MXL 990 Circuit Upgrade Kit

$ 129.00

This is the original drop-in upgrade circuit kit for the MXL 990. Beware the lesser products from copycats. The version sold here was developed as a full implementation of the Schoeps CMC5 circuit, using high-quality, hand-picked parts. We've done extensive revisions to this product over the years, most recently in April 2021. The latest revisions has improved noise filtering at several critical points in the circuit.

Included with kit

  • New, custom designed, 2-layer circuit board; lead-free ENIG finish
  • 100% of PCB components necessary to build the microphone, including matched capacitors and a pre-biased JFET to maximize headroom while minimizing distortion
  • Additional capacitors to allow you to tune the high-frequency response of the circuit to accommodate brighter-voiced capsules such as our RK-87 and RK-12.
  • Full-color, step-by-step instruction booklet
  • Cardioid/Omni switch. Note that this switch is internal; you will need to unscrew the microphone's base to access it.

You must supply the MXL 990 for modification. You will also want to purchase one of our large-diaphragm capsules.


This kit is compatible with the MXL 990 only. Customers have reported that these boards also fit the MXL 990 XL, although we have not personally tried. Our PCB should fit in all generations of MXL 990, whether the original mic was an older "through-hole" or the newest "surface mount" style. In all cases, the MXL PCB will be left over at the end of the process, intact. We don't modify it with this kit, but replace it outright.


We take no shortcuts on these DIY microphone kits. We do not compromise on quality or on parts selection. See the 1000+ customer reviews on this site for enthusiastically reported success stories. 

Strongly recommended

We strongly recommend that you replace the small-diaphragm MXL capsule with one of our large-diaphragm replacement capsules. All of our premium large-diaphragm capsules will find a lot more applications around your studio than the stock capsule. They will all also give your mic lower self-noise, and the possibility of Omni mode with our 990 PCB kit. (The stock MXL capsule is a single-diaphragm model that cannot produce an Omnidirectional pattern.) Our most popular capsules for this kit are the RK-47 and RK-12. Click these links to order a capsule for use with the PCB kit.

990B/RK-47 vs. Neumann U 87 Ai

We tested the newly modified MXL 990 (with our 990B PCB kit) against a $3300 Neumann U 87 Ai. The point of this test is not to suggest that the two microphones should sound exactly the same, although in truth they're pretty close. The point is that for about $300 you can build a mic that performs on a par with a studio standard that costs 11 times more (!).

The RK-47 capsule in the 990B is a bit leaner from 200Hz–1kHz, ever so slightly more present from 1kHz–7kHz, and then rolls off the top end sooner. These are subtle differences, but you'll hear them.

You might also hear that the amount of detail is comparable. The noise performance is similar. The 990B is every bit as usable, and in fact, I'd prefer it on most voices as it is less likely to capture mouth sounds and sibilance.

Then, too, remember that for the price of the U 87, you could build 10 990B's — 5 stereo pairs with different capsules, to cover a huge variety of sources and applications. Long live DIY!


This kit contains a bare circuit board and a bag of components. You must solder the circuit together. This is not a suitable kit for beginners; please don't buy this for yourself if you have never soldered before.

We can build this kit for you, in our Sonoma County (California) headquarters. See our service policies page. You might also be interested in our boutique 990 upgrade service, which includes personal consulting and access to circuits and capsules not sold to the DIY market.

If you're able to confidently form good solder joints, you should have no trouble with this kit. The documentation is excellent; it shows how to install the components, step by step, with many photographs. Customers tell us our manuals are the best DIY manuals they've ever seen. See also this series of livestream videos and tutorials. Although the 990 has not been featured in a video, all of the techniques for soldering components illustrated in the videos apply to the 990 kit.

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