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

[SebC]

Picture 2

 

[SebC]

Picture 3

 

[59flame]

i need help with  a question on the psu pcb.  R6  and r8 are reversed as far as the labeling on the pcb  vs danys psu schematic. Can someone tell me if the pcb labeling or the schematic is right? thanks

 

[thesoundmotel]

What's the wiring configuration for the Cinemag CM-5701 transformer ? Could someone point to it . Thanks in advanced .

 

[HeavyHand]

this is what i came up with for the cinemag transformer.  emailing them about it might help.

 

[bremusound]

Hi,

the genuine Neumann  U67 (Re-issue) Bv.12  transformer is available @Sennheiser UK:

https://spares.sennheiser.co.uk/neumann/neumann-u67/neumann-u67-transformer-bv12-576521.html

I just stumbled across it so this may be interesting for some guys here...

 

[emrr]

I’ve heard the new Neumann BV.12 and the Moby BV.12 back to back in the same mic and found them to be extremely close to one another.  I’ve no problem recommending the Moby version, and it’s definitely at some real savings. 

 

[myers1st]

Hi everyone-

I'm working on finishing up my D-U67.  I've got Max's BV.12 and trying to figure out how to connect it correctly.  Does anyone have a diagram or know which ioaduio bv12  pins correspond to the AMI?  I do not have the MKBV12 PCB set, so will have to wire it directly to the mic PCB.  Kinda confused as the ioaudio has 16 pins and the AMI has only 12.  Thanks!

 

[emrr]

Diagram of the ioaudio bv12?  The extra pins are additional shield connections. 

The 8 pairs of contacts, a pair being one on each side of the winding, are each a winding.

Looking at the U67 schematic, and with the transformer center vertically oriented relative to yourself ( a row of contacts on left and right relative to you, either correct or upside down, looking at the bottom of the transformer), you will be able to tell based on DC resistances:

1 Secondary / tie to 5 - 18DCΩ
2 Tertiary / tie to 6 - 45DCΩ
3 shields, either side, brought out to 2 points
4 Primary to ground / tie to 8 - 1040DCΩ
5 Secondary / tie to 1 - 18DCΩ
6 Tertiary / tie to 2 - 45DCΩ
7 shields, either side, brought out to 2 points
8 Primary to C9 / tie to 4 - 1040DCΩ

 

[Murdock]

Quick question about the "R2 problem". 
I have a power transformer with 20V/500mA secondary.
But I just saw, that one should not use a power transformer with more than 300mA...
So am I right that a 5W resistor for R2 is not enough for this transformer?
Do I need one with at least 7W rating?
The 5W 25 Ohm resistor I have in now gets really hot. About 100 degree C and in the datasheet it says that the power rating goes down by 40% at this temperature...
Is there any other solution? As a 7W resistor barely fits in there...

 

[TLRT]

Quick question about the "R2 problem". 
I have a power transformer with 20V/500mA secondary.
But I just saw, that one should not use a power transformer with more than 300mA...
So am I right that a 5W resistor for R2 is not enough for this transformer?
Do I need one with at least 7W rating?
The 5W 25 Ohm resistor I have in now gets really hot. About 100 degree C and in the datasheet it says that the power rating goes down by 40% at this temperature...
Is there any other solution? As a 7W resistor barely fits in there...

The problem was that Neumann used a 14 or 16V trafo, not 20 in NU67. I remember we recalculated it with Dany ,so you need 40ohms/5watt for R2, otherwise you overload the zener and it will unsolder itself, and a short circuit will happen between the heater and the B+ line. (fuse will break)
Use this small size 40ohms/5W wirewound from Mouser, and lift it up to the air:605-RWHSE09TU040R0FS, I have spares if needed.

 

[Murdock]

The problem was that Neumann used a 14 or 16V trafo, not 20 in NU67. I remember we recalculated it with Dany ,so you need 40ohms/5watt for R2, otherwise you overload the zener and it will unsolder itself, and a short circuit will happen between the heater and the B+ line. (fuse will break)
Use this small size 40ohms/5W wirewound from Mouser, and lift it up to the air:605-RWHSE09TU040R0FS, I have spares if needed.

Thanks for the info!
Now I also see it. Schematic says 20VAC secondary and -19V after rectifier which is not normal.
Thats good to know!
But I just realised that I have a 300mA secondary. Mixed it up with another transformer...

I took some measurements
I get about -23V for H- after the rectifier and about -16V after R2 (25 Ohm). So there is 280mA running through this resistor, right?
7*0,28=1.96W
I need a minimum Resistor Wattage of 1,96W and I have a 5W Resistor.
I don't understand why it get's so hot.

Also I have a slight hum problem... I tried different grounding schemes. Some make it worse but none makes it dissapear.
Could a damaged zener be the source or a hot resistor?

 

[TLRT]

Thanks for the info!
Now I also see it. Schematic says 20VAC secondary and -19V after rectifier which is not normal.
Thats good to know!
But I just realised that I have a 300mA secondary. Mixed it up with another transformer...

I took some measurements
I get about -23V for H- after the rectifier and about -16V after R2 (25 Ohm). So there is 280mA running through this resistor, right?
7*0,28=1.96W
I need a minimum Resistor Wattage of 1,96W and I have a 5W Resistor.
I don't understand why it get's so hot.

Also I have a slight hum problem... I tried different grounding schemes. Some make it worse but none makes it dissapear.
Could a damaged zener be the source or a hot resistor?

I use also 20V/500mA secondary, so with 22R,it is both the R2 and the zener overloaded P=2.88W for R2 and 5,74W calculated on the zener. This is the reason why R2 gets so hot and it melts the soldering and make a short circuit with the B+ resistor which is very close. So if you raise R2 to 40 ohms then the R2 P goes down to 1,7W and the zener P is 3.54W which is just good and safe. If you change the zener to 18V or higher then you would need to change the filtercaps also to higher value.

 

[Murdock]

I use also 20V/500mA secondary, so with 22R,it is both the R2 and the zener overloaded P=2.88W for R2 and 5,74W calculated on the zener. This is the reason why R2 gets so hot and it melts the soldering and make a short circuit with the B+ resistor which is very close. So if you raise R2 to 40 ohms then the R2 P goes down to 1,7W and the zener P is 3.54W which is just good and safe. If you change the zener to 18V or higher then you would need to change the filtercaps also to higher value.

Thanks alot, I will try the 40 Ohm resistor.

I also just noticed that the hum doesn't get attenuated when I switch in the -10dB pad... It seems to be even louder but that is probably due to the hum not being overwhelmed by the surrounding noise.

Is that any indication on where the hum comes from?

 

[TLRT]

Thanks alot, I will try the 40 Ohm resistor.

I also just noticed that the hum doesn't get attenuated when I switch in the -10dB pad... It seems to be even louder but that is probably due to the hum not being overwhelmed by the surrounding noise.

Is that any indication on where the hum comes from?

Check your grounding and the caps, no other idea...

 

[Murdock]

Hmm, will try the 40 Ohm resistor and a new diode.

Another question: When I ordered the parts for my U67 build, Mouser only had 68M resistors instead of the 60M ones.
I saw in one schematic that Neumann once also used this value...
How critical is it? 

EDIT: The 25 Ohm resistor definately wasn't the right one. It constantly tried to draw 320mA of current although my transformer is only rated for 300mA.
I've put in the 40 Ohm resistor of my second PSU and a new diode while I wait for the new resistors. Now the diode doesn't get that hot anymore but the hum is not really better. Maybe a little bit but it's still there...

Could the power transformer be damaged after being driven into saturation for some time?

I'll also try  to increase the filter capacitors to 47uF and 1000uF.

I'm puzzled why the hum doesn't decrease when I switch in the -10dB pad....

 

[TLRT]

Hmm, will try the 40 Ohm resistor and a new diode.

Another question: When I ordered the parts for my U67 build, Mouser only had 68M resistors instead of the 60M ones.
I saw in one schematic that Neumann once also used this value...
How critical is it? 

EDIT: The 25 Ohm resistor definately wasn't the right one. It constantly tried to draw 320mA of current although my transformer is only rated for 300mA.
I've put in the 40 Ohm resistor of my second PSU and a new diode while I wait for the new resistors. Now the diode doesn't get that hot anymore but the hum is not really better. Maybe a little bit but it's still there...

Could the power transformer be damaged after being driven into saturation for some time?

I'll also try  to increase the filter capacitors to 47uF and 1000uF.

I'm puzzled why the hum doesn't decrease when I switch in the -10dB pad....

No matter you can use 60M or 68M either.(68 is cheaper I think)

 

[myers1st]

Awesome, this was exactly what I needed, thanks!

Diagram of the ioaudio bv12?  The extra pins are additional shield connections. 

The 8 pairs of contacts, a pair being one on each side of the winding, are each a winding.

Looking at the U67 schematic, and with the transformer center vertically oriented relative to yourself ( a row of contacts on left and right relative to you, either correct or upside down, looking at the bottom of the transformer), you will be able to tell based on DC resistances:

1 Secondary / tie to 5 - 18DCΩ
2 Tertiary / tie to 6 - 45DCΩ
3 shields, either side, brought out to 2 points
4 Primary to ground / tie to 8 - 1040DCΩ
5 Secondary / tie to 1 - 18DCΩ
6 Tertiary / tie to 2 - 45DCΩ
7 shields, either side, brought out to 2 points
8 Primary to C9 / tie to 4 - 1040DCΩ

 

[myers1st]

So I've got my PSU and mic built.  Unloaded PSU voltages check out.  Plug in mic with no tube and I get the same voltages on the mic... H- at -16V, trimmed B+ to 210V no problem... But I stick the tube in and my H- drops to -3.2V with the pot up all the way and my [a] voltage is 107V (think I read it should be 75?).  I've looked over everything and can't find a wiring problem....

Could it be a bad tube?  It's the only one I have currently so I can't swap out to test, but got a couple more on order.  Something else maybe?

Edit: I also verified that the -16V is reaching pin 5 of the tube socket and there is continuity between pins 4 and 5 on my tube, so it seems the problem must  be after that?

Edit:  ‘Twas a bad tube.  Got a couple Mullards to test out and I’ve got -6.3V.!!! Excited to report back with how great this sounds!  IO transformer and Heiserman capsule.

 

[Murdock]

What exactly is R4/R7 in the PSU good for?
Is it just a bleeder resistor?
If so, why did they change it that often (68k, 100k, 160k)?

I have a 220V 10ma secondary. Should I change the value to a lower resistance?

Edit: I think I got it. They changed it according to the power transformer power rating. The bleeder resistor acts as a kind of voltage regulator. In one schematic you can see, that the 220v secondary is rated with 4ma. The 68k bleeder resistor, while shunting 3ma to ground, ensures that no more than 1ma is going to the anode.

Is that right?