Project: True Condenser Capsule Repair and DIY Microphone

Thread Starter

Willen

Joined Nov 13, 2015
313
Project: DIY True Condenser Microphone


The project includes a 34mm large diaphragm condenser capsule, an audio preamp with audio balancing circuit board, a high voltage booster oscillator circuit board and a NW-700 studio microphone case. The microphone has a similar design which is popular around the internet. One of the examples is MXL microphones. This design is pretty close to MXL990.


When I got the 34mm condenser microphone, I saw it was very well made capsule. But I don't know how the manufacturer forgot to clean the backplate before installing diaphragm on it. Some of the little thicker (thicker than the space between backplate and diaphragm) dust grains of brass was pushing the diaphragm up, making some noticeable pimples around the surface.

Before.png
It means it disturbs the frequency response a lot. So I decided to do some critical (stupid?) repair.

Capsule Disassembly

I dissembled the whole capsule! I don't know the fact but I guess I am among few people in the world who dissembled the condenser capsule and successfully re-assembled. :) I am still not getting any web page or YouTube video about capsule disassembly. To repair, I removed the all screws, removed the thin diaphragm from the capsule, cleaned the dust of backplate and tried to fit back the diaphragm. And I knew that without stretching the diaphragm, it is impossible to fit it back to the capsule. Then I prepared a plastic sheet, made a hole little smaller than the diaphragm and glued the diaphragm which covered the hole of the sheet. Now I stretched the sheet (with diaphragm at the centre) in a ring (double in diameter than the capsule), glued the stretched sheet around the ring. Then placed the stretched sheet in the capsule, set all screws and peeling the outer unnecessary plastic sheet, I got my capsule back. :cool:
2.jpg 3.jpg 8.jpg

DIY True Condenser Microphone

Ok, back to the project, the project is based on the schematic:
Schematic.GIF

I designed the PCB in ExpressPCB, etched using toner transfer method, and soldered all of the components.
PCB Silk Artwork.jpgtwo boards ready.jpg


While testing, as a quick test I didn't use microphone case and I recorded like 200% noise and 2% audio. I thought I made some silly mistake around PCB or around capsule. During the troubleshooting, I made the delicate diaphragm puncture, Ops.....! So painful twist of the project! I had repaired the capsule VERY carefully by spending my 6 hours. :(
Capsule Punctured.jpg


Next day, with the punctured diaphragm, I fit all the boards and capsule in the Microphone case. Amazing! All the horrible noise disappeared! I learned that without microphone case, it is impossible to use condenser microphone. The metal case is serving as a perfect shield (for hum/RF disturbance). Then I got it recording audios amazingly.
Mic with Capsule.jpg Grill mic preamp.jpg

But I noticed a problem. It was recording with very low level than normal. So turning up the gain would increase the noise level badly. Within few weeks, I modified some suspected components around the oscillator board. I was suspecting to oscillator because who donated me these components have genuine MXL microphone and he is getting around 40V DC out from his oscillator. I was getting very low voltage (6V only) on my version of oscillator. The capsule needs high voltage (as I read somewhere 50V to 100V) to perform correctly. After some fails, when I replaced 150uH (L1) very thin (3mm) inductor with 120uH thicker inductor (5mm) and 22nF 50V ceramic capacitor (C11) with 22nF 100V non-polar green cap... amazing! Got 36V DC from oscillator. After replacing C9 from 10pf to 15pf, I got 39.7V DC. Now I am getting amazing powerful gain so I need to turn down the preamp volume knob.

Performance

You know I dissembled the capsule, stretched the critical diaphragm as my wish, and punctured the diaphragm too. It means I lost capsule's quality too badly. However I can make pretty attractive recording through it. Except ambient noise, it is completely quiet. Recorded sound makes me feel so deep, wide and booming. The microphone performs well around low frequency but not good around high frequency. So every time I boost high frequency using recording software to make it attractive. As I found around internet, the frequency response of the capsule also depends on the stretchiness of its diaphragm. Careful manufacturer carefully tunes the diaphragm with correct level of stretchiness to get flat frequency response. Now my capsule is little far from such tuning.


Overall this is one of the most exciting project to me. It's hard to believe that the thin metal coated plastic sheet over a brass plate can record so nice audios! I want to thank to Robert Jenkins (Engineer of JRW Developments, UK) for the donation all of these necessary parts and devices. Because of no access of eBay, Amazon etc of mine, I would never get chance to play with true condenser microphone.


March 12 2019

Willen (Raj)

NEPAL
 
Last edited:

ebeowulf17

Joined Aug 12, 2014
3,282
Sounds like a great learning experience. Well done!

It's a shame the capsule tension and damage have compromised its performance, but still very cool that you've got a working mic now and you've learned a lot along the way!
 

Thread Starter

Willen

Joined Nov 13, 2015
313
Really, I was completely unknown to XLR system, sound card (audio interface), condenser capsule, balancing circuit etc and it taught me everything!
 
Last edited:

Wolframore

Joined Jan 21, 2019
2,318
It seems like this would be a good secondary mic to use for bass amp or drums now.... Full frequency response is better because you can cut and filter but good bass is always nice to have on a separate channel. Microphones are tools to get the recording you want. I have a large collection for all kinds of reasons. Can't have just one... :) Nice work!
 

Thread Starter

Willen

Joined Nov 13, 2015
313
It seems like this would be a good secondary mic to use for bass amp or drums now.... Full frequency response is better because you can cut and filter but good bass is always nice to have on a separate channel. Microphones are tools to get the recording you want. I have a large collection for all kinds of reasons. Can't have just one... :) Nice work!
Thank you! Yes, it would be a nice bass microphone as a secondary microphone. Problem is I am out of the international market (actually my whole country). So I am compelled to use it as 'primary microphone'. The project made me addicted to Audio Electronics! Wow!
 

Wolframore

Joined Jan 21, 2019
2,318
There are some issues with phasing when boosting treble with EQ. I hope you get another mic with a good capsule to play with. The filtering causes the sound to get out of phase and can cause phase cancellation while mixing. Many times the sound is layered and phase issues add and cancel certain frequencies. There’s a lot of discussion about this topic it seems. Phase relationships are very important when recording and mixing. For most people it won’t matter. It’ll just sound like a mediocre recording.
 
Last edited:

Thread Starter

Willen

Joined Nov 13, 2015
313
There are some issues with phasing when boosting treble with EQ. I hope you get another mic with a good capsule to play with.
Being self learner hobbyist, I do not know deeply about disadvantage of using software based digital equalization. I use high frequency faders fully up to get normal voice. Yes, I just hope for the nice capsule.
 
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