Transformerless SDC Circuit Kit


This kit is not stocked in the UK warehouse, but can be ordered from the US warehouse; click here.

Low-cost small-diaphragm microphones suffer from the same problems we've seen in large-diaphragm microphones: pricing pressure results in compromised quality.

Introducing the Transformerless SDC Mod Kit

We have spent years optimizing transformerless microphone circuits. We've measured and heard hundreds of components. We know what sounds good.

And, against surprising odds, we've packed our favorite parts into this tiny microphone.

The new circuit is modeled on a design pioneered by Schoeps -- a circuit that has been used in tens of thousands of microphones, by dozens of manufacturers. It is a well-proven design. When properly built, it delivers very low distortion, high sensitivity, and low self-noise. That's what this circuit kit will give you.

It will give you a few extras, too; read on...

The Pad Option

If you plan to use the mic primarily for loud sources, we've included parts to hard-wire a permanent 6dB pad into the circuit. This gives the mic 6dB more headroom so that high-SPL sources won't clip the mic's electronics.

Better yet, this is not a capacitive pad, so the pad does not add more parts (or noise) to the circuit.

Corrective EQ Options

EQ EffectsMost of MXL's small-diaphragm capsules exhibit a fairly sharp peak at 16kHz -- as much as a 6dB boost. We've built an EQ option into the circuit, so that you can neutralize the capsule's high-frequency peak within the circuit.

Two "tone" capacitors are included. Choose the one that makes your mic sound its best. You could use a more aggressive rolloff if you want a darker mic, or a milder rolloff if you prefer a neutral top end. Or, if your capsule sounds good already, build the circuit in "linear" mode and simply enjoy the lower distortion and less cluttered sonic palette provided by the new circuit.

In the box

Each kit includes:

  • Our custom, made-in-the-US two-layer circuit board, designed specifically for DIY with an accessible layout, reasonably sized pads, and thermal cutouts
  • 100% of components needed to build the circuit
  • Additional parts to create the optional 6dB pad and high-frequency EQ, if desired
  • a full-color, step-by-step, 28-page build/installation guide
  • fast and friendly tech support (via email)


You provide: solder, tools, and a couple hours' time.


The circuit board is known to be compatible with the MXL 603S, 991, and 551.

This kit is not recommended for the MXL 604 at this time.

The PCB will physically fit the CAD GXL1200, but would require you to manually drill 4 holes in the board to accommodate the mounting screws.

Skills Required

If you have never built a kit from a PCB before, we recommend getting more soldering experience before you attempt this kit. Although many first-timers have successfully built these kits, our strong preference and recommendation is that you develop your DIY skills on inexpensive "learn to solder" kits first.

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