Useful Arts SFP-60 Vacuum Tube Preamp  From Useful Arts

The SFP-60 is a two-channel, full-featured all-tube mic pre designed to make microphone signals pop in a mix without the need for other enhancement.  We view the microphone and the preamplifier as a single unit – the signals coming out of a microphone are useless by themselves, and we only know what the microphone is doing after the signal is presented by the preamp.  It seems a shame to use a great microphone and never hear the majesty of its sound, so we designed the SFP-60.

It’s easy to get relatively clean, flat sound out of a preamp costing a fraction of the price –  the preamp in your phone does an astonishingly good job of that. And to most ears, the bare sound of a signal from an average microphone through an average preamp, standing by itself, is pretty good.  Yet dropped into a mix, that perfectly “fine” sound is dead, lifeless and lost.  So begins the cycle of processing, the quest for the one magic box or plugin (or series of them) that will make things sound like they’re supposed to.  The result is usually a louder, heavily-effected but even more lifeless signal that can’t quite find its place in a mix. 

It turns out that fewer, not more, knobs and buttons are the key to incredible sound.  In the early days of recording, the overprocessing phenomenon didn’t exist.  Records were recorded to one or two tracks, and whatever was heard in the room during the performance was heard by the public – simple as that. And it sounded great — with no post processing at all.  The key was that the sound was preserved in all its natural glory before  recording.  It’s a lot like the difference between fresh food and microwaved food.

In a preamp, we are looking for a creative tool that complements the other main tools available: EQ, compression and time-based effects.  None of those tools can make one preamp sound like another.  So let’s get to what’s going on under the hood.

The sound of the SFP-60 is more clear, present and edgy than dark and creamy, and is specifically designed to make vocals sound forward and intimate.  We accomplish this by using tubes for what they are best at – accentuating harmonics that make sound more clear and pleasing to the ear and brain in the real world.  Different tubes have different properties.  Properly used, pentodes add third-order harmonic content that contributes the the “clarity” and “bite” associated with the sounds of the great early radio recordings.  Triodes contribute more second-order harmonic components, which add a richness, or “thickness” to the sound without sounding dark.  (Note: These are subjective impressions.  People can and do debate these adjectives for a lifetime).  The SFP-60 input tube is an EF-806 pentode, and the final 2 gain stages are high-current ECC82 triodes.  12AX7s, which are wonderful in guitar amps and form the basis of many mass-marketed tube mic preamp circuits, are nowhere to be found in the SFP-60.

The character of the harmonic distortion in a preamp has a spectral effect that we intuitively think of as EQ-related, but in reality is completely different from EQ.  EQ filters give you more or less of what you put in (and normally introduce undesirable phase shifts too), while the preamp is actually adding signal that isn’t present in the original signal.  Because this addition is not a perfectly faithful reproduction of the input signal, it’s called distortion (even though it sounds nothing like the clipping distortion generated by a guitar amplifier).  Every audio device that has ever been made introduces distortion, so we might as well make it a good thing.

It turns out that distortion can be quite useful. Every sound we record will have to be reproduced by a mechanical speaker of some sort, and the right kind and amount of distortion can actually help the speaker do its job by compensating for the inadequacy of the reproducing device.  In this way, distortion can actually help make a recorded sound seem more real at playback. 

Enter the color knob on the SFP-60.  Unlike most preamps, which are designed to maintain a predetermined ratio of distortion to gain, the SFP-60 puts the artistic choice of distortion in your hands.  At a setting of 0, the preamp is very, very clean – it sounds “polite.”  A setting of 10 will add about 10db of gain and increase the distortion artifacts by nearly ten times.  The result is an agressive, full sound that stands out in a mix and grabs the listener’s attention.  That’s an incredible creative range, and it gives the SFP-60 the ability to offer very different sounds for different tracks and different sources.