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

[Spencerleehorton]

Im using the NU-67u power supply schematic from Gotham Audio corporation so i don't know what the SHLF is?

Looking at Danys schematic i see now:

Ive rewired and to my mind it disconnects 0v from chassis whilst still keep chassis to mic?

 

[dmp]

Yup, but the 0V to chassis is permanently connected in the mic, so the SHLF is a redundant connection (creates a ground loop in the mic)

 

[Spencerleehorton]

well i'm pretty certain i have it now as per the schematic.

One other question was the polarity switch, when i touch the switch i get a hum? any ideas? its not connected to anything gnd wise as the actual bit you touch isnt connected to any part of the rest of the metal of the switch?
It may not do it now i have rewired it!!! i will check.

 

[dmp]

well i'm pretty certain i have it now as per the schematic.

One other question was the polarity switch, when i touch the switch i get a hum? any ideas? its not connected to anything gnd wise as the actual bit you touch isnt connected to any part of the rest of the metal of the switch?
It may not do it now i have rewired it!!! i will check.

Strange - you'd want to get rid of that. What kind of switch is it? Might be a low quality switch and the hum is coming from the capsule polarization being sensitive. If it is a good switch like grayhill make sure the metal of the switch bushing has conductivity to the chassis. Might have to scrap off a bit of paint.

 

[Spencerleehorton]

its a good quality switch that i have used the same one on my other version and no noise/hum?

on another note i have changed the txf in this other one to correct config, as in one toroidal 230/ 2 x 115v and a 20v.
I had made myself a transformer which did both but this was only temporary until the other came.
It made a bit of noise so good to put these others in.

The only thing is now im over voltage on the B+, i have 6.8k then 8.2k but only 100k drain resistor R7 and im getting 229v and i cant dial down any less.
Should changing R7 to 160k give me the drop to 210v?

 

[poctop]

IMPORTANT:
You Should always have the SHLF Jumper in position in the PSU PCB ,  this will ensure that your cable shield is tied at both end of the cable and connect to 0V
you Should Also have the 0V star grounded to your case and then from there to you IEC earth ground ,

 

[Spencerleehorton]

There seems some vagueness about the 0v and cable screen, I will try and detail how I have done it so you can tell me I’m wrong and then I can put it right!!!!

 

[Spencerleehorton]

for B+ now i have D1 diode, 6.8k, 8.2k, 20k pot then the 160k drain resistor.
i start with 351v at D1, and end up with 250v at B+.

as i understand it, i should be putting 210v AC in, but i have 2 x 115, so i'm +20v AC, which after diode is 368v.

changing the drain resistor from 100k to 160k has made the B+ go up from 229v to 351v so maybe i need to go down a little on the drain resistor?

any ideas?

 

[Spencerleehorton]

i've now tried 82k as drain and i'm down to 220v, so will try 68k next, will post my setup.

Ok my voltages are all over the place.

Ive worked out that i must be delivering 291v to D1!!!! to have 350v on the other side?

i need to test my toroidal to make sure i've wired it correctly!!

 

[Spencerleehorton]

Here is what i think i should do once i have sorted my toroidal to get 230v:

230v x 1.2 = 276v after diode
R1 = 6.8k
R3 = 6.8k
R7 = 33k

this should hopefully drop 66v right?

 

[Spencerleehorton]

Could someone look at my drawing please and hopefully understand what ive done and let me know the correct way please as i feel there is confusion about the 0v to chassis.
Dany is saying it need a connection to chassis from 0v and others are saying there doesnt need a connection?

 

[poctop]

OK , here is the briefing

For More Information on Grounding
https://www.dropbox.com/s/6y121dasm3iw4ba/Sound_System_Interconnection.pdf?dl=0

i always have the habit of never float 0V on a PSU i always star ground it to the chassis and then IEC plug , 
about the shield it is imperative at all time that the shield has a path to the chassis as well so it will protect YOU and Your investment. in the mic PCB the shield and 0v are on the same level meaning they are connected togheter and the mouting hole of the pcb are connected to the chassis , this lead to always have the shield protecting you because i ALWAYS star Ground 0V to the power supply case so in all everything always goes to the earth no matter what , So in essence this SHLF is somthing i wanted to implement as a ground lift but it was not done properly and i am glad it turned that way because it is safe this way ,
i guess i am an habit person i always did it like this and never had any problem.

if you have hum problem  you can start floating thing arroundb like the 0V from PSU to in respect to Chassis or the shield but it is very something of last resort and i dont recommend it as it is to me potentially unsafe to do so ,
Hope this helps,  above is a good read about how to properly ground the equipment togheter,

best,
Dan,

 

[dmp]

Dany is saying it need a connection to chassis from 0v and others are saying there doesnt need a connection?

Since the 0v and shield are connected in the mic, I assumed the SHLF connection should be open.
Typically the 0v should be connected to chassis ground in a single location, not multiple. And the Neumann schematic shows the connection in the psu as open, indicating that is the typical operation I assume.

 

[Spencerleehorton]

Again your not being specific with this as  it sounds like your saying to connect pin3 to gnd along with pin 7?
Is this forming a gnd loop? As you’ve said already they are connected in mic?
Could we have clarity with this please.

Also my B+ is 229v so I need to reduce 8.2k down, gonna try putting another 8.2k in parallel with it to give 4.1k and see if I can then tweak down to 210v.
My toroidal is 100va 2 x 115 but outputs 260vac!

put in 20k in replacement of 8.2k and now sitting at 210v!! and have now taken out GND lift switch and connected both pin 7 and pin 3 to chassis.

 

[dmp]

"0v"=audio ground=pin 7 of mic cable
"shield"="chassis ground" = pin 3 of mic cable

For safety the mic3 connector coming into the PSU should be hardwired to the PSU chassis and the "mic-3" pad - and the IEC gnd should also go to the chassis. I think everyone agrees on this.

0v and shield are connected on the mic pcb (look at the schematic). The SHLF connects them optionally on the PSU pcb (a second connection). I did build a neve preamp that had a slightly lower noise floor once with multiple connections between audio and chassis ground. I am throwing in my two cents, but Poctop is the designer of this project.

Ultimately, as long as the mic is safe, you can try different things out for the ground scheme. As I've posted my U67 works great and has a low noise floor at my recording space but had a bad ground hum at another location once.

 

[poctop]

"0v"=audio ground=pin 7 of mic cable
"shield"="chassis ground" = pin 3 of mic cable

For safety the mic3 connector coming into the PSU should be hardwired to the PSU chassis and the "mic-3" pad - and the IEC gnd should also go to the chassis. I think everyone agrees on this.

0v and shield are connected on the mic pcb (look at the schematic). The SHLF connects them optionally on the PSU pcb (a second connection). I did build a neve preamp that had a slightly lower noise floor once with multiple connections between audio and chassis ground. I am throwing in my two cents, but Poctop is the designer of this project.

Ultimately, as long as the mic is safe, you can try different things out for the ground scheme. As I've posted my U67 works great and has a low noise floor at my recording space but had a bad ground hum at another location once.

+1000
Thanks for beeing so clear,  The important thing here is safety and the premise of this post is just that ,
there is so much that can be said about grounding and grounding scheme but it always seems to comes at this principle wich is different strokes for different folks but Safety First.

Best,
Dan,

 

[poctop]

I did based my work on this PSU ,
hoping it is a good schemo if not then remove the jumper and try again but in all time make sure you have a solid connection from the mic body to earth so it will protect you and your investment ,

 

[HeavyHand]

Here's how I wired the Cinemag transformer into the mic. I'm losing some top end in omni and figure 8.  Between the schematic and the layout there is a discrepancy between wires 7 / 8  and 9 / 10 and their relative polarity. Could that be it? In fact, the drawing on the left side no longer appears when you go to Cinemag's website and look up the CM-5701 pdf. Maybe they realized a mistake and removed it? Any ideas?

 

[mohausler]

Just a quick note to say good this project is. I am stunned. Thank you poctob!

Fab Transformer & Heiserman capsule (plus a Valvo EF86) turned out to be a great combo. I have used many original 67s and this clone sounds stunning. Thank you!

 

[Hank Dussen]

Just a quick note to say good this project is. I am stunned. Thank you poctob!

Fab Transformer & Heiserman capsule (plus a Valvo EF86) turned out to be a great combo. I have used many original 67s and this clone sounds stunning. Thank you!

Hey Mohausler, congratulations with your build.
Do you have an inside picture of your finished microphone? I'm kind of puzzling to fit in the FAB transformer and a Valvo tube.