MXL 990, 991 and Oktava 319 Mods

[kirk97132]

Right on.  I've been reading this stuff too.  And it seems you took everything out that you could replace.   What did you replace C1, C2, C3 & C4 with?  Are they ceramic?  It also looks like you changed the values of D4 & D5?  And did you just cahnge them out from ceramic to metal film or did you change values too IE: C12.    Thanks for sharing your work, Kirk

[Chuck]

Right on.  I've been reading this stuff too.  And it seems you took everything out that you could replace.   What did you replace C1, C2, C3 & C4 with?

C1    WIMA FKP2 - .01uF /63v (Polyester film)
C2    WIMA FKP2 - .01uF /63v (Polyester film)
I chose those capacitors because Marik had recommended that value for those, and they fit well.

C3   WIMA MKS2 - .47uF /63v (Polyester film)
C4   WIMA MKS2 - .47uF /63v (Polyester film)
Because that's what was available and they fit. I may try different values, if I can find bigger ones that fit.

C11   Panasonic - .033uF/ 100v (Metal film)
C12   Panasonic - .033uF/ 100v (Metal film)
I chose those, because I already had them and again, they physically fit.

It also looks like you changed the values of D4 & D5?

I didn't change those.

And did you just cahnge them out from ceramic to metal film or did you change values too IE: C12.

The resistors I used are good metal films. When I was first testing the mics, I noticed pretty wide tolerances for some of the stock resistors. So, I decided to replace most of them. Then I hand matched everyone, so that the values were (nearly) identical in each mic. I hoped that would make the mics exactly the same in every respect, but that didn't work, since one mic has more gain than the other now. 

[Chuck]

I'm wondering if someone would be willing to provide some circuit design help?
I have attached a schematic for the circuit for the mic. It's not very accurate. But the section of the circuit around the 2SK170BL is what I am concerned with now.

The stock values for the 2SK170BL biasing resistors in my mics are:

R13   91K ohms
R14 470K ohms

Another circuit that uses the same transistor has the values at:

R13 240K ohms
R14 270K ohms

I'm wondering what the difference is and if either set of values is better than the other. For kicks, I changed the values in one mic to 240k and 270K and didn't notice any big difference. It didn't change the noise or output level by much. I'm hoping someone can give some guidance on what the best values would be.


edit to add schmatic for another mic that uses the 2SK170BL transistor (GenChinaMic.pdf).

[illconditioned]

^^^ Thanks for the schematic!


Looks pretty standard for modern mics, in particular, transistor at the upper left to generate the 60V or so polarization voltage.  There is a similar circuit in the Studio Projects C4 and many others.


  Richard

[Chuck]

No sweat Richard.

I wonder if the lower gain from one of my mics is because there is a 4v difference getting to the capsules of each mic? I don't think either is getting 60 volts. Off the top of my head, I thought it was more like 40v for one and 36v for the other. I don't know how to tweak the circuit to make sure both mic capsules get the same voltage.

[Chuck]

Jon, thanks for the reply. Is that something I could, or should do? Replace the resistors with a 500k or 1 megohm cermet trimpot?
But, I don't have a scope to view distortion with and how do you feed signal to the mic to even measure it?

Also, why does one circuit use completely different values for those resistors? Is there an ideal range? It's always been my understanding that high value resistors contribute more noise than lower values. Again, I'm a neophyte here just looking to learn and make these mics as good as I can with limited equipment.

[Chuck]

I use my DAW and converters to generate/analyze test signals.

I'd love to read the details of how to do that if you don't mind sharing. Even a link to a source I could read about doing that would be great.

I can generate signals, I just don't know how to send them to the transistor then monitor it for distortion. If I used a pot to figure out what the values should be, I could then find fixed value resistors to permanently install.

How close of a tolerance does the resistor ratio need to be? Also, if I get one 2SK170 done would I have to do the same thing for the other mic? Is there a big difference between individual transistors? If not, I'd think the best values would be posted somewhere.

Edit to add:
I just measured the voltage at R9 on the GenChinaMic.pdf circuit. I got 20.1v on one mic and 24.3v DC on the other.

[kirk97132]

So as I follow this thread with EXTREME interest.......ANd if you could see this, I now have 4 mics sitting apart on my desk ....I want to piggyback in on Chucks questions.  General question (to Jon?  Thanks in advance who ever answers)  Is there a big difference if I sub a .047uf capacitor for a 0.033 one in the generic C3 position of a mic?  What will the difference do?   Then is there a big sound difference between a Wima polyester style and a metal film style?  Thanks everybody...and a BIG BIG thanks to Chuck for posting this and sharing his work.   

[Chuck]

That's odd.  Why bother with a step-up circuit when ~34V is available straight off the XLR pins 

I thought the same thing. Maybe it would be possible to just bypass the step-up circuit. It might make the mic less noisy that way too.

[Chuck]

That's odd.  Why bother with a step-up circuit when ~34V is available straight off the XLR pins 

I thought the same thing. Maybe it would be possible to just bypass the step-up circuit. It might make the mic less noisy that way too.

If the polarization voltage is really 20V, then an increase to 34V will certainly increase sensitivity (5dB or so), which will also lower relative noise as a hotter signal will hit the FET.  That, however, will decrease max SPL correspondingly.

The advantage of the step-up is for low-voltage phantom; it will tolerate some degree of low supply voltage before dropping out of spec.  That's why it might be good to revise the step-up circuit instead.  But if you don't need that, then yeah, trying bypassing the step-up (just remove R9, and route new 1M resistors from pins 2 and 3 to C9).

Really?
So, I could just tie XLR pins 2 + 3 to the positive side of C9 and that will bypass the step up circuit? I've read that Marik advocates bypassing the step-up circuit. I will try it. But, just to make sure, I can remove R9 (1M), tie that to PIN 2 and C9, tie another 1M resistor from XLR PIN 3 to the positive side of C9?

That can't be right, it would combine the in phase and out of phase signals.

I just want to make sure I have it right, as I don't want to blow the thing up Can I also remove R12 (1.5k)? That resistor is also tied to the step up circuit.

[kirk97132]

Jon, Thanks for the reply.  Thanks for sharing your knowledge.  I should have known better than to say something is generic when all these mics are just slightly different between manufacturers.  But I believed you answered my question.  I'm reading the article and working on digesting that.   At this point I'm thinking I am almost ready to try and work on my mics.  The SDC's are first.  Then we'll see about the LDC's.  Based on Michael Jolly's observations he states that most of these SDC's have a HF bump due to the vent configuration.  While I am not ready to start cutting the vent area to open it up the Samson CO2's had a very tightly meshed screen behind the open.  So first I removed that and I removed the small foam piece above the mic capsule.  There was no easy way to get the capsule more forward in the housing due to it's design.  And there was no easy way to remove the screen with out really tearing things apart.  So I left it alone.  But it does look like the vents are a lot more opened up now.  Here are some before and after pix.  Next will be the actual soldering

[Chuck]

Hmmm... that cap isn't the same cap I have on mine.
The GXL1200's and MXL 603's have the shorter capsules. They don't have the vents like your's. The vents are on the mic body. I've inquired about the capsules you have pictured and have been told by several sources that those capsules are not interchangeable with the GXL1200/ MXL603 bodies.

[illconditioned]

Jon, Thanks for the reply.  Thanks for sharing your knowledge.  I should have known better than to say something is generic when all these mics are just slightly different between manufacturers.  But I believed you answered my question.  I'm reading the article and working on digesting that.   At this point I'm thinking I am almost ready to try and work on my mics.  The SDC's are first.  Then we'll see about the LDC's.  Based on Michael Jolly's observations he states that most of these SDC's have a HF bump due to the vent configuration.  While I am not ready to start cutting the vent area to open it up the Samson CO2's had a very tightly meshed screen behind the open.  So first I removed that and I removed the small foam piece above the mic capsule.  There was no easy way to get the capsule more forward in the housing due to it's design.  And there was no easy way to remove the screen with out really tearing things apart.  So I left it alone.  But it does look like the vents are a lot more opened up now.  Here are some before and after pix.  Next will be the actual soldering

Hey now.  That Samson C02 is a Transsound brand, electret capsule.  It is similar to those found in AT2020/21 and Behringer C2.  (Note: The Behringer B5 is an externally polarized cap).


There is nothing wrong with electrets -- some great mics use them.  The problem is Transsound are pretty much middle of the road.  You'll get much nicer/smoother and more detailed sound out of some of the other electrets, including: Sennheiser MK40 (KA10 capsule), AT822/825, AT3031/4021, etc.


As for externally polarized capsules, there seem to be lots of options, MXL, APEX, CAD, SP, AKG, etc.  Too many to mention, but they all have a decent chance... provided the quality control is there.

Nothing wrong with modding.  I might just wait until you have a better capsule in there first.  And, in the meantime, please keep the pictures coming!

  Richard

[Chuck]

Jon, is this what you have suggested?

[Chuck]

Cool. Thanks Jon. I'll try that.
I bought another mic over the weekend. I'm going to keep one mic that I can experiment on.


edit to add:
I revised the drawing and attached it here.