T-84 Circuit Kit

$ 189.00

The T-84 is a premium transformer-coupled microphone circuit kit, designed as a DIY alternative to commercial single-stage JFET/transformer microphones such as the Mojave Audio MA-201/MA-301 FET.

The core design is Neumann's KM84 circuit. We've taken it to the next level by implementing it with audiophile-grade components, a painstakingly selected and manually biased NOS JFET, and a custom-wound, US-made output transformer. We've discarded the parts of the circuit that were doing more harm than good, and added a few bits that Neumann probably would have if they'd had room inside the KM84 body.

The finished mic has high output, carefully contoured harmonic coloration, and great tone.

Pair it with our RK-47 capsule or RK-12 capsule to create a fantastic studio instrument.

Mod Kit vs. Microphone Kit vs. Microphone

This circuit kit is sold as a standalone unit because it works as a drop-in upgrade for numerous inexpensive donor-microphones; see a partial list below.

If you're interested in building this into a boutique microphone of your own, see our "T-series" DIY microphone kits, which include an unbranded mic body and chassis, a capsule, and this circuit kit.

If you love the idea of this circuit but can't build it yourself, see the finished T-47 microphone (built by hand, to order, in California).


Read the SonicScoop review of the T-12 kit.

Read the Ask Audio review of the T-47 kit.

(Both the T-12 and T-47 microphone kits use this T-84 circuit.)


The T-84 circuit boards are known to fit:

  • MXL V67G
  • MXL V63M
  • MXL 2001
  • MXL 440
  • MCA SP1
  • CAD GXL2200
  • Apex 430 (but see notes below)

T84 PCB dimensions

If you'd like to verify fit in your microphone, click the image at right to download the dimensions (as a PDF). Print the document, check that the printed dimensions match your hardcopy, then cut out the paper PCB and hold it against your chassis.

Note that the T-84 is a 2-board set; your chassis would ideally accommodate boards on both sides of the chassis. That said, the second PCB just holds the transformer. If your mic chassis (e.g. Apex 430) has mounting tabs on only one side of the chassis, you can still use this kit -- just plan to tape or zip-tie the transformer in place behind the audio PCB.

This circuit is also available within our all-inclusive T-47 microphone kit and T-12 microphone kit.

Pristine, vintage signal path

Capsule→JFET→output capacitor→transformer. That's it.

We use no electrolytic capacitors and no tantalum capacitors anywhere in this circuit.


  • Custom, US-made, 2-layer circuit boards
  • Custom, US-made 6.5:1 output transformer
  • 100% of necessary components to build the mic
  • 2 internal switches: Cardioid/Omni pattern, and -10dB pad
  • Comprehensive, step-by-step DIY instruction guide

 Skill Level

This circuit kit is easier to build than our transformerless circuit kits; it has approximately half the parts count of our MP-V57, 990, and TL3 circuits. Further, it has only 2 polarized components, whereas the transformerless kits have 11 or more.

If you've never built a microphone circuit before, this kit is a good first project. We've had many first-timers successfully complete this circuit.

But: you must be able to solder. If you cannot reliably form sound solder joints, please do not buy this kit.

If you're looking for a less ambitious project, we recommend starting with a capsule upgrade, or our Studio Projects C1/C3 upgrade kits.

Related Alternative

Our T84-55 circuit kit employs the same circuit as the T-84, but incorporates a few changes:

  • Different capacitor complement (not better or worse. just different.)
  • Supports external pad/filter switches.
  • Fits GT55, ST55 donor bodies.

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