MXL 990 mod question from Norway

[Frdfrode]

Thank you all for the enthusiasm in your answers, you motivate me for this project even more.

Best regards
Frode

 

[RuudNL]

A K67 capsule is not the best idea in a circuit without frequency correction!

 

[micolas]

Building a tube mic sound exiting, but may be a to big project to start with. I would have to build the PSU also I guess.
I was thinking about a sweet vocal mic, sparkling without being to rough in the top end, with a solid mid/low mid.

I do not know if the k67 is the right for that? With or without the beyer transformer?

Maybe there is someone on this forum that have pcb´s for i nice circuit, or I can solder on project plate. I have plenty of components, wima condensors and metal film resistors etc. Some I am sure I will have to buy.

I have 4 pieces of the 5840 tubes if someone would want some, I do not 4 of them.

So on to a tube mic for next project, but one at the time

Frdfrode,
do you happen to have another microphone besides the MXL that can be used as a donor to build various microphone circuits?

 

[kingkorg]

Thank you all for the enthusiasm in your answers, you motivate me for this project even more.

Best regards
Frode

You can swap the capsule for the k67 you have, and then add filter correction to existing stock circuit. There are several ways to do it, from adding one cap, to more complex solutions. This would bring you more towards u87 territory. Hilsen fra Nesodden

 

[Frdfrode]

Frdfrode,
do you happen to have another microphone besides the MXL that can be used as a donor to build various microphone circuits?

 

[Frdfrode]

There are also nice bodies on ali express. Pity with the high shipping costs, but I think some of them look nice.

This cad is same age as my AKG 414. Big mic with small capsule. Some screws are missing so its falling apart, but it can be fixed of course

 

[Frdfrode]

You can swap the capsule for the k67 you have, and then add filter correction to existing stock circuit. There are several ways to do it, from adding one cap, to more complex solutions. This would bring you more towards u87 territory. Hilsen fra Nesodden

Sounds like a good idea, hopefully the pcb is easy to work on. I will se when the donor mic arrives

Hilsen fra Nærbø
Frode

 

[Paul W]

I too have an MXL 990 to mod... actually I've collected three of them, one older through-hole and two newer SMD, and am looking for really easy mods, like a simple capsule swap and possibly a very easy paint-by-numbers capacitor upgrade. (I've never done any delicate soldering and am not savvy about analog circuitry.)

I also have collected 3 AKG Perception 200s and a Behringer B2 Pro (older through-hole version), so I guess I'm good for K67-type mics, and looking for mics with different strengths to complement them. (Can you tell I'm heavily influenced by kingkorg?) Oh yeah, I also have a couple of BM-800s and have ordered kingkorg-recommended electrets for those.

I'm leaning toward putting Arienne's flat 47 in the through-hole 990, leaving open the option of further through-hole component upgrades but hoping to just swap capsules and then leave well enough alone.

I'm also looking for simple capsule swaps for the SMD 990's, probably identical to make a pair of something. Maybe something CK12-ish?

Does that even make sense? Are the cheap fake CK12's that are really just edge-connected K67s flatter than regular K67's, and a reasonable thing to put in a mic with a flat Schoeps circuit? If I'm willing to EQ the on-axis sound, would they give me something usefully different about the off-axis sound?

 

[Frdfrode]

Hi again
So the donor mic came in. It is a SMD version, I guess that makes it harder to do any modifications on it.

I decided to save the K67 capsule I got for a later project. It came to my mind that I bought this as an C12 capsule thinking I would have a try at the tube mic on the danish site Gyraf. But that was about 30 years ago, and a lot of water under the bridge.

I am studying the schoeps circuit and try to figure out what is what to gat some basic understandings, but it will take some time. I retire in June so there will be plenty of time to work on this then.

I bought a K47 capsule from Ali express to put in to the mxl 990. It seemed to me like a thrustworthy seller with clear description of the capsules properties. Hopefully it will be good, if not there is a lesson learned.

https://vi.aliexpress.com/item/1005...0319312.1005005133810554&gatewayAdapt=glo2vnm

So now there is 4 weeks wait for the capsule. Is there any recommondations or links from you guys on anything that will improve the standard circuit that can be done even if it is a SMD?

Best regards
Frode

 

[micolas]

@Frdfrode

can you post some pictures of the inside?

 

[Frdfrode]

Hi
Thank you for replying

Hopefully this will help

 

[micolas]

@Frdfrode

How does it sound , Does it have high self noise?
Does the PCB have circuit paths only on the side with SMDs?

 

[micolas]

@Frdfrode

OK. After you have done some sound tests with the stock mic, to be able to compare later with the modded mic, disassemble the PCB and take clear, detailed pictures of the capsule and XLR connections.

 

[micolas]

@Frdfrode
Here's a modification idea for the SMD version:
https://audioimprov.com/AudioImprov/Mics/Entries/2013/4/6_MXl_990_Surface_Mount_Version.html
https://audioimprov.com/AudioImprov/Mics/Entries/2013/6/28_Lo-Cap_Front_End_for_SMT_MXL_990.html
https://audioimprov.com/AudioImprov/Mics/Entries/2013/6/24_Shortening_the_circuit.html
https://audioimprov.com/AudioImprov/Sound_Checks/Pages/MXL_990_RK-12v2.html

 

[Frdfrode]

Thank you so much

I will do some tests and look into this.

Regards
Frode

 

[micolas]

I will do some tests and look into this.

Is the capsule electret or externally polarized?
Measure if there is voltage on the capsule terminals, but quietly.
I didn't have mxl990 so I don't know
There were several versions, for example in the picture, there are two inductors needed for a DC/DC converter, that is, the capsule is externally polarized, which would simplify things. You would connect the new large capsule directly instead of the stock one.Possibly increase the voltage by changing a Zenner
Mechanical adaptation is not complicated.

 

[Frdfrode]

Hi Again.
Did some voice recording with the mic, very bright.

I measure 50mV on the capsule.

Even though they do not look the same, there might be some inductors on the bord.

It seems like like the red lead was nor connected to the xlr socket, or maybe it just came loose when i opened it ?

Where is the zener diode on the bord?

I have downloaded the circuit for the mxl 990

Thanks for all help !
Best regards
Frode

 

[Khron]

I measure 50mV on the capsule.

Even though they do not look the same, there might be some inductors on the bord.

It seems like like the red lead was nor connected to the xlr socket, or maybe it just came loose when i opened it ?

Where is the zener diode on the bord?

Due to the large impedances involved, the "mV on the capsule" is a very unreal number. You'll want to measure between ground and the D6-R12 junction.

Yes, there are inductors on the board (the black blocks in the left in the first photo, marked "680" and "151", 68uH and 150uH respectively.

You'll wanna reconnect that XLR wire then.

Most likely D3 ("south" of the two black inductors in the first photo).

 

[Frdfrode]

Hi again.
New measurement following your instructions gave me 24.5V across.

So some filtering with 22nF over the xlr capsule. Maybe they are already there, C1 and C2 ?
A 12 V zener diode to increase the voltage to the capsule (60V ?)

I am planning to use a single sided K47 capsule, so no need for a LPF then ?

Best regards
Frode

 

[Khron]

Assuming that's a real value, makes one wonder why they even bothered with adding a bias oscillator, when 24V could easily be derived straight from the phantom power