U47-style bodies - who can make one?

[Moby]

Unfortunately I cant have 6 meters of 63.5mm brass tube as free sample  :'(

 

[Buttercup]

Hi

Hopefully, this is the link for my U47 pictures........

Sorry it's taken so long - just lost my job so life has taken on a new urgency!

http://s542.photobucket.com/albums/gg416/gilesjeremy/

As the original uses Aluminium tube ( and is mostly aly ) is that easier to get hold of?
I got some and got the engineer to change the inner bore as the wall thickness was a bit over
the 1.5mm needed.

 

[Moby]

Looks great.  Are you happy with aluminium? Do you have some hum or buzz issues. I's much easier for me to "hunt" aluminium but we decided to go with brass as original vintage Mic's.

 

[Buttercup]

Hi Moby

Yes, there is still an annoying buzz which I am tearing my hair out about!
I have checked all the ground connections many times and also tried a ground-lift to no avail.
Sounds great apart from that............the search continues........

 

[Moby]

Bit off topic but did you tried to shield inside the AL tube with copper sheet? Sheet should be grounded of course 

 

[Martin B. Kantola]

Bit off topic but did you tried to shield inside the AL tube with copper sheet?

Aluminum works fine as such, I frequently test my microphone prototypes in a cardboard tube shielded only with thin aluminum foil, with NO hum whatsoever. The problems are elsewhere. Once built a monster of a microphone with 25W of heater power, that one needed some careful grounding and PSU filtering...

Martin

 

[Moby]

Thanks Martin, it's great you want to share your experience with us  What about stainless steel? how good is that material for mic bodies?

 

[Kingston]

Aluminum works fine as such, I frequently test my microphone prototypes in a cardboard tube shielded only with thin aluminum foil, with NO hum whatsoever.

That's interesting considering aluminum is not ferromagnetic. I've been told aluminum is a really bad shield, especially for blocking transformer induced hum.

 

[Buttercup]

But not forgetting of course that the original U47 was mostly Aluminium apart from the brass headgrille ( steel or brass mesh - not sure which ) and 3 steel struts inside.

Well, the buzz is pretty annying but I'm sure that it's the rather dodgy preamp I'm using to test the mic but here is what may be a useful run down of what I have done.....

PSU - Thougt I'd try an Ela M 251 circuit as I had some pcb's lying around from a previous project.

Transformer - custom made 300mA 9V heater and 125V 4mA HT - These could need a bit more power.....?
With the standard 4x 47uF caps, 4.7k,5.6k,27k with 270k across the output I get between 105 and 108v
The heater is a pretty standard LM317 circuit with a 5k pot to adjust.
PSU box is grounded to the mains supply earth and both the xlr output and mic inputs are grounded to solder lugs fixed to one of their respective fixing screws.

Mic uses an EF14 and is grounded first to one of the struts, then the output transformer/c3, then r3 (29ohms), then the EF14 ground then terminating on a lug screwed into the top plate of the mic body ( that the head connects to with its two little blades ) like the original. Checked with my meter and there is a good ground continuation throughout.

I was trying to negociate an Oscilloscope from my old job which would have really helped but they wouldn't let me have it.

Lots of lovely output from the transformer - which wouldn't have happened without the massive help of Max ( ioaudio ).

Back to the buzz - if anyone can spot my maybe obvious mistake...........
Still learning!

Thanks

Giles

 

[Martin B. Kantola]

That's interesting considering aluminum is not ferromagnetic. I've been told aluminum is a really bad shield, especially for blocking transformer induced hum.

Blocking the field from a power transformer is not what we're talking about here, hopefully...  For that neither aluminum, copper nor brass works. I use mu-metal.

There's a reason why the power supply is always separated from the microphone, my Finnish friend

Martin

 

[Martin B. Kantola]

Thanks Martin, it's great you want to share your experience with us 

Thanks Moby, this forum is definitely one where I have more to learn than share... 

Martin

 

[Martin B. Kantola]

Back to the buzz - if anyone can spot my maybe obvious mistake...........

Hi Giles!

Probably can't help, but I can always try offering some very basic mike troubleshooting 101.

- does the buzz continue when you cut the mains power (while the tube is still operating for a while)? If it doesn't, your PSU filtering is not good enough, or you have a stray field from the transformer.

- have you tried shorting the tube grid to ground and putting the whole microphone into a metal box? (or aluminum foil wrapped cardboard box, my favorite...) That will give you a clearer idea of what the buzz sounds like without other distractions.

Martin

 

[Buttercup]

Hi Martin!

I was just writing you an email.

You may be right about the filtering as the buzz does stop when the mains is switched off.
I have used this psu scheme with great success before so I thought it would be fine....... :'(
Ok, I will try to improve the filtering first and, if that fails I will try the Aluminium foil wrapped box - which will
probably make my girlfriend think I am more mad than usual!!

I did try putting the mic a good distance away from the PSU ( another room ) just in case there was a source of EMI in the room but it
didn't improve matters at all.

Would increasing the filter caps help as I'm hoping that I don't need to start completely from scratch - although I will if I have to of course.

I have always found PSU stuff the most difficult to get right so now is my chance to learn something really useful.



Thanks!!

Giles

 

[Martin B. Kantola]

I was just writing you an email.

Great, please finish it!

You may be right about the filtering as the buzz does stop when the mains is switched off.

Next thing is to check if the buzz is on the HV or LV. Easiest to simply cut one while listening, since the caps will hold enough voltage for a while. Sometimes it's only that you're loading a small transformer too much.

OK, another thing I forgot to ask, is the preamp grounded too? Watch out for ground loops.

Ok, I will try to improve the filtering first and, if that fails I will try the Aluminium foil wrapped box - which will
probably make my girlfriend think I am more mad than usual!!

Does she have a metal breadbox in the kitchen, steal that instead. Perfect size for a mike.

Would increasing the filter caps help as I'm hoping that I don't need to start completely from scratch - although I will if I have to of course.

Of course I don't know your problem really, but increasing capacitors is usually not a solution, only painting over the problem with another layer of filtering...

If you find the buzz comes from your badly filtered PSU, try series resistance before the filter caps instead.

I have always found PSU stuff the most difficult to get right so now is my chance to learn something really useful.

I agree, the PSU is such an important part of audio gear, I have learned by my mistakes mostly. Hum, smoke and sparks.  ;D

Martin

 

[Moby]

But not forgetting of course that the original U47 was mostly Aluminium apart from the brass headgrille

All vintage microphones have Brass bodies made then nickel finished. Neumanns, AKG's and similar. On mine C24 everything is brass even the end cap  Same with U47.
Copper and brass are good for EMI/RFI shielding not for hum. As Martin stated mumetal is ferromagnetic material used for magnetic shield, also, steel can be used but mumetal is better of course. 

That's interesting considering aluminum is not ferromagnetic. I've been told aluminum is a really bad shield, especially for blocking transformer induced hum.

Same with brass. It's not ferromagnetic so that means that you have to keep your mike far away from big transformers

 

[Buttercup]

Yes.....metal bread box!

At least she will get her boyfriend back when I have finished the mic ( been focussing on it rather than her lately ;D ).

Thanks for your checklist Martin, which I shall work through tomorrow after whatever sleep our darling teething baby gives us.

Could still be the preamp though and a ground loop so I might just have to send it to my customer to try on his system ( while I upgrade mine!! ) when I have exhausted every other avenue.

Will post back here with what I find.

Thanks!

Giles

 

[Buttercup]

Sorry to hijack this thread temporarily but.......sorted the buzz!!!!

Whilst being very carefull to avoid a ground loop in the PSU, I missed the fact that I had not grounded the negative rail of
the supply board - silly but true!

Also, the PSU transfomer is too low powered so I will get my local winding company to make me a more beefy affair.

The filtering seems fine but I do not want to be lazy so I will uprate this too and continue trying to make the best mic I can.

And I've got Martin's metal box treatment to go next!!!!!

Finally, be aware that to find a quiet EF14 ( for those of you planning to use one ) you might need to try a few.................

Thanks!

Giles

 

[mad.ax]

Hello,

Buttercup wrote: "Transformer - custom made 300mA 9V heater "

The EF14 datasheet says: heater 6.3V / 470 mA ! Looks like a bit much of "underheating"...

And don't forget that LM317 like a 3V voltage drop between input and output to regulate properly... If you adjusted it to 6.3V, or if your line voltage hapens to be a bit weak, then the regulation falls...

You may try to substitute a big heafty 12V transformer...

Axel

 

[ENS Audio]

Probably asking a redumbdant question but has anyone had any luck PMing one of the fellows over at CNCzone.com??? <<Pro and Hobby Machinist hangout

Theres even a Employment Opportunity and RFQ (Request for Quote) section on the site (for those that didnt know)

http://cnczone.com/forums/forumdisplay.php?f=74

Just post exactly what you would want them to do...I dont see why those guys wouldnt be able to what's needed to be done.....


Hope this helps,

 

[guavatone]

That's a good idea ENS

has anyone tried this?
www.emachineshop.com

Though it seems freelance guys may give a better price.