This article originally appeared in the February, 2017 issue of Pro Sound News as "Innovations: The Manufacturer's View - Pushing the Art of the Possible: Focusrite Red Range." Innovations is a monthly column where different pro audio manufacturers are invited to discuss the thought-process behind creating their products of note.
Focusrite’s audio interfaces are based on three “pillars” that can be seen across all the products in the range: high-quality microphone preamplifiers, precision digital conversion, and low latency. In the case of the Red Range, their importance is clearly evident. The Red range—currently consisting of the Red 4Pre with four mic preamps, and the newly-introduced Red 8Pre with eight—is aimed squarely at the professional end of the market, with Thunderbolt and direct Pro Tools connections, along with Dante-based audio over IP capability.
REBUILDING THE CLASSIC PREAMP
When it came to designing the Red preamps, Focusrite’s design team took as its inspiration the sound quality of the original transformer-input mic preamp found on the Focusrite Red Range—a design itself based on the very first product the company produced, the ISA (Input Signal Amplifier). The team then determined to push “the art of the possible” to the extreme in terms of audio performance, using the very highest quality modern components. The result was the “Red Evolution” mic pre—so called because it was a modern development of the goals of the original Red preamp.
The highly-regarded sound of the original Red mic preamp rested on several features. The transformer input delivered a high common-mode rejection ratio, resulting in extremely low noise. The secondary provided linear, low-distortion gain. The transformer shapes the low frequency end of the curve, while the transformer’s resonance provides a lift at higher frequencies that adds life and openness to the sound, particularly on vocals and acoustic instruments. This combination of input impedance and resonance is what Focusrite customers have traditionally referred to as “Air.”
One design goal for the new Red preamp was to recreate this aspect of the original mic preamp, using modern components to exceed the capabilities of the original. The design was based around a highly-linear, low distortion amplifier, an accurate analog model of the original Red transformer behavior and a passive frequency shaper to add the resonance. The concept, originally created for Focusrite’s Clarett line of Thunderbolt interfaces, was refined and developed with a higher performance especially for the Red range. The remote-controlled Red design employs a gain control based around a precision resistor network providing 1 dB steps, allowing accurate setting recall and channel matching. A high-precision balancing amplifier provides improved CMRR and signal-to-noise ratio, and in fact, the entire signal path from this point is balanced, right the way to the converter inputs.
THE FINEST COMPONENTS
We paid particular attention to specifying the components used in the design, including the selection of 0.1 percent thin film resistors throughout, with audiophile-grade MELF (metal electrode leadless face) resistors in the mic input—more than 100 times quieter than conventional resistors—and electrolytic capacitors that provided the best possible low-frequency performance, as well as highly-specified op-amps. The design ensured that where resistor noise was a significant factor, the impedance was kept low to minimize it. Components that were stable over a wide temperature range were specified to maximize improved long-term reliability. The combination of specially-selected components, careful circuit design and board layout, and the provision of separate analog and digital power supplies to minimize the risk of digital noise getting into the analog circuitry resulted in an Equivalent Input Noise (EIN) of -129 dB—right on the edge of theoretical performance for analog components—and a THD + Noise of a remarkable 0.0009 percent. We also gave the preamp enough headroom—19 dBu—to take whatever might be thrown at it in the studio, and with a wide gain range, from 0 to 63 dB.
While the analog model of the original transformer design provides the “Air” effect beloved of ISA owners, the Red preamp also boasts a “flat” mode, which attempts to get as close as possible to the theoretical “piece of wire with gain” amplifier that audio engineers seek, offering the best possible performance for professional recording. In this mode, a high-input impedance reduces the microphone-to-input coupling, resulting in an extended low-frequency response, while the removal of the resonance circuit allows HF performance to be essentially flat all the way up to 35 kHz.
When it came to converter selection for the Red range, three criteria were established that had to be balanced without compromise: wide dynamic range (up to 121 dB in the DAC), low distortion (0.0009 percent THD+N) and extremely low latency. To deliver this level of performance, the conversion system in the Red 8Pre employs a ‘parallel path summing’ configuration, where two matched converters are run in parallel and their outputs summed, providing a 3 dB improvement in the noise floor. The DACs are also DC-coupled to the output stage for maximum linearity and flattest possible low frequency response, as well as minimum distortion.
When we designed the Red range, we set ourselves the goal of pushing audio performance to the limit of what was possible in silicon. We believe we have achieved that goal with a product range that delivers on all three of our pillars of interface design: high-quality microphone preamplifiers, precision digital conversion and low latency.
Richard Elen is a Focusrite Technology Specialist.