Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)

[TillM]

I noticed a hum (at around 50hz) when I used my M49 and take a higher input gain from my pre as I usually do.
Is there any way to reduce the hum except the pin wiring 3 to 7 and 7 to three ?
It seams like a ground loop to me.
Appreciate any suggestions.

 

[Delta Sigma]

I had issues with hum in my M49b. I had the PSU circuit board grounded by the 0V connection by the transformer at one of the PCB standoffs' chassis hole. If I moved the ground near the choke the hum would increase. I thought the case was too crowded (Hammond enclosure) so I rebuilt it in a larger case from dandeurloo. Same issue.

I turned out to be a ground loop because I grounded the PCB from the transformer side of the PCB to roughly centre of the chassis. I moved it to the mic cable connection to the PCB and grounded it on the 7 pin XLR mounting screw and the hum was gone.

Grounding is hard to describe so I made a rough sketch. I made a forum member mad once for suggesting a ground lift as shown in Neumann schematics; so to be clear:
1) the power supply chassis is always bonded to earth ground via ground connection at IEC outlet
2) the mic body is always bonded to the power supply chassis via mic cable shield (and sometimes extra "screen" conductor bonded to shield)
3) the Neumann grounding scheme (see U67/NU67 schematic) has 0V, screen/shield and mic body all bonded inside the mic
4) the U67/NU67 has a switch on the screen (pin 3) which is probably preferred over switching 0V
5) I put the switch on the 0V because the shield will always be bonded to the power supply chassis via XLR housing and I omitted the screen (pin 3 in U67 schematic) on my M49 to add relay control over omni only mode

 

[TillM]

Thank you so much !
I will try it !

 

[Delta Sigma]

Good luck! It's a frustrating thing to troubleshoot. I think the key is grounding the 0V to the chassis at the same location as the shield since they're connected in the mic. You have no choice with the shield because the mic cale connector is bonded to the chassis where it is mounted.

 

[TillM]

the thing is, I've a M49c and a M49b.
The only problem with the 50hz hum makes the M49b version.

 

[Delta Sigma]

I didn't have a problem with hum on my U67 or Elam 251. It wasn't until I built an M49b that I had an issue. It may be because of the choke. It may be because of the bias. Not sure why but changing the grounding corrected the problem. I can't remember where I grounded the 0V on my U67 but I believe it was at a PCB standoff. My 251 supply is grounded to a lug in the centre of the chassis. I referred to the U67 and 251 supplies when building my M49b supply and ended up with hum.

It's been a while since I've read it but Rane talks about the XLR "Pin 1 problems" here:
https://www.rane.com/note165.html

The difference to note is that your mic cable isn't coming into a preamp chassis that has a small power supply relative to the chassis. You're bringing your mic cable into a chassis that is 90% power supply components. My guess, is that the idea of having a supply ground (chassis ground to wall), 0V/signal ground and a shielding ground all going to different places in the chassis SHOULD work but doesn't seem to because of the choke.

There may be some benefit to experimenting with 0V/shielding connections in the mic body as well (i.e. not connecting 0V to the mic body - only connecting it to the power supply chassis) but I think we need someone to modify a mic that travels a lot to do experiments. The RF/grounding situation in one building may be different than another.

Unfortunately, my experience with grounding is in communications. I'm sure I have a lot of bad habits in audio grounding!

 

[arsmusic]

I am really interested in building the D49 (m49) rev c round pcb mic.  There are a few things I cam unsure of.  First is the accrilic plate included with the pcb?

This one:

pichost

Also, if the transformer included with them round kit?  This one:

http://www.tab-funkenwerk.com/id82.html

That is not on the mouser list. 

Then I see you can order the mic body from Studio939

So roughly:

$60 for pcb
$45 for mouser mic parts
$150 for mouser PS parts
$175 for m49 body
I already have a neumann k49 capsule
Then you need to buy a ac701k tube $900+
Or a JAN 5703WB around $10

Is there anything else you need to buy?

I do not see the capsule mount anywhere so I guess I would also need to buy this?

 

[rmburrow]

The late Oliver Archut discussed the use of a triode connected 5840 in place of the expensive AC701k.  Both the 5840 and the 5703 have 6.3 volt heaters.  Anyone done any noise studies on either of these tubes with the grids terminated with 150 meg or higher?

 

[TillM]

After getting in Trouble with my EF47 build I get much help from David (TLRT).
His help also sorted out my M49b noise problem.
I attached the grounding scheme to everyone. It helped me a lot !

 

[Delta Sigma]

After getting in Trouble with my EF47 build I get much help from David (TLRT).
His help also sorted out my M49b noise problem.
I attached the grounding scheme to everyone. It helped me a lot !

Good news that your hum is gone Till!

It's nice that you put the effort into a clear representation of the wiring - looking back at my sketch, I probably made things more confusing.

A few things though:
If you're not using the optional switch, where are you making the 0V connection to the power supply chassis? Via mic?

I agree bypassing the PCB with the audio pair is smart but also keep in mind to twist it and keep it short.

 

[TillM]

Yes, I leave 0v out and make proper grounding inside the mic.

Maybe I’ve to say, for testing the Voltages for the mic with a load resistor it’s better to connect the 0v from PCB to chassis.

By the way, I also want to thank you for your help in this thread.
So it’s a graphic with much help from DeltaSigma and TLRT !

 

[JMPGuitars]

What do people generally see for output level with the M49c?

I find that I have to turn my preamp gain really high to get any real signal from this mic. I brought it to a friend's studio, and his preamp gain wasn't significant enough for the mic to seem that useful to me.

Thanks,
Josh

 

[Phrazemaster]

What do people generally see for output level with the M49c?

I find that I have to turn my preamp gain really high to get any real signal from this mic. I brought it to a friend's studio, and his preamp gain wasn't significant enough for the mic to seem that useful to me.

Thanks,
Josh

There's def something wrong then; I've no problem with this build with levels...I built the b version but you should not be having this issue...

 

[JMPGuitars]

There's def something wrong then; I've no problem with this build with levels...I built the b version but you should not be having this issue...

I agree, something's definitely not right. Hopefully somebody has some insight what to troubleshoot.

 

[Delta Sigma]

I agree, something's definitely not right. Hopefully somebody has some insight what to troubleshoot.

Low output could be anything from power supply,  transformer (wiring or defective unit), tube, Hi-z feedback (C4, R3), capsule and everythign in between. Start with measuring voltages and posting here. Also, what transformer are you using? Post the colour code of how you wired it.

 

[JMPGuitars]

Low output could be anything from power supply,  transformer (wiring or defective unit), tube, Hi-z feedback (C4, R3), capsule and everythign in between. Start with measuring voltages and posting here. Also, what transformer are you using? Post the colour code of how you wired it.

My voltages are within spec, and the transformer is oriented correctly.

 

[Le Roux]

Just finished mine after years of being on the shelf.

Sounds amazing!! RCA tube

PROBLEM THOUGH

Cardoid does not work

Using pattern switch on psu

fig 8 and omni both work

The switch seems to be functioning properly checking continuity

Any suggestions?

Cheers
Neil

 

[Delta Sigma]

From your power supply, measure the voltage on the common terminal of your pattern switch in omni (0v), cardioid (1/2 of B+) and fig 8 (same as B+)

 

[Le Roux]

Voltage checked out ok.

I decided to cut the link in the attached pic to see if it works in cardoid only

Cardoid works now 
Thought I needed that link to have all the patterns working???

Fig 8 is louder than both omni and cardoid. But omni and cardoid are completely acceptable comparing other mics I have.
Omni is much louder when speaking into rear capsule compared to front, which is odd.

Still some issues

 

[Delta Sigma]

Measure voltages at the mic. Make sure pin 6 is changing with pattern selection changes.

Did you look at the schematic? See switch S2, that's the jumper you show in your photo. If your mic is wired correctly, there will be no pattern selection switch changes with the switch open. It connects the rear diaphragm with the front through cap C1. Cap C1 blocks the DC so that the rear diaphragm can be polarized differently than the front.

You probably either have something wrong with your power supply wiring (maybe B+ and pattern voltage are mixed up?) or something wrong with your front and rear diaphragm wiring.