Whistle Rock Audio

ML12 Discrete Mic Preamp PCB


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The ML12 is based on a classic topology that has stood the test of time. Nothing beats a couple of transformers and a discrete opamp to bring you closer to that sonic nirvana! This simple project is ideal for those looking for an easy-to-build and great sounding preamp without breaking the bank.

With a wide range of transformers and opamps to choose from, this preamp can be built in various combinations to meet your personal tonal requirements.

The ML12 is fully compatible with most opamps with the 2520/990 footprint. Additionally, the ML12.DI1 and ML12.DI2 direct inject input modules are designed to interface directly with the ML12 to add a high impedance instrument input to the preamp.

Finally, the ML12 preamp is meant to be powered by the ML12 PSU. Several preamp modules can be powered from the same PSU (up to 8 per PSU) via a single power harness. This makes the wiring and troubleshooting a snap while also helping keep your build clean and professional looking.


  • This PCB is supplied with 6 high-quality Mill-Max opamp sockets and necessary hardware to mount most standard-size output transformers to the board.
  • All ML12.DI2 boards come with a complimentary 2N3819 JFET.


  • All Discrete: The ML12 has a completely discrete signal path from input to output. No integrated circuits here!
  • The Essentials: The ML12 provides +48V phantom power for your condenser mics, -20dB input pad for those loud drums and polarity reversal to help resolve phase problems. While these are all activated via push buttons, the pad and polarity reversal are implemented using sealed, telecom-grade relays to shorten the signal path traces.
  • Gain Control: Gain setting is done via a Bourns 51-series sealed potentiometer which allows for a smooth and continuous adjustment of the preamp's gain from 32 to 70dB*.
  • Discrete Opamp: The ML12's main active gain stage is built around a socketed discrete opamp (DOA) which can easily be swapped out for servicing or experimentation. The choice of DOA used in the preamp will have a perceivable effect on the overall sonic footprint of the preamp. Go ahead and experiment at will!
  • DI Input: An optional DI module can be easily attached to the ML12 via a flexible connection at the edge of the PCB. There are currently two DI modules available that were specifically designed to interface with the ML12's DI header: The ML12.DI1 passive module and the ML12.DI2 JFET active buffer module.
  • Input Transformer: Several input transformers are compatible with the footprint on the ML12 PCB. These include the EA-2622 (AP2622 recreation), Cinemag CM-75101APC, CMMI-8PCA, CMMI-10PCA, Altran C-3402-2, Jensen JT-110K-HPC and JT-115K-EPC. For dual primary winding transformers, the transformer's ratio can be set using the jumpers at the input.
  • DC Blocking Caps: Because some DOAs can have significant DC bias under certain operational conditions, the ML12 provides DC blocking caps at the opamp's output to protect the output transformer. If not required, these capacitors can be defeated by installing a simple jumper to directly couple the DOA to the output transformer.
  • Aux Output: When using an output transformer with four interleaved windings (referred to as 'quadfillar'), only three of those are traditionally put to use. The Aux Out connection gives you access to that fourth winding that you may use as an extra output for monitoring or maybe to feed a VU meter. The choice is yours.
  • Size: The ML12 PCB has a width of only two inches. This makes it possible to have four channels built into a single 1U chassis, including an internal PSU. If building the PSU externally, one can potentially fit eight channels in a single 1U case!

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