Circuit: Microphone Preamp

Most PC “mic” inputs are pretty average for noise and distortion, and they’re usually mono. Plus, they can’t properly take balanced, pro microphones that often need 48-V phantom power. This preamplifier fixes both issues, delivering clean, balanced, phantom-powered recording.

The Circuit

Let’s consider Joseph Kreutz’s design. Although it might have seemed outdated, using a transformer (TR1) made it possible to design a simple circuit with effective input isolation and strong common-mode signal rejection. The transformer substantially reduced interference, and microphone transformers with a 1:10 ratio were easy to find. This type stepped the signal voltage up by 20 dB, which considerably improved the preamp’s signal-to-noise ratio.
 
To preserve these benefits, the Kreutz specified a FET-input op-amp with negligible noise current. The AD743 was considered ideal thanks to its very low input noise, but it was pricey. The more affordable OPA134 from Burr-Brown (TI) was an excellent alternative, and the prototype was built with a TL071, delivering surprisingly good results. Dual op-amps such as the OP2134 or TL072 were noted as well-suited for stereo projects.

“To apply the signal to TR1 primary, the mic should preferably be connected using a connector to the usual professional XLR standard or via a 6.35-mm / 0.25” stereo jack,” Kreutz noted. “In this case, the tip of the jack corresponds to the ‘hot’ (left channel), the ring to the ‘cold’ (right channel), and the remainder to Ground. Condenser mics require a 48-V supply voltage, applied via 6.8-kΩ resistors R2 and R3. The precise value of R2 and R3 is not essential, but these two resistors must be matched to 0.4% or better. Get out your hi-res ohmmeter to pair them. They must be rated at 0.5 W at least to avoid their burning out in the event of a short to ground.”
  The op-amp (IC1) is powered using the 12- and −12-V rails produced by the lower part of the circuit. Hammond cases were selected to house the two PCBs. The preamp was placed in the smaller
case.

If you want to build a similar project, you will need a few PCBs, about 5 hours of work time and then a bit more for fitting things into a case, and around €300 of parts.
 

The Microphone Preamp Project

The original article, “Microphone Preamp with professional specs,” appeared in Elektor May 2017. You can read the article for free during the two-week period following the publication of this post. Enjoy!

Editor's Note: This article first appeared in a 2017 edition of Elektor. Given the project’s age, the components might not be available. Nevertheless, we think the design will inspire you to start a project of your own.