The FXR Power Meter Tube Microphone Preamplifier

There are some projects that are a little bittersweet to have to let go of, the FXR preamp is certainly one of them for me. This may not look like much, but inside is quite a powerful device. The order called for a vocal preamp for ribbon mics, that combination of high gain and low noise isn’t easy to achieve with any active device and certainly not tubes. Add to that, an EQ circuit was needed, after all those pretty knobs are just asking for something to do! Then, to top it all off, an output control to be able to overdrive the unit which means even more loss of precious gain.

Therefore, a special power supply was needed. This uses a switch-mode supply which makes DC heaters a breeze but brings its own set of high frequency issues. Luckily, most of these are above the range of human hearing. With judicious use of filtering, the supply performs very well.

As the original unit contained a UTC A25 that was, of course, shoehorned into the circuit and it makes a really nice sounding output transformer also when saturating a bit. The EQ is active with fixed low and high frequencies, adding a little bass and treble to a vocal mic does wonders! Finally, the switched output attenuator allows for nice control of the saturation as it comes after the output transformer.