Useful Arts SFP-30 Tube Microphone Preamp

Tracks that punch through the mix

Inheriting all the attractive sonic attributes of its 2-channel stablemate, the Useful Arts SFP-30 is a full-featured, all-tube, 1-channel mic preamplifier designed to make microphone signals punch through a mix without the need for additional processing. Useful Arts views a microphone and preamp as a single unit, the logic being that mic signals are quite useless by themselves, we only know what a microphone “sounds like” after the signal is processed by a preamp. Why degrade the sound of a great microphone by using it with a mediocre preamp?

Getting a reasonably clean, flat sound out of a budget preamp is easy. Indeed, to most ears, the sound of an average mic through an average preamp is more or less acceptable. Yet in the context of a mix, that “acceptable” signal is lifeless and utterly undistinguished. And so you turn to processing, trying in vain to create sonic magic in a signal that’s completely devoid of redeeming qualities. The result is invariably an over-processed, even more lifeless sound that’s hard to place in a mix. Enter the Useful Arts SFP-30.

Artfully curated tubes

Sonically, Useful Arts’ design brief for the SFP-30 was the same as it was for the SPF-60: clear, present, and edgy as opposed to dark and creamy. Specifically, they designed it to make vocals sound forward and intimate. They employed tubes to do what tubes do best — bring out harmonics that increase clarity and euphonics. Each type of tube has its own properties. Correctly used, pentodes add third-order harmonics that contribute to the perception of clarity and bite that make vocals punch. Triodes are known for producing second-order harmonic content, adding richness and thickening up sources without sounding dark. Useful Arts curated the tubes for the SFP-30 accordingly, choosing an EF-806 pentode for the input tube, and high-current ECC82 triodes for the final two gain stages. Nowhere to be found is the pedestrian 12AX7 — a popular preamp tube choice for guitar amps and mass-market tube mic preamp circuits.

Amazing-sounding harmonic distortion

A preamp’s harmonic distortion has a psychoacoustic spectral effect that humans may perceive as equalization, although in reality, it’s quite different from EQ. While filters output more or less of selected regions of the input signal (along with unwanted phase shifts), the tube preamp is actually adding harmonic content that isn’t in the original signal. The added content is called distortion because it isn’t a faithful reproduction of the input signal. As every audio device ever made introduces distortion, Useful Arts set out to make the SFP-30’s distortion sound amazing. And Sweetwater thinks they succeeded quite nicely.

From flat to exuberant: dial it in with the Color knob

Unlike preamps that are designed to maintain a fixed distortion-to-gain ratio, the SFP-30’s Color knob puts this artistic choice in your hands. At a setting of 0, the preamp sounds squeaky clean — polite, even. Dimed, it adds around 10dB of gain and increases distortion artifacts nearly tenfold, delivering a full, aggressive sound that sits boldly in a mix and commands the listener’s attention. That’s an incredibly broad creative range, and it lets the SFP-30 serve up very different sounds for different sources, tracks, and musical genres.

Useful Arts SFP-30 1-channel Tube Mic Preamp Features:

• Each gain stage is single-ended Class A
• Audio Magnetics input and output transformers, plate inductor (custom-wound by CineMag)
• Output transformer with 50% nickel core
• Flat frequency response: 20Hz–30KHz, ±2dB
• Switches: Input selector (rear-panel XLR/front-panel instrument input), highpass filter, input pad, Phase (polarity) Reverse, phantom power, Color knob for variable harmonic distortion
• Switches pass no signal; they control sealed, gold-plated relays for many years of faithful operation
• Input and output levels of the triode stages are separately controllable via detented attenuators; combined with the Color knob, the SFP-30 delivers just over 60dB of usable gain
• New-manufacture JJ EF806 and ECC82 tubes for long life and easy replacement
• Fully modular construction, with gold-plated connectors used throughout audio path
• Gold-plated rear-panel XLR connectors for the transformer-balanced inputs and outputs
• Output level VU meter, op-amp buffered to ensure that the signal is not loaded by the meter
• Audio power supply features toroidal dual-primary transformer and linear regulation for lowest noise
• Voltage selector for 115/230-volt operation
• Faceplate milled from 0.190"-thick aircraft-grade aluminum for maximum rigidity
• Laser-engraved front-panel markings
• Handcrafted in Phoenix, AZ

Tech Specs

• Preamp Type:Tube
• Number of Channels:1
• Phantom Power:Yes
• Pads:-22dB
• Polarity Switch:Yes
• EQ:Color control, Highpass filter
• Tubes:1 x EF806, 1 x ECC82
• Analog Inputs:1 x XLR
• Analog Outputs:1 x XLR (line out)
• Power Source:Standard IEC AC cable
• Height:4.5"
• Depth:10"
• Width:9"
• Weight:8 lbs.
• Manufacturer Part Number:UAA-SFP-30