Useful Arts Audio SFP-30 Single-Channel Tube Microphone Preamplifier

Bringing the heralded variable second-order harmonics from the dual-channel SFP-60 into a single-channel tabletop format, the Useful Arts Audio SFP-30 tube microphone preamplifier offers tracking engineers, musicians, and vocalists a hand-built, USA-made preamp designed to rival units from vintage consoles and boutique models. The Class-A circuitry includes EF806 and ECC82 JJ vacuum tubes, custom-wound CineMag input and output transformers, and up to 55 dB of usable gain.

The SFP-30 provides adjustable harmonic distortion via its Color control, allowing you to dial in the perfect amount of character for your tracks. Separate switches and LED indicators are present for phantom power, polarity reverse, the -22 dB input pad, and a 120 Hz high-pass filter—helpful functionality for optimizing your signal quality. To avoid undesirable noise and extend longevity, the switches control sealed gold-plated relays rather than having audio routed through them.

Each gain stage is single-ended, inherently Class-A with Audio Magnetics input and output transformers custom wound by CineMag. From 20 Hz to 30 kHz, the SFP-30's frequency response is flat with 2 dB.

The only active devices in the signal path are the tube gain stages, consisting of an EF806 and an ECC82. New-manufacture JJ tubes are used for long life and easy replacement.

Absolutely no signal passes through the four channel switches (phantom power, input pad, polarity reverse, and high-pass filter) to avoid the noise that develops over time in even the most expensive switches. Instead, the front-panel switches control sealed gold-plated relays to ensure years of operation without pops and clicks.

The input and output levels of the triode stages of the SFP-30 are separately controllable by detented attenuators and combined with the Color control, the SFP-30 provides just over 55 dB of useable gain.

With the Color control setting of 0, the preamp is ultraclean, it sounds polite. A setting of 10 will add about 10 dB of gain and increase the distortion artifacts by nearly ten times. The result is an aggressive, full sound that can stand out in a mix and grab the listener's attention.

The SFP-30 offers switches with LED indicators for phantom power, polarity reverse, a -22 dB pad, and a 120 Hz high-pass filter.

Just like the dual-channel SFP-60, the single-channel SFP-30 features separate regulated DC power supplies for tube filaments, phantom power, and high voltage. The high-voltage, soft-start power supply puts out over 270 volts of clean, regulated power to ensure maximum headroom. The audio power supply features a toroidal dual-primary transformer and linear regulation to achieve the lowest noise possible.

The front panel features a balanced mic input and a balanced line output, both on gold-plated XLR connectors. The rear panel provides an IEC mains input and a voltage selector for 115/230 volt operation.

The SFP-30 features an output level VU meter, which is op-amp buffered to ensure that the signal is not loaded by the meter.