UM57 & UM70 observations

[rodabod]

Bernd - go for it.

That circuit might be quite good with the little Beyer transformers. Make some measurements!

 

[Gus]

I did not post not to build.  I am all for building and testing and learning.

 

[bernbrue]

I did not post not to build.  I am all for building and testing and learning.

 

[rodabod]

Whatever you build won't be a CMV at the end of the day. It's very quick to adapt these circuits and PSUs to other designs as most are so similar with the exception of fixed-biased designs.

 

[mpc3000limited]

I was doing this design to start and learn myself, thats why i was wondering what kind of sound it has.. i know its not a u47 but might be easy to modify and make a decent mic! Sure there are plenty of complicated maybe better designs out there but the um57 i thought looked a good quick and easy place to start learning. I am open to hear if someone else has suggestions!

 

[Rossi]

I did some recordings with my UM57 yesterday. I still like it much more with the 47u cathode bypass cap, it also appears to be a bit lower in noise. I also did comparisons to my U47 (which is K47 equipped). The U47 has thicker bass response, and more presence, it sounds closer to the source and more "real". The UM57 sounds nice, but you never get that "unmediated" feel I get from my U47. That said, the UM57 is very usable in the mix now.

Regarding the UM57/CMV circuit: It is interesting and may be worth some experimentation. But if I were to build something based on this design, I'd make some changes. For one, I would impelment a filter stage for the rear diaphragm polarization within the mic. The original schematic only has a filter stage in the PSU, and the pattern selector pin goes directly to the rear diaphragm. Increasing R1 may also be a good idea to get a bit more bass response. I would also think about different tubes. And, as Zebra has shown, it's worth experimenting with the transformer hook-up etc.

 

[zebra50]

Nice!

, I would impelment a filter stage for the rear diaphragm polarization within the mic. The original schematic only has a filter stage in the PSU, and the pattern selector pin goes directly to the rear diaphragm.

I think this is a good idea. It would make it more compatible with some DIY power supplies - this has fooled me in the past!

Increasing R1 may also be a good idea to get a bit more bass response

I didn't measure or hear a change going from 220 to 470 Meg for R1 & R2. But if R1/R2 are old and dirty you may get problems!

 

[Rossi]

I'm not sure a filter stage for the backside makes a real world difference, but if there is some noise in the mic cable, it could get to the capsule and to the tube grid. A resitsor and a cap are so cheap, I wouldn't want to take chances. The only reason I didn't put a filter stage in my mic is that I'd like to keep it original.

I don't know what would be the higest sensible value for the grid resistor, but upping R1 to say 1 gig shouldn't be a problem.

My mic must be very old given the serial number, but the resistors seem to be okay after so many years. The only things that I had to replace were the tube and the output cap.

 

[zebra50]

Hi!

A quick bump and a shout for info. Does anyone have a value for the primary inductance for the UM57 / CMV transformers?

Thanks!

Stewart

 

[Gus]

I think there is a thread were CJ tested a CMV transformer

 

[zebra50]

Thanks Gus.

I found the thread but most of the links are dead / missing.

http://www.groupdiy.com/index.php?topic=9310.msg111151#msg111151

 

[zebra50]

I'm being thick here aren't I? We can calculate it from the turns ratio and the secondary inductance.

Doh!

 

[terry setter]

Hello,

Thank you very much for you helpful post about the UM70 and its relationship to the various mic bodies.

I am currently half way into my UM70.  I've got the plastic plugs out, the three screws that hold the bottom of the unit to the upper half.  I survived the spring loaded center falling out, and removed the single screw that was on the bottom the upper capsule housing. 

BUT  the thing doesn't want to release the screen components from the capsule base.  I'm afraid to be too aggressive with it, and don't see anything else to unfasten.

Any advice would be MUCH appreciated.

Thanks in advance,

Terry Setter

 

[Sredna]

Hi Terry!

The two "pegs"or positioning pins with a small hole in them are actually screws. 

Use a paperclip or similar trough the hole and slowly rotate the pin.
When removing the last pin, be careful not to harm the capsule.

Good luck,

Anders