MXL 990 microphone upgrade

[rjenkinsgb]

Hi all,

I've just rebuilt an MXL990, surface mount version, to eliminate the ceramic caps in the audio path.
I've not seen anything on modifying this version, only the older through-hole type, so I've taken a lot of photos in case anyone else is interested.

For info, multilayer ceramic capacitors are extremely microphonic and appear to cause significant distortion when used for audio coupling.
In one mic I got to play with and mainly for the casing (an NW700), I completely disconnected the electret capsule - and the microphone still gave output from being tapped or shouted at, the PCB itself was so microphonic due to the surface mount ceramic caps..

The 990 surface mount "update" [or costcutting] version has also done away with the PTFE insulated standoffs that the through-hole version and most other condenser mics use.
The bias feed resistors are one gigaohm so avoiding possible leakage is critical. I've added pillars and also replaced the surface mount bias resistors & input FET.

edit - Links to a schematic of the mic [on gearslutz.com], for completeness.
The surface mount one one I have seems identical other than R6 & R7 legends being reversed.
https://www.gearslutz.com/board/att...eplacement-how-far-off-schema-sp-1-mxl990.gif

Edit 2 - some missed photos added, showing the drilling details for the through-hole cap installation.

And the mod photos:-

Before modification:
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Capacitor drilling locations, after removing the ceramics and cleaning the pads with solder wick.
1mm holes drilled near the edge or corner of the original cap pads & also the three test point pads.
The new cap leads then folded across the pads and cut to fit within the solder area.
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First stage, relevant ceramic caps replaced.
The remaining ceramics visible on the board are part of the bias generator and not in the audio path.
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Second stage, the high impedance points moved to PTFE standoffs to eliminate leakage:
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And the board reassembled in to the mic frame:
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[tomizett]

Very interesting, and beautifully documented. Thanks for posting.

 

[Willen]

Always love this kind of improvements! WOW! One was removing ceramic and used film in YouTube, and got so warm and crisp sound. It would be mind blowing if you recorded before and after audios too, for the comparison.