T-84 Circuit Kit

$ 229.00

Style: Transformer-coupled JFET
Distortion: Moderate second harmonic
Noise: Low
Character: Saturated under moderate SPL
Sensitivity: High
Circuit Frequency Response: Flat
Polar Patterns: Cardioid / Omni (via internal switch)
Pad: 10dB (via internal switch)


The T-84 is a premium transformer-coupled microphone circuit kit, designed as a DIY alternative to commercial single-stage JFET/transformer microphones.

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 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).

Bundle Discount Option

For a limited time, get a $50 discount on the LDC capsule of your choice via our LDC Bundle Discount plan.


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.


  • High-quality 2-layer circuit boards, designed in-house for DIY
  • Custom-made 6.5:1 output transformer
  • US-made, premium film capacitor for audio output (retail cost: $30)
  • 100% of necessary components to build the mic
  • 2 internal switches: Cardioid/Omni pattern, and -10dB pad
  • Comprehensive, step-by-step DIY instruction guide

Transformer Options

The "MP6.5" transformer is our custom-wound BV8 style transformer. It is the standard transformer used in this kit since we first developed it. It offers smooth, flat frequency response from 20Hz to beyond 20kHz, with musical and predictable harmonic coloration.

The "T6.5" transformer shares the size, turns ratio, and performance of the MP6.5. We developed this transformer for our 990XF kit. Within the T-84 circuit, it delivers very slightly higher second harmonic coloration (~2dB), and equally smooth/flat frequency response (deviation less than 0.5dB) from below 20Hz to above 30kHz.


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.

Related Products