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

[youngshu]

Alright I have replaced the entire big transformer and all of the caps in the power supply and I am still having a 120hz hum its faint and I honestly could work with it and no one but us engineers would notice but I want it to be perfect.

Can someone point me to where I should look next? Otherwise this mic sounds amazing with the beeskneez capsule and ami t67.

also i have tried it on multiple outlets multiple interfaces in multiple rooms and am getting the same 120hz hum

 

[Khron]

also i have tried it on multiple outlets multiple interfaces in multiple rooms and am getting the same 120hz hum

Try disconnecting the output 3-pin XLR's pin 1 from ground/earth, in the power supply.

 

[youngshu]

Try disconnecting the output 3-pin XLR's pin 1 from ground/earth, in the power supply.

I tried disconnecting pin 1 from ground on the output and no difference other than more High Freq noise with it disconnected. The hum is exactly at 120. could it be as simple as my cable? From what I read online the caps in the power supply points to 120hz specifically but they along with the power transformer were replaced. Same issue.

Could it be a different component on the pcb? I have a duplicate of every power supply component so i could replace everything on the board if you guys think that would make any sense.

Could it be in the mic itself?

For grounding I followed the build guide. I scrapped the paint off the chassis and made sure it was touching bare metal as well as added a wire going to the lid to make sure that had sufficient connection to ground.

 

[Delta Sigma]

Could be a ground loop.

https://groupdiy.com/threads/neuman...lm49-conversion-to-m49-b-c.51862/post-1188606

Have you read this? You can have a ground loop from the power supply to preamp, but also within the power supply if you have multiple 0V to chassis grounds. If you are inexperienced, please make sure you have mic body to earth ground continuity before turning this thing on.

 

[eso]

Have you confirmed everything in the mic is grounded correctly? shell, headbasket, base, internal rails, everything.

When I first powered mine up I had really bad hum. With the modifications I made to the boards and the body I had defeated a couple ground connections, especially the board with the switches. I just installed a couple small jumpers to ground everything. With the mic plugged into the power supply you should have continuity of the ground from all of the mic body through to the power supply.

eso

 

[youngshu]

 

[youngshu]

I did read the post you sent and i think i am following all that that post said. here are some pictures anything stand out obvious?

 

[youngshu]

I have pin one on the xlr disconnected from the board and had tried it completely disconnected then i jumped pin 3 from the 7 pin to the 3 pin ground and pin one just to try it no difference.

 

[eso]

You need your meter for this. Just checking continuity. Connect one lead to the ground in the power supply and then with your probe confirm that you have a continuous ground path. Also check from the power input to the power supply to make sure everything that should be grounded is.

I have a cheap old multimeter that has a tone for checking continuity. When I'm assembling I check the grounding frequently.

eso

 

[Delta Sigma]

There's a lot of long wires in your power supply. I wouldn't bring your audio pair to your power supply PCB. straight from the 7 pin to 3 pin, twisted tight and uniformly. My brain is not leaving work mode to think straight, but I think I terminate pin 1 of the 3 pin to the chassis immediately - on the connector mounting screw. Either way, your three pin, pin 1 wire is flying around the world looking for RF. I also can't tell how many times you connected 0V to the chassis. I see one at the three pin header that has tape on it.

 

[youngshu]

There's a lot of long wires in your power supply. I wouldn't bring your audio pair to your power supply PCB. straight from the 7 pin to 3 pin, twisted tight and uniformly. My brain is not leaving work mode to think straight, but I think I terminate pin 1 of the 3 pin to the chassis immediately - on the connector mounting screw. Either way, your three pin, pin 1 wire is flying around the world looking for RF. I also can't tell how many times you connected 0V to the chassis. I see one at the three pin header that has tape on it.

I check continuity from the IEC plug - chasis - xlr and it all looks good. Anything else i should check?

Any guidance on which wire from the 7 pin go to which on the 3 pin? Going to try it now though to see if i can figure it out.

As for 0v I only have one wire from the PCB 0v header going to my ground point on the case.

 

[youngshu]

I check continuity from the IEC plug - chasis - xlr and it all looks good. Anything else i should check?

Any guidance on which wire from the 7 pin go to which on the 3 pin? Going to try it now though to see if i can figure it out.

As for 0v I only have one wire from the PCB 0v header going to my ground point on the case.

I connected audio from 7 pin straight to 3 pin i figured that out thanks for the tip. same 120hz. very faint but audible when the mic is on and idle maybe 6 db higher than it should be

 

[youngshu]

Rechecked ground continuity from supply to microphone and everything in between and got the beeps.

Could it be Diode related?

I replaced all the capacitors for B+ and H- so I'm wondering if there is another component that could cause it. I have no idea personally just wondering based on what I have been googling. I trust you guys more than google lol

 

[eso]

How clean are your solder joints, especially in the high Ω section?

Have you thoroughly cleaned all of the flux and other residue from the circuit?

eso

 

[Delta Sigma]

Rechecked ground continuity from supply to microphone and everything in between and got the beeps.

Could it be Diode related?

I replaced all the capacitors for B+ and H- so I'm wondering if there is another component that could cause it. I have no idea personally just wondering based on what I have been googling. I trust you guys more than google lol

My guess is shield or 0V issue. Ground loop. Try shortening any related wire. Open your current 0V chassis connection. Connect 0V to chassis at the 7 pin connector mounting screw.

Again, hard to tell in the photos, but it looks like you have a PCB to 0V connection at the transformer side. Connect to chassis from the PCB headed for the 7pin connector to the 7 pin mounting screw. Paint removed obviously.

 

[youngshu]

How clean are your solder joints, especially in the high Ω section?

Have you thoroughly cleaned all of the flux and other residue from the circuit?

eso

yes I cleaned everything very thoroughly before installing the styroflex caps

 

[youngshu]

My guess is shield or 0V issue. Ground loop. Try shortening any related wire. Open your current 0V chassis connection. Connect 0V to chassis at the 7 pin connector mounting screw.

Again, hard to tell in the photos, but it looks like you have a PCB to 0V connection at the transformer side. Connect to chassis from the PCB headed for the 7pin connector to the 7 pin mounting screw. Paint removed obviously.

Just Tried What you recommended and same 120hz persists.

any other ideas my friends haha I appreciate the help so far guys!