floor.
AT67 seems right connected, except for the reversed yellow label, but marks seems correct.
At last,  i've got a tedious hum at 100 Hz noise (why?) as u can see in the attachment.
I've modded as suggested some pages ago, with switch for hard ground, better on RF, not on this Hum.
Anybody experienced that?
thank you very much.
Corrado

 

[poctop]

i had lowered the voltage on this psu unit on the heater to prevent extra dissipation on the heater line ,
but i realized that the 16V might be a little short in power so i have replaced the power specification in this build,
it was 6VA 16V and i changed it to 12VA , can you let me know what PN number did you use for the heater traffo ,
try and lower the voltage of the heater to 5.8V to see if it dissapear , do you have the 16V traffo or the 20V ,
Let me know utlimately you will be needing the 600ma rated 16V traffo to solve this problem ,
best,
Dan,

 

[tamtamstudio]

thank you Dan, here my traffo it's 6VA @ 16V
detail https://www.mouser.it/ProductDetail/530-DP-241-4-16

probably isn't enough, later today i try to lower to 5.8V and let see.
at last i'll try to find a traffo 12VA here to check it.
Or I'll order a couple of new one, following specs.

 

[TLRT]

You can mofidy the resistors and then the 16v tranny is fine. I have custom wound 200V/16V -6VA, R2 needs to be lowered to 18ohms, the 47R paralell with the pot back to 22-27R, the 18ohms to 14, and even the 5,1R to 4(so all back with 20% approx) in the meantime I get an original NU67 and the secondary is 14,4V only, so the neumann schematics is not correct what you can see on the web , or they calculted with DC after rectification(so then the 20 is fine but not AC but DC)

 

[tamtamstudio]

I tried to lower down the heater to 5.8V, no changes.
at last i tried also to cut away the small light on 16V rail and no changes.
Unfortunately i haven't got a 16V 12VA here around to test it.

To TLRT, I'm try to find some resistors here around to adapt as you written, let's  see.

I will order a new on e at mouser (same family of that but with 12VA) and let see.

here's some snapshots of my tests.

 

[tamtamstudio]

I found here around a 18V 9VA, I tried to change, unfortunately there's no differences at all.
Maybe the noise floor seems a little bit better, but it's dependent  'cause i'm on a different preamplifier.
100Hz hum still there.

 

[TLRT]

I found here around a 18V 9VA, I tried to change, unfortunately there's no differences at all.
Maybe the noise floor seems a little bit better, but it's dependent  'cause i'm on a different preamplifier.
100Hz hum still there.

R2 is critical,reduce it to 15-18ohms.(under 18ohms you will need 5W) make sure the zener's load should not go above 5W also because of R2's change! You can increase C2 from 500uf to 1000uf also.

 

[poctop]

i am still investigating this ....... i know for sure the 20V 6VA will work with no issue but it has to respect and be limited to 6VA not more not less than that it has been working for years ,  i would recommend to increase R2 to 5W as the dissapation is about 1.8W with the 6VA , to make sure this problem will go away use the 20V 6VA only ,

i am still yet to have some answer on the 16V 12VA  exploration continues , since i wanted to get this thing to run cooler a just opened a pandora box , i know later U67 were retrofitter with 10W zener and R2 5W , although the zener 10W is a much different package altoghter ,

TLRT is right about the 16V it can be used here but all the resistor need to be tweaked about 20% down , to allow proper regulation ,  but 20V 6VA will work with the stated resistor in the pcb and bom.

So in the meantime what i recommend are the new 5W 22ohm or 25ohm resistor option i added in the BOM , lift the 5W zener off board a little and R2 as well of the board and use the recommended hammond 20V traffo wich is 6VA not more than that to make sure R2 do not dissipate more than needed,

Sorry for the trouble it caused , i was just trying to improve thing wihout changing too much of what it is ,
but i was wrong ,

Best,
DAn,

 

[tamtamstudio]

So in the meantime what i recommend are the new 5W 22ohm or 25ohm resistor option i added in the BOM , lift the 5W zener off board a little and R2 as well of the board and use the recommended hammond 20V traffo wich is 6VA not more than that to make sure R2 do not dissipate more than needed,

Hi Dan, thank you!
what do u think this one can be ok?

https://www.mouser.it/ProductDetail/Hammond-Manufacturing/186B20?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGyR%2fQB%252b1BzrOY%3d

 

[poctop]

yep this is the one , best is to have dual primary in all case 
best,
Dan,

 

[tamtamstudio]

yep this is the one , best is to have dual primary in all case 
best,
Dan,

thank you, I'll update as i receive it!

 

[tamtamstudio]

I changed the R2 to 25R 5W and the traffo with the Hammond 20V 6VA and seems perfect!
the 100Hz Hum is gone, thank you again for the support!
all the best, Corrado.

 

[poctop]

I changed the R2 to 25R 5W and the traffo with the Hammond 20V 6VA and seems perfect!
the 100Hz Hum is gone, thank you again for the support!
all the best, Corrado.

 

[winzer]

Hi!

I have a general question regarding the +210V B+ line in the power supply:
Is there a reason for the  differences to the original values? the neumann schematic ( http://www.tuberadio.com/robinson/museum/Neuman_U67/NeumanPSUcct.gif ) uses a 100k resistor for R7 instead of 160k in Danys. The R3 ist 8,2k fixed instead of 8,2k+56k+0-25k(Poti)....
Wouldn´t it work with a 100k R7 Resistor and only the 25k Poti?

 

[dmp]

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!

If I understand correctly, you are lifting the shield (p3) and 0v (p7) from the chassis ground, which includes lifting the metal mic body. This is unsafe if so!  Everybody that builds a tube mic should make sure the mic body is earthed before turning it on with a multimeter. 
The metal mic body should always be connected to earth since the mic has +210v inside. If that high voltage were to accidentally short to the mic body it NEEDS a path to earth so the fuse will blow. Otherwise the metal mic body will go to 210v and potentially fry someone. 

In the Neumann U67 mic schematic, p7 and p3 are connected together and to the mic body, within the mic itself. In the Neumann PSU, the lift switch disconnects audio ground (P7) from the chassis, which is a redundant connection with the P3/P7 connection in the mic. The XLR out connector ground is always lifted in the Neumann schematic if I am reading it right.

In poctop's schematics, the XLR p1 is always connected to chassis (no lift) and the lift (SHLF) disconnects P3 (shield) in the psu ( from chassis ground), which is incorrect in comparison to Neumann schematic ,as you pointed out.

A ground lift switch could lift pin 1 of the XLR out. That would be the first thing to try after doublechecking the mic body is grounded to earth through a single path.

I know I had a hum using my build at a friend's house so my mic has an issue too. I'll look into this further

 

[Delta Sigma]

In mrerdat's grounding, the power supply chassis is grounded to the wall outlet at the IEC inlet. The mic body is connected to the mic cable's shield. The mic cable's shield is connected to the power supply chassis. Safe!

As in the Neumann schematic, the 0V return is connected to the cable shield inside the microphone. It is, however, only connected to the power supply chassis via switch. In other words, the power supply board's 0v path to earth ground with the ground switch open would be from the power supply pin 7 to the mic to the shield / pin 3, to the power supply chassis, to earth ground.

This gives audio only one path to ground, opening a ground loop if one is present.

FYI, on my M49 I did the opposite of the Neumann schematics to try kill hum. I isolated 0V and the shield / mic body but connected 0V inside the power supply chassis. It did not kill the hum. Apparently those Neumann engineers knew a thing or two!

 

[dmp]

In mrerdat's grounding, the power supply chassis is grounded to the wall outlet at the IEC inlet. The mic body is connected to the mic cable's shield. The mic cable's shield is connected to the power supply chassis. Safe!

He says:

another position that lifts BOTH 0V and cable shield from each other and the chassis.

Pretty clear that the mic body is being lifted from the chassis & earth connection. I don't think this should be done for safety reasons.
Easy to check anyway - use a multimeter to make sure the mic and psu chassis are earthed before turning the power on. It is very important in tube mics with dangerous voltages!

 

[Spencerleehorton]

I am glad this noise issue had been brought up as I have built 2 and I’m experiencing hum too on both.
I want to build another two but want to solve this noise issue.
I will look into it further and try what has been suggested.

Should the noise floor be comparable to the u87?

Regards

Spence.

 

[Delta Sigma]

He says:
Pretty clear that the mic body is being lifted from the chassis & earth connection. I don't think this should be done for safety reasons.
Easy to check anyway - use a multimeter to make sure the mic and psu chassis are earthed before turning the power on. It is very important in tube mics with dangerous voltages!

I agree that lifting pin 3 and the shield sounds bad but a DMM may still show continuity through the cable's connector. If the cable shield is wired to the connector housing that should ground the mic. mrerdat's post is confusing because of the U67's pin 3 being a shield connection as well as the cable shield itself.

I really should re-read his posts as I've posted a link to them a couple times. Once I understood mrerdat's point I referred to the Neumann schematic when installing my ground lift.

What you are saying, though, it VERY good practice. Setting a DMM to continuity and looking for continuity of grounds and to check for shorts is smart. Sorry to derail your point!