Useful Arts SFP-30 Tube Microphone Preamplifier

The concept behind the SFP-30 is simple: Take the exact audio and power circuitry from the SFP-60, put a single channel into a tabletop format, and make it less than half the price of the SFP-60. Done. Everything about the SFP-30 signal path is identical to the SFP-60, except that the SFP-30 lacks the DI input.

Just like the SFP-60, the 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 and the lowest noise possible.

Each gain stage is single-ended, inherently class A.

Audio Magnetics (Input and Output Transformers, Plate Inductor) custom wound by Cinemag

Output Transformer with 50% nickel core

Frequency response is flat within 2dB from 20Hz-30KHz:

Each channel of the SFP-30 features 4 switches:

High Pass Filter

Input Pad

Phase Reverse

Phantom Power

Absolutely no signal passes through these switches, because we hate 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 55dB of useable gain.

The rear panel is simple: it offers a voltage selector for 115/230 volt operation in the IEC fuse block. The gold-plated XLR connectors are on the front panel.

The only active devices in the signal path are the three tube gain stages, consisting of 1 EF806 and 1 ECC82 per channel. We use new-manufacture JJ tubes for long life and easy replacement.

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

The audio power supply features a toroidal dual-primary transformer and linear regulation for lowest noise.