Circuit Upgrade Kit for MXL 990

$ 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.

Revised & Upgraded in 2021

We've done extensive revisions to this product over the years, most recently in April 2021. The latest revision provides improved noise filtering at several critical points in the circuit, and a more stable DC oscillator. 

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
  • Internal switch: configure it for either pattern (Cardioid/Omni) or pad when you build the microphone. 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 circuit kit was designed specifically for the MXL 990. Read on for specifics:

13mm Standoffs

The most common version of the 990 chassis uses 13mm standoffs. (The "standoff" is the plastic or metal cylinder that holds the PCB away from the underside of the capsule deck; the mounting screws go through the PCB, through the standoff, into the threaded holes in the capsule deck.) For these mics, our 990 circuit board is installed with components on the capsule side of the PCB.

7mm Standoffs

A small percentage of 990 mics uses shorter standoffs, approximately 7mm in length. For installation into these microphones, you will need a small metal file or Dremel tool to remove approximately 1.5mm of material from the chassis side rails, to provide clearance for components on the board.

990S / 770 / USB - Not Compatible

This kit is not compatible with the 990USB or 990S (the one with switches). It is not compatible with the MXL 770, although we have a circuit kit for that (see the TL3).


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.

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

We tested the newly modified MXL 990 (with our 990 circuit 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 MicParts 990 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 MicParts 990 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 MicParts mics — 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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