Oktava ML52 modification?

[SoulFood99]

Hello!

I got this mic as a gift. Searching the forums for reviews, I found out alot can be done to improve the sound.

I don't know why, but I can't seem to find the long threads where modification is described. I read about them on other forums.

Could someone give me a link or could we start from ground up for people like me?

Does anyone have any pictures uf changing the trafo with a Lundhal or changing the grill?


Thank you all!

Borut

 

[David Curtis]

There was a long thread on rec.audio.pro:

http://groups.google.com/group/rec....read/795f598fb9a3a068/e1e9507697434691?rnum=1

Here are some images of my modified ML-52:

Grill and top with fins/struts cut out (they ring like the tines on a kalimba):
http://resources.edgeofmagic.org/cut.jpg

Grill replaced with perforated brass and an open-weave mesh on top:
http://resources.edgeofmagic.org/newgrille.jpg

"Silks" glued in place:
http://resources.edgeofmagic.org/assemble2.jpg

Ribbon motor with goal post/blast shields removed and wire replaced with heavier gauge (AWG 18):
http://resources.edgeofmagic.org/motor.jpg

I also designed and built a phantom-powered head amp, to make it an active ribbon. This mod included replacing the transformer with a Lundahl 2911.

The PCBs, stuffed and soldered:
http://resources.edgeofmagic.org/pcbs.jpg

Assembled PCBs:
http://resources.edgeofmagic.org/pcbassy2.jpg

This assembly just barely fits into the bottom cavity of the ML-52.

Here is the schematic:
http://resources.edgeofmagic.org/arp.jpg

Some explanation is in order --

1) IN+ and IN- are connected to the output of the ribbon transformer. In my project, I replaced the Oktava xfmr with a Lundahl 2911-- a worthy upgrade all by itself.

2) OUT+ and OUT- go to pins 2 and 3 of the XLR connector. You might need to check polarity/phase on this (or just rotate the mic 180 degrees...). GND goes to pin 1.

3) A1A (first half of the LT6234) give a gain of 5x, and A1B inverts that signal for balanced output. The total gain is 10x (20 dB).

4) The +10V, +5V and GND rails act more or less like +-5V rails for powering the amp, with +5V being a 'virtual ground'.

5) VR1 and VR2 are 5V LM4040s.

Selection of the op amp was critical, to get a usable combination of low power and low noise. The LT623x voltage noise is 1.9 nV/rtHz, and the quiescent current draw is 1.2 mA per section, which makes it possible to use phantom power. Most ultra-low noise op amps are current hungry. Here is a link to the LT623x data sheet:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P2398,D2333

The real trick was getting all of this into the bottom cavity of the ML-52. It's a tight fit, but it works. Everything is sandwiched between 2 round PCBs--one holds the transformer and the other all of the circuitry. If anyone cares, I can post some pics and the artwork for the PCBs. The sound is nice, very clean (yeah, yeah, I know, IC op amps are the work of the devil...). It helps to use quality components, particularly the output coupling caps (C5 and C6) -- I used Nichicon Muse KZs.

I'm going to try replacing R1 with 300 Ohms and R2 with 1.2K Ohms, in a effort to reduce noise. The noise floor is OK, but any reduction would help. I'm also planning on re-working the power section.

Any comments or criticisms would be appreciated.

Cheers,
Da5id

 

[stoff]

Hello,
David, very well done! :thumb:
Has anybody a link or informations about modfifications for the ML-53?
Of course, the LL2911 will be one...

Greets Stoff

 

[SoulFood99]

Thank you so much david!

You did an icredible job! How does it sound?
Did you get rid of that HF cut?

It looks amazing!!!! :thumb: :thumb: :thumb:

 

[rodabod]

Great post. This needs to go into the Mic Meta....

Ribbon motor with goal post/blast shields removed and wire replaced with heavier gauge (AWG 18):
http://resources.edgeofmagic.org/motor.jpg

WTF is going on with the baffle in front of the ribbon assembly and the small drilled holes? Is this mic fig. 8? Hmmmm. Can you not remove that baffle?

 

[Kingston]

Thanks for this David Curtis,

inspires me to mod my t-bone rb-500 (shinebox/nady) ribbon a bit further.

 

[David Curtis]

Thanks to all for the kind words. I'll try to answer some of the questions raised:

SoulFood99 wrote:

You did an icredible job! How does it sound?
Did you get rid of that HF cut?

The sound is a definite improvement. I'm not sure what you mean by the HF cut. If you could expound more, I'll try to answer as best I can.

Removing the external wire/sheet metal assembly from the motor (referred to in various posts as "blast shields" or "goal posts") had two noticable effects--1) it reduced the response above 10KHz (more on this in a minute) and 2) it cleaned up the high end, which was previously smeared. Apparently one of the purposes of the blast shield thingy is to serve as a resonator to boost high end response. It does this at the expense of clarity. Some of the posters on RAP thought that it introduced group delays (as would any resonant filter, whether electronic or acoustic). I don't know enough to comment on this effectively.

The net result is a cleaner, but reduced high end. The mic is more useable because I can boost the high end with a decent EQ and get it back, without the smearing. The lundahl makes a load of difference, in overall clarity and more particularly in a deeper, cleaner low end.

The amp is very transparent, the main benefits being the additional 20dB of signal, and effectively eliminating the usual ribbon mic's sensitivity to to the input impedance of the preamp. The noise is relatively low (I'm planning on making some measurements, which I'll post), but I'm working on further noise reduction.

A few other notes:

The brass perforated metal (from McMaster) looks nice but there is a steel alternative that has a higher % of open space (and thus, greater acoustic transparency).

The ringing of the grill/shell is much improved, but more can be done here. I've tried various damping materials on the inside of the shell--still working on this problem.

rodabod wrote:

WTF is going on with the baffle in front of the ribbon assembly and the small drilled holes? Is this mic fig. 8? Hmmmm. Can you not remove that baffle?

The plate on the front with the holes is actually part of the magnet/pole structure, so it can't be removed. There is an identical plate on the reverse side. There are two ribbons (one beneath each of the vertical rows of holes). The mic has a figure-of-8 respose pattern. If you haven't seen the motor in an unmodified ML-52, it looks even stranger--a Rube Goldberg-esque contraption that look like miniature goal posts (American football...) is glued to the front and back. This is the resonator discussed above.

I have 2 of these mics. One of them needs re-ribboning, so I dissected the motor to see how it's built. If anyone cares, I could take some photos of the internal components and post them.

Thank again for the compliments,
Da5id

 

[rodabod]

[quote author="David Curtis"]
rodabod wrote:

WTF is going on with the baffle in front of the ribbon assembly and the small drilled holes? Is this mic fig. 8? Hmmmm. Can you not remove that baffle?

The plate on the front with the holes is actually part of the magnet/pole structure, so it can't be removed. There is an identical plate on the reverse side. There are two ribbons (one beneath each of the vertical rows of holes). The mic has a figure-of-8 respose pattern. If you haven't seen the motor in an unmodified ML-52, it looks even stranger--a Rube Goldberg-esque contraption that look like miniature goal posts (American football...) is glued to the front and back. This is the resonator discussed above.
[/quote]

I assume that these plates form a magnetic return circuit? They are not magnetic are they?

I personally don't like the look of it. I wonder if it could be further drilled out? Have you tried removing one side and seeing the effect on the sound (obviously this would lose the balance side-to-side and therefore the frequency response though)?

I have 2 of these mics. One of them needs re-ribboning, so I dissected the motor to see how it's built. If anyone cares, I could take some photos of the internal components and post them.

Yes, yes! This would be good. Please post any pictures in the "ribbon dissection" thread. You should have a look at this yourself too!

Thanks,

Roddy

 

[Gus]

Some cast metals are next to impossable to damp because of the air voids inside of them. Some Oktava bodies seem to be cast.

In the past I worked on a large optical scanners and recorders. The first ones used steel welded mirror optic mounts, for cost reduction some parts were made from cast Al, problems problems problems.
The unit had an air bearing and air support honeycomb optical table etc. The 1600lb unit had vibration problems with the early castings, when the castings were cut open it was found they had air voids when cast correctly the problem was minimized.

Thing were tried to use the early casting but only a little can be done IMO.

 

[David Curtis]

rodabod wrote:

I assume that these plates form a magnetic return circuit? They are not magnetic are they?

I personally don't like the look of it. I wonder if it could be further drilled out? Have you tried removing one side and seeing the effect on the sound (obviously this would lose the balance side-to-side and therefore the frequency response though)?

They are in fact part of the return circuit, not the actual magnet. I haven't tried removing one of the plates, but I think it would be problematic. They are also important to the overall mechanical integrity of the motor.

Thanks for the tip about the ribbon dissection thread. I'll post photos there.

Gus wrote:

Some cast metals are next to impossable to damp because of the air voids inside of them. Some Oktava bodies seem to be cast.

Thanks for the knowledge. I once worked on a weapons targeting system that had a high slew-rate telescope mount. They ended up making the structure out of beryllium, because it was the only material that didn't have resonance problems.

I can see it now--the next exotic ultra-high-priced microphone feature: machined beryllium bodies. If we can only solve the toxicity problems...

Cheers,
Da5id

 

[Gus]

I also retubed the argon laser used in the recorder. I was very careful when handling the tube because it had a Be oxide tube inside the larger glass body. Beryllium is some nasty stuff I believe it was the navy that had a study about the toxicity when making props.

 

[SoulFood99]

David Curtis, thank you for sharing your knowladge!

I was thinking about the curve i got with the mic. It's very simmilar to the one shown here:
http://www.oktava-online.com/ml52-01.htm

When i tested the mic, the sound was very dark. smooth but dark. Maybe I was misusing it! How far would you normally put the mic, when recording vocals or saxophone?

This thing has a really big proximity efect!

And another question about the grill. The ML52-01 has 3 layers of it. The first being the vertical metal lines, and the other two a "wire net", the last being the finest (smallest).

Would I do anything good with removing the vertical grills and the first "wire net" and try to damp it the best i can?

I assume it is very light, but I haven't opened the mic yet...

 

[Kingston]

[quote author="SoulFood99"]I was thinking about the curve i got with the mic. It's very simmilar to the one shown here:
http://www.oktava-online.com/ml52-01.htm

When i tested the mic, the sound was very dark. smooth but dark. Maybe I was misusing it! How far would you normally put the mic, when recording vocals or saxophone?

This thing has a really big proximity effect![/quote]

Yeah, well, that's how ribbon mics are supposed to behave. They are always dark, with curves similar to the oktava, and depending on the ribbon motor/baffle structure, the proximity effect can be pronounced. Use it to your advantage, since there's no way to fight against it.

How far to put the mic? Well, what sounds best of course, or fits in the mix easiest. :green:

Just make sure you DO NOT get any air movement in the ribbon itself, or it's an ex-ribbon soon.

 

[SoulFood99]

Thank you for the tips Kingston!

 

[Marik]

I modified quite a few of those puppies. The very first mod would be re-ribboning and fine tuning both elements to the same frequency--nothing what you can see coming from the factory. The tolerances of motor structure are quite... loose. Usually, I measure difference in gap width as much as 0.03". With a gap of appr. 0.1" it is about 30%. There is not much you can do about it mechanically, so to compensate I cut the ribbon width accordingly for each gap, with much tighter clearance.

Very often at least one of the original ribbons is slacked, so response is whether obviously f***** up, or is all over the place.

The magnet structure is quite interesting, where two magnets are put face to face with the same polarity, and piece of iron in between. Changing the magnets for neodymiums will somewhat boost the output. The perforated iron pieces you can see outside should be left alone, not to waste any magnetic strength.

To dampen the grill I am using hot glue and strips of self-sticking linoleum.
Here is the pic what I did:

 

[khstudio]

Cool thread... I did the mods also, from the artical that was in Tape-op.
The damn shell ruins the whole thing... RING, RING, RING, TONE, TONE, TONE. When I take it off it just sings but you can't leave a ribbon open like that so...

Marik,
Where can I get the new grill like yours? How did you do it?, I'm ready to cut mine open & shut that thing up.
It's a great mic with the new Lundahl (forget which one) tranny & posts gone but sits on the shelf because of the shell

I'd really like to make the whole thing a grill but yours looks about right. :wink:


Kevin

 

[Marik]

[quote author="khstudio"]
Marik,
Where can I get the new grill like yours? How did you do it? [/quote]

Just put it on a rotary table standing vertically, and zzzhhhhhik-zzzhhhhhik-zzzhhhhhik--cut it with a mill. After that it will still ring. The linoleum I was talkng earlier should take care of it.

 

[khstudio]

That's cool but where can I find the grill?

 

[Marik]

[quote author="khstudio"]That's cool but where can I find the grill?[/quote]

I used original grill... or do you mean the mesh?

 

[khstudio]

Really... ok. I haven't had mine out for a while & thought it was black. I also didn't think it was that big, the grill that is...oh well.

Thanks