Warmth, Color, Harmonic Distortion, its all inside the SPF-60
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.
The spectrum analyzer plots show the difference:
SFP-60 – Color = 0
SFP-60 – Color = 10
The harmonic content generated by the color control is all second harmonic.
British Hybrid (SS preamp)
British Hybrid (With Tube stage)
The harmonic content generated with the tube “coloration” is predominantly third and higher-order odd harmonics. Yuck.
The next characteristic that separates preamps is impulse response – in other words, how does the preamp react to brief transients? Here, we do NOT want distortion – in the time domain, we strive to preserve every detail. We want to capture the dynamic detail in a signal, because there is absolutely no way to recreate what we lose before A/D conversion. Here too, the SFP-60 excels. Compare the impulse response of a well-loved tube preamp currently manufactured by a major company with that of the SFP-60:
Popular Tube Preamp
The mass-produced preamp reproduces the transient, but “rings” before settling down. The result is a slight blurring of the dynamic content of the sound, which translates to “cloudiness.” The SFP-60 reproduces the transient with a minimal settling time, greatly increasing the perceived clarity of the sound.
How do we do this? We load the output tube with a massive inductor (custom wound by Cinemag) rather than a resistor — the same technique used in the design of classics like the Telefunken V76. Sure, a resistor costs a few cents and that one inductor costs us nearly $40.00 per channel. The inductor also permits an internal voltage swings of over 500 volts, requiring the use of very high-grade film capacitors in the signal path. But once you hear what that design technique does for dynamics and the perception of space in a recording, you’ll know why we did it. (You can also imagine why most manufacturers wouldn’t dream of increasing their costs by such amounts)
Useful Arts SFP-60 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–20KHz, ±2dB
- Per-channel 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-60 delivers just over 60dB of useable gain
- New-manufacture JJ EF806 and ECC82 tubes for long life and easy replacement
- Fully modular construction with gold-plated connectors used throughout the audio path
- Gold-plated rear-panel XLR connectors for the transformer-balanced inputs and outputs
- Each channel features an 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