wstratton

MK-012 pair - constant hiss
« on: January 13, 2021, 10:56:33 PM »
Hi all,

I have a couple of MK-012 microphones (not a matched pair). These ones are sort of odd--I bought both from Microphone Boutique, and they look more like the "Oktamic" microphones that you can buy on Reverb--different logo, et cetera. Not sure if Oktava's Tula site has split production in two, or what. Both have a considerable amount of self-noise, a static sort of hiss beyond what I would consider acceptable for any microphone--definitely too much for recording acoustic stringed instruments. I've tried dehumidifying the capsules to see if that was the problem, but it didn't change or lessen the noise. I've tried reheating most of the solder joints, and that didn't do the trick either. What is weird to me is that they both have the noise and that the noise is basically the same in both microphones. I'm attaching some photos I took of the internals of one mic--one photo here, one in the next post. Does anybody have any thoughts or suggestions?

Best,

Will
« Last Edit: January 13, 2021, 11:02:16 PM by wstratton »


wstratton

Re: MK-012 pair - constant hiss
« Reply #1 on: January 13, 2021, 11:01:20 PM »
(second image)

radardoug

Re: MK-012 pair - constant hiss
« Reply #2 on: January 13, 2021, 11:32:52 PM »
The important question is what is the signal to noise? Put as loud a signal in there as you can and then measure from that level down to the noise. I have never been impressed by Oktava stuff.

dbelousov

Re: MK-012 pair - constant hiss
« Reply #3 on: January 14, 2021, 08:52:28 AM »
The board is authentic. This is the last generation of this mic. I've never seen the front part (gray plastic) like this before, though, it's something new. Is it glued?

What are the JFET and BJT models?


Dmitry

wstratton

Re: MK-012 pair - constant hiss
« Reply #4 on: January 14, 2021, 10:11:18 PM »
Hi Dmitry, thanks for your help. The gray plastic piece is not glued down. There is a Fairchild BC308 and a Toshiba K170 in each mic. I think that's it for JFETs and BJTs, but I don't know this circuit well--if I should be looking for more, let me know?

Radardoug, after reading about it, I'm still not sure how to measure the signal to noise ratio properly. But compared to other SDCs I have used, the noise level relative to the ability to pass signal on these microphones is very prominent. It hasn't always been this bad on these microphones, though--when I first got them, they were moderately noisy but useable.

squarewave

Re: MK-012 pair - constant hiss
« Reply #5 on: January 14, 2021, 10:59:26 PM »
Is this the schematic?



If yes, I would say try cleaning around the 1G resistors. It could be that phantom is drooping and you're cranking the gain to compensate. Specifically, get some 99% isopropyl alcohol and clean thoroughly around the 1G resistors. Then take a lens wipe and some plastic tweezers and polish those areas. It has to be crazy clean.

Then plug it directly into a USB audio interface, put it in front of a speaker, play a tone from the speaker fairly loud, adjust the level to -10dBFS, record 10 seconds, pause the recording, turn off the tone and record another 10 seconds. Then listen to the recording so that the level is about what you recorded at and when the 10 seconds of silence is played, is there hiss?

wstratton

Re: MK-012 pair - constant hiss
« Reply #6 on: January 14, 2021, 11:20:40 PM »
Thanks squarewave--this is the correct schematic--I'll try this tomorrow. I think you may be onto something...the amount of signal that the mics are putting out is not what it used to be. If this does end up being the problem, is this a design flaw that can be fixed, or just something that would require periodic maintenance, or something else, in your opinion?

squarewave

Re: MK-012 pair - constant hiss
« Reply #7 on: January 15, 2021, 12:40:26 AM »
I wouldn't call it a design flaw. Pretty much any condenser mic is going to have 1G+ resistors to charge the capsule.

Microphones are not really supposed to last indefinitely. After a while the capsule will get crapped up. I don't know what the capsule is like in this mic but clearly the rest of it is an economy design. I wouldn't get too deeply invested in these things.

wstratton

Re: MK-012 pair - constant hiss
« Reply #8 on: January 15, 2021, 12:48:26 AM »
I wouldn't call it a design flaw. Pretty much any condenser mic is going to have 1G+ resistors to charge the capsule.

Microphones are not really supposed to last indefinitely. After a while the capsule will get crapped up. I don't know what the capsule is like in this mic but clearly the rest of it is an economy design. I wouldn't get too deeply invested in these things.

The thing is, these two mics are barely three years old, and haven’t seen much use! I’m just hoping to get them to where they function as they should—I like them timbrally much more than the Mikteks and AKG 451s I’ve had over the years. But I suppose eventually I’ll plunk down the cash for some Soyuz, Josephsons, or Beyerdynamics if I can’t whip these two into shape.

dbelousov

Re: MK-012 pair - constant hiss
« Reply #9 on: January 15, 2021, 08:21:33 AM »
I rebuilt a handful of them. The capsule quality is quite stable, I had five of them at the studio once, I mean simultaneously. A pair from 2000-2001, some from 2010, and another pair from 2015. It was hard to find any difference in sound with the same body.

PCB, components, connectors, and metalwork are a very different story. It looks like they used everything that they have at the moment at their storage until this revision with the newest PCB and Vishay caps. My ones had the BC557, though.

After rebuilding, the mic is easily comparable to the Schoeps CMC6, and yes, I have it to make this comparison.

https://yadi.sk/d/WnHCNePJyNLf8A

The files don't have names on purpose.

2017 was a rough year for Oktava, though. The company was almost bankrupted, and as the result, it was moved under the direct management of Rostec, a state-owned, high-tec/defense industrial holding. It's possible, that its capsules from that period have some quality issues, who knows. Checking with a different body would help.

Careful cleaning is always a good idea.

« Last Edit: January 15, 2021, 09:37:29 AM by dbelousov »
Dmitry


Gus

Re: MK-012 pair - constant hiss
« Reply #10 on: January 15, 2021, 09:55:28 AM »
Surprised no one posted the following

First find out if it is the circuits or capsules or preamp

Sub in a cap for the capsule make sure you install the circuit in the body and shield the capsule. Test the circuit.

Is the noise still there? if the noise is reduced it could be the capsules.

If it is the circuits take some DC measurements and search for 012 etc. at this forum. This circuit has been discussed here in the past

It is not clear from the posts. Do you have good SDs available you can use with the microphone preamp you are using, maybe it is an issue with the phantom supply or preamps. If I understand your posts you used other SD microphones before, can you try other microphones now?

wstratton

Re: MK-012 pair - constant hiss
« Reply #11 on: January 16, 2021, 12:00:17 PM »
Surprised no one posted the following

First find out if it is the circuits or capsules or preamp

Sub in a cap for the capsule make sure you install the circuit in the body and shield the capsule. Test the circuit.

Is the noise still there? if the noise is reduced it could be the capsules.

If it is the circuits take some DC measurements and search for 012 etc. at this forum. This circuit has been discussed here in the past

It is not clear from the posts. Do you have good SDs available you can use with the microphone preamp you are using, maybe it is an issue with the phantom supply or preamps. If I understand your posts you used other SD microphones before, can you try other microphones now?

All good suggestions. I a/b’ed with other condensers and other preamps, but not with other SDCs. I have a pair of Beyers on the way—I will see if they have any problems with noise. At the point I’ve done what I can short of replacing components. Eventually, I’ll try out some other capsules to see if the issue resides with them.

radardoug

Re: MK-012 pair - constant hiss
« Reply #12 on: January 16, 2021, 03:08:32 PM »
One of the problems with that board is that the fet has all legs soldered into the board. Smarter manufacturers usually keep the gate leg away from the board. Leakage on the board is a problem in a circuit sitting at one gigohm.


 

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