Oktava ML 51 ribbon microphone new . Need advise.

[vanakaru]

Although I am not a sound technician by any means I am hooked on old soviet microphones. I use these for recording my sound installations. Here in Estonia one can get some of these mic's for few euros still. Most of these are crap. But once in a while one finds a marvel.
So I got this beast few days ago. Oktava ML51 made in 1986 #294. It has not been used at all. Someone bought it when russian money lost its value on 1990 just to exchange paper to something at least. I always wanted to see what the ribbon mic is like. So here I am.
I wired it up, found a 3V battery and tested with my cheap Yamaha mixer. As expected the mic's preamp is noisy, but not too awful(I have heard worse). Mic works otherwise. Low's and mid's are rich and clear but there are no high's almost at all.
What I should look for when continuing my exploration?

I studied these topics here
http://www.groupdiy.com/index.php?topic=41856.0
http://www.groupdiy.com/index.php?topic=21096.0
http://www.groupdiy.com/index.php?topic=5743.msg73762#msg73762

 

[vanakaru]

So I did remove the horrid preamp. Actually when I tested the mic direct from transformer it sounded pretty normal. It seems that this piece of electronics was that distorted the sound so bad.
Since I was at it already I rewired the toroid as well. I think it sounds OK now.
But my big problem is hum. If I hold the metal grille the hum goes away - almost. Also by moving the cables I can reduce it a lot. So I wonder if there is a procedure to wire the cable shield and line.

 

[ricardo]

But my big problem is hum. If I hold the metal grille the hum goes away - almost.

The grille & metal body MUST have good electrical contact with p1 of the XLR and the cable shield.

 

[vanakaru]

I think I got it mostly. With trial and error I ended soldering shields/ grounds as follows.
motor: shield to magnets/frame, signal+ to ribbon
tranny: shield to pre, signal+ to pre, metal cover to frame
XLR male sec signal+ to pin2, signal- to pin3, shield to pin3 - just like balanced XLR.
When connecting pins 1 and 3 (unbalanced wiring) the noise in terrible.
So however I got it into useable state it picks up interference when moving cables around and gets very noisy if I touch the XKL barrel.

 

[ricardo]

Can you draw us an EXACT diagram (not a picture) of what you did for earthing?  Your post is unclear & confusing.

 

[Marik]

So I did remove the horrid preamp. Actually when I tested the mic direct from transformer it sounded pretty normal. It seems that this piece of electronics was that distorted the sound so bad.
Since I was at it already I rewired the toroid as well. I think it sounds OK now.
But my big problem is hum. If I hold the metal grille the hum goes away - almost. Also by moving the cables I can reduce it a lot. So I wonder if there is a procedure to wire the cable shield and line.

The preamp in ml51 is pretty louzy. Keep in mind, the transformer is a high ratio, so once you bypass the preamp the output impedance is something like 1.5KOhm (from memory).

The output connector is DIN, so you should properly wire a XLR adapter and make sure the shield (pin 1) is connected to the mic ground. The microphone body is made out of (kinda) metalized plastic--something to keep in mind. With no exception all ML51's I ever saw had corroded metal, so you should make sure there is a proper ground connection--you need to clean everything well.

Best, M

 

[vanakaru]

I have been trying to get rid of hum again and again. I replaced wire from ribbon and added direct wire from ribbon chassis. I connected all shield points to one location, added XLR to the body and what not. Still not much of results. Time for ground lifter, I guess.

One more experiment. I have a broken Oktava MD41 that looks like crap. However it has output toroid in it with rather similar rate to my ribbon toroid. And it does pretty good job. It is larger diameter 40mm but fits inside the housing fine.