MXL 991 Capsule Mod?????

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Paul678

Well-known member
Joined
Mar 3, 2013
Messages
97
Ok, replacing the caps looks easy enough:

        http://www.groupdiy.com/index.php?topic=5156.0

And grinding out the gill slots seems easy enough, although there is some debate over if you should
remove one rib, or two, or all three.

But modifying the capsule is a bit confusing here:

        http://www.audioimprov.com/AudioImprov/Mics/Entries/2012/12/5_Modifying_the_MXL_991.html

It's not very clear what I need to do with the capsule.  From what I understand, the clamp that holds
the mesh should be left out upon assembly????

Does anyone have links to better instructions on this?
 
Oh, nevermind!

Once you take the capsule apart, it's easy to see what he's talking about!

ok, thanks anyways!

I'll let you all know if the mod improves this little guy!

;D
 
Please do! I have been reluctant to mess with the casules on a pair of Nady CM90's. The back vents have been opened up but I have read that taking apart the capsule renders them useless. Something about not being able to reassemble the diaphrams to the correct tension. Please let me know if this is true  or not. Also, what did you use to bevel the edge of the capsule housing? I have read the article link you provided several hundred times. I still do not get how to "shave" down the capsule though! Any help (or pictures) would be great! Thanks!
 
mabell313 said:
Please do! I have been reluctant to mess with the casules on a pair of Nady CM90's. The back vents have been opened up but I have read that taking apart the capsule renders them useless. Something about not being able to reassemble the diaphrams to the correct tension. Please let me know if this is true  or not. Also, what did you use to bevel the edge of the capsule housing? I have read the article link you provided several hundred times. I still do not get how to "shave" down the capsule though! Any help (or pictures) would be great! Thanks!

Like everything else, once you take an actual unit apart, you'll see what he's talking about.

There is a clamp ring that holds everything together.  They tell you not to tighten it too hard, but
mine was on quite tight, and was a bit difficult to get off, so I tightened it pretty well on re-assembly.

I will experiment with tightening and loosening it, to see it's effect on the frequency response.....
 
Turns out, I have a back-up capsule from my
SP1, where I had installed an RK-47.

So now I have two capsules for my MXL991, and I can
fully experiment with one of them.

So question:  has anyone tried leaving out one
of the very thin plastic spacer rings?  There
were two of them in my first capsule.

These are the spacer rings between the diaphragm
and the brass backplate, so the thickness will
affect the capacitance, and the frequency
response of the mic.

Also, has anyone tried enlarging the holes in the clear
plastic delay plate?  Or making more holes?  I know this
is getting really experimental...

Any advice appreciated...
 
Paul678 said:
Turns out, I have a back-up capsule from my
SP1, where I had installed an RK-47.

So now I have two capsules for my MXL991, and I can
fully experiment with one of them.

So question:  has anyone tried leaving out one
of the very thin plastic spacer rings?  There
were two of them in my first capsule.

These are the spacer rings between the diaphragm
and the brass backplate, so the thickness will
affect the capacitance, and the frequency
response of the mic.

Also, has anyone tried enlarging the holes in the clear
plastic delay plate?  Or making more holes?  I know this
is getting really experimental...

Any advice appreciated...


Certainly ADDING a spacer ring to one capsule,
for a total of 3, would decrease the capacitance,
and the sensitivity, so the S/N ratio would
probably suffer.  But how would the frequency
response change?

And if you remove a ring, you'll greater sensitivity,
but you risk the diaphragm hitting the backplate earlier,
so that the signal will clip earlier, and distort at a
lower sound pressure level....
 
What did you use tool-wize to cut down and bevel the capsule housing? Cutting I am not worried about, it is the bevel on the edge of the capsule housing. How is the capsule housing beveled? I should say with what tool?
 
Do a search on CAD GXL1200 capsule modifications. There is a rather lenghty how-to on it, but in a nutshell, they use a dremel and a flat head screwdriver to do the job. I wish I had the time to look up the site for you, but acoustic foam does not hang itself (maybe it would, if it were suicidal, but I digress).

BTW, Marik does, from what I hear, awesome mods to those capsules. For his reasonable costs, you might want to consider letting him take the wheel (I will when I can afford to have all six of my capsules done at once). He says he can tune them to sound more like a KM-54/84.

-James-
 
Yes, a dremel tool, and various grits of sandpaper will do.

Takes some time, but is worth the effort.

See my original post for the links.....
 
Ok, I put a very large 47nF cap across
the bases of the output bipolars, just
to see how much of the high end sibilance
I could get rid of.

And it definitely works, although it may
be too much, as expected.  I did a "she
sells seashells" test against my AT4047, and
they are now similar in sibilance, but
I end up EQing the AT4047 at around 10kHz,
with a little +3dB bump, just to give it
a little more presence to cut through a
mix.

So I'll try 22nF or 10nF instead, just to
get some high end back.

Also, amazingly enough, now that I have
1uF monolithic ceramics for C3 and C4, I
have a huge bottom end, almost like the
AT4047, and certainly more than my
Studio Projects C1 (whose bass did not
increase for the same 1uF values, for
some reason).  So, I might try 0.47uF
instead, as that might be enough bass
for me.  I can't believe you can get such
good bass response out of a medium sized
capsule.

But wow, I'm REALLY shaping the sound of this
mic!!  Very empowering, and I cannot imagine
someone not modifying these cheapie mics. 
Some people actually like these stock, but
with just a little work, I think they sound
MUCH better.  I don't have the Oktava MK-012,
but it wouldn't surprise me if what I have
now could hold it's own against that model.
 
Good stuff! Now I am sure not to screw it up! Thank you Paul. Very informative. I have been searching 603 capsule mods to no avail. Well, nothing clearly stating what to do. CAD 1200 search did the trick!  1000~4700pf is a good range for the caps you added. The caps are absent on the Chinese SDC's but are usually present on the LDC or the SDC that look like a LDC. THe caps go across R30 & R40 on the MXL 603 schematic which is the same as any of the other Chinese SDC's. I hope this is clear to you as it is early and  am nursing a pretty nasty hangover. Mmm, German beer.........
 
Paul, it seems you've got the bug for good now.  ;D
Since this is incurable, you might as well keep us posted.

I can't believe you can get such
good bass response out of a medium sized
capsule.

There's no reason why smaller capsules would have compromized bass response.
When it comes to truly extended bass (large organ pipe territory) pressure mics (omni) rather than directional ones make a much bigger difference.
Character is another story, though.


Henk
 
mabell313 said:
Good stuff! Now I am sure not to screw it up! Thank you Paul. Very informative. I have been searching 603 capsule mods to no avail. Well, nothing clearly stating what to do. CAD 1200 search did the trick!  1000~4700pf is a good range for the caps you added. The caps are absent on the Chinese SDC's but are usually present on the LDC or the SDC that look like a LDC. THe caps go across R30 & R40 on the MXL 603 schematic which is the same as any of the other Chinese SDC's. I hope this is clear to you as it is early and  am nursing a pretty nasty hangover. Mmm, German beer.........

    Well, just as a disclaimer, no matter how informed you may be, you can always STILL screw
a mic up if you are not careful!  The diaphragm is delicate, and you can easily lose the two plastic
spacer rings if you breathe too hard!  Also, it's VERY easy to leave dust or tiny particles of dirt
between the diaphragm and the back plate (you'll see the mylar get deformed).  In fact, I had to
disassemble and re-assemble 3 times before the mylar was clean and flat.

    Yes, you can put the caps in parallel with R30 and R40, but since they are virtually in series, you
can also simply use one cap of 1/2 the value, across both resistors (across the two bases of Q30 and Q40).

    It's amazing to tone-form a microphone!  It's almost like modifying an instrument! 

 
micaddict said:
Paul, it seems you've got the bug for good now.  ;D
Since this is incurable, you might as well keep us posted.

I can't believe you can get such
good bass response out of a medium sized
capsule.

There's no reason why smaller capsules would have compromized bass response.
When it comes to truly extended bass (large organ pipe territory) pressure mics (omni) rather than directional ones make a much bigger difference.
Character is another story, though.


  Yes, I'm hooked for good now!  haha!

  So omni directionals are generally better for pipe organ, low frequency sources?

  When you say character, you mean the overall frequency response, right?

  Thanks for your time.....
 
Omnis are known for having a flatter, but extended low end response. So the bass isn't going to be boomy (lack of proximity effect), but it will extend down another octave or so (generally).

-James-
 
HellfireStudios said:
Omnis are known for having a flatter, but extended low end response. So the bass isn't going to be boomy (lack of proximity effect), but it will extend down another octave or so (generally).

-James-


Perhaps I will try temporarily plugging up the holes on the plastic acoustic delay plate,
just to see what sort of omni the 991 makes.....

 
Actually, you can make it omni by covering up the slits on the sides. There are also omni and hypercardioid capsules available on ebay for the MXL 604s (pretty much the same mic). Personally, I prefer the latter option.

-James-
 
Actually, you can make it omni by covering up the slits on the sides.

You probably wouldn't make it absolutely airtight anyway, but if you do you may want to have a miniscule opening to allow for air pressure shifts (changing weather). Any experts on this?
 
When you say character, you mean the overall frequency response, right?

OK, let me try to elaborate in my own words. Large versus small diaphragm.

Just about the only technical (!) advantage of an LDC I can think of would be the lower noise floor due to the larger surface (everything else being equal). Oh, and arguably the fact that you can fit larger electrical components in the larger housing that usually comes with it.

In all other respects SDCs take the cake, I'd say. Again, technically speaking. E.g. polar patterns are more accurate from a pencil mic with changeable heads. LDCs (dual diaphragm) have the patterns created electronically. And the omni setting isn't a very true omni, especially when compared to pressure mics. (Neumann M50 is a special omni, but that one doesn't have a big, dual diaphragm.) You could make a pencil housing for one inch diaphragms, but it would be a fat pencil, in any case the bit at the head end (think opposite shape of a measurement microphone ). So the bigger housing would still be a con.

Also, large diaphragms break up in the audible frequency range (more so than SDs). Kinda like woofer speakers do. But the latter have the artifacts filtered off in the cross over. We're talking non-linear distortion here, so EQ will not help. Again, SD is the winner, technically speaking. But why then do singers never use superior measurement microphones? Well, in practice we like the subtly more distorted sound of LDCs (on studio vocals anyway) and call it character.
Tubes, trannies, big PIOs etc. can add even more.

A bit of a long winded journey towards the character matter, but I hope it helps.
If it's all too obvious, well, no one gets hurt. Carry on.  :)
 
Paul678 said:
mabell313 said:
Good stuff! Now I am sure not to screw it up! Thank you Paul. Very informative. I have been searching 603 capsule mods to no avail. Well, nothing clearly stating what to do. CAD 1200 search did the trick!  1000~4700pf is a good range for the caps you added. The caps are absent on the Chinese SDC's but are usually present on the LDC or the SDC that look like a LDC. THe caps go across R30 & R40 on the MXL 603 schematic which is the same as any of the other Chinese SDC's. I hope this is clear to you as it is early and  am nursing a pretty nasty hangover. Mmm, German beer.........

    Well, just as a disclaimer, no matter how informed you may be, you can always STILL screw
a mic up if you are not careful!  The diaphragm is delicate, and you can easily lose the two plastic
spacer rings if you breathe too hard!  Also, it's VERY easy to leave dust or tiny particles of dirt
between the diaphragm and the back plate (you'll see the mylar get deformed).  In fact, I had to
disassemble and re-assemble 3 times before the mylar was clean and flat.

    Yes, you can put the caps in parallel with R30 and R40, but since they are virtually in series, you
can also simply use one cap of 1/2 the value, across both resistors (across the two bases of Q30 and Q40).

    It's amazing to tone-form a microphone!  It's almost like modifying an instrument!

Indeed! Even if I screw the capsules up too bad the mics are dirt cheap. If I break it I'll just replace it. Don't you mean double(x2) the capacitance?
 

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