The Focusrite design for the original ISA110 EQ was commissioned as a custom rack for Air Montserrat, and later at George Martin’s personal request, for the custom Neve at Air London, (these units are still working at Air Lyndhurst today.) But what makes the Focusrite EQ so special?

The design, like most great designs, is relatively simple; a classic six band EQ, with shelving high and low bands, parametric low mid and high mid bands, and low and high pass filters. The HF and LF shelving EQ’s are unique – they are an implementation of the legendary Baxandall circuit designed in the 1950’s. The EQ is inherently expensive to build as a circuit; most typical shelving filters are variable resistance, fixed capacitance, but the design featured instead variable resistance for gain and variable capacitance for frequency. The positive benefit of this high-end design is that there is no interrelation between EQ bands, (making a change to one band has zero effect on another band,) and that the EQ is virtually noiseless.

The HF and LF EQ features switched capacitor circuits, rather than a variable resistor circuit. (The shape of the shelving filter in the case of a variable resistor circuit varies according to the frequency, thus shelving response is not pure, it’s shallow at one point, steep at another.) With the ISA 110, this switched capacitor circuit means the Q is constant, and the EQ slope is constant, meaning accurate, predictable, beautiful results.

The high-mid and low-mid EQ bands are fully parametric, and are a full implementation of the state variable circuit configuration – separate amplifiers for the Q, and for low-mid/high-mid gain. The usual cheap way to simulate this is to implement a compromise – a gyrator mid-band peaking circuit – but cutting corners in this way invariably means a poor imitation of the real thing. The Focusrite way has, from the beginning, been to choose the solution that sounded best then minimise cost through elegant design.