_Linear 87 Circuit Kit

$ 269.00

The "Linear 87" circuit kit has been discontinued. Our T-84 circuit, also available within our all-inclusive T-12 and T-47 microphone kits, delivers equivalent performance at a much lower price point.

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The "Linear 87" is our premier vocal mic, created with a "no compromise" approach to parts selection. It combines a vintage, single-stage FET circuit design with a custom output transformer. Capsule not included; order our RK-47 or RK-12 capsules separately.


• Circuit replacement fits CAD GXL3000
• 3-pattern mic with pad and filter
• Custom PCB (US made)
• Custom output transformer (US made)
• Simple, clean, high-gain circuit design
• No-compromise parts selection process

The "Linear 87" is our premier DIY microphone kit. We based the design on the best single-stage JFET/transformer microphones of years past, including the U87 and KM84. We created a custom transformer for this circuit.

To keep the cost down, we do not provide the "donor" microphone body for this kit. Bigger companies can negotiate better prices from their Chinese suppliers than we can, so we'd have to charge more for the exact same thing you can buy from companies like CAD.

This kit replaces the donor microphone's audio circuit. We strongly recommend that you order a replacement capsule, too, such as our RK-12 or RK-47 (see notes below). That way you would be replacing all the components you can hear. We retain the DC converter circuit from the donor mic, because it works well and is easier to buy than build.

The resulting microphone is a fantastic hybrid of vintage design plus modern performance. The sensitivity is around 22mV/Pa, better than the U87Ai and significantly better than the original U87i. The tone is wonderful; choose from either a Fet47 sound profile (via our RK-47 capsule) or a C414 sound profile (via our RK-12 capsule). We've heard wonderful results from this mic on guitar, vocals, voice, even horns.

The New "Linear '87"

The kit described on this page is a second-generation design. The first generation approach retrofit our new circuit onto the stock audio PCB, and therefore necessitated some compromises in parts selection. This 2nd-gen design includes a custom audio circuit board, which allowed us to make room for the best-sounding parts we could find. As part of the redesign we tweaked the component values to improve headroom. And we sourced a vintage/NOS JFET from Texas Instruments, which we then select for IDSS and headroom.

Because proper JFET biasing is critical in any single-stage JFET circuit, we put each of these parts into our oscilloscope rig so that we can provide the precise bias resistor needed to optimize the transistor's performance for your microphone.

About the Build

Stuffing the new PCB is easy; there are only 14 components. But it is not simple to wire this board into the microphone. The mic has 3 switches with 2–3 wires apiece that must be precisely placed and soldered in order that the mic functions properly. Due primarily to the wiring, we consider this kit a more advanced kit than our other PCB kits.

See a few in-process photos from Kyle Sweet.

What's in the box?

  • Our custom PCB for the audio circuit, designed and manufactured in the US
  • 100% of the components to populate the audio board, including the specially selected and biased JFET (NOS/vintage) and high-quality components from Wima, Panasonic, and Clarity Caps
  • Custom 10:1 output transformer made in the US by Samar Audio
  • US-made, military spec silver/Teflon replacement wire for mic's internals
  • Comprehensive 40-page DIY installation manual with step-by-step instructions, circuit schematic, and wiring guide

Required but Not Included

You will need to supply a CAD GXL3000 or Golden Age FC3 microphone to host this mod kit. Find sale prices on the GXL3000 (currently $99).

You will need a large-diaphragm capsule such as our RK-47 or RK-12.

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