Focusrite EQ

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.

Sound On Sound Magazine review quotes
Jan 2004
Hugh Robjohns
"The ISA 430 MKII has been designed to provide the most complete and superb-sounding channel path available, and it does that in spades... I should think a similar volume will be sold to home and project studios that need just one really good front-end unit, and with the optional A/D card, the ISA 430 MKII has to be one of the best and most versatile units on the market today. I can’t think of anything negative to say about the ISA 430 MKII, it really is that impressive. Committed recording engineers now have a new British standard-bearer when it comes to high-end analogue recording channels."

Pro Audio Review Magazine review quotes
March 2004
Russ Long

"Holding true to the original Focusrite vision, the Class A Focusrite ISA 430 MKII offers an unequalled, uncompromising recording channel that includes a mic/line preamp, equaliser, compressor, expander/gate, de-esser and limiter."

"Several new features significantly improve the performance of the $2,995 MKII over the now discontinued ISA 430."

"I found that the ISA 430 MKII works extremely well with drums and percussion. I had nice results using the box along with an AKG D112 on kick drum with a Shure SM-57 and a BLUE Ball on snare. I also had good results using the box along with a BLUE Bluebird to record percussion."

"I had fantastic results using the ISA 430 MKII to record vocals. I used the unit with the BLUE Cactus and Bluebird, and every instance had pleasing results. I found that the vintage compressor mode worked extremely well with vocals, especially when using a tube microphone. Plus, the de-esser worked substantially better than my old faithful dbx 902."

"I found that the Blend feature worked extremely well on bass guitar and vocals. This allowed me to create a more compressed track that actually had the feeling of being less compressed."

"I had good results recording bass guitar through the instrument input. The instrument had a nice bottom end but still maintained its presence and definition. I also had good results using a Royer R-122 to record electric guitar and an AKG C28 to record acoustic guitar."

"The ISA 430 MKII’s A/D converter sounds fantastic. At 96kHz, they are every bit as good as anything I’ve encountered. I used the box during a mix to simultaneously EQ a vocal whilst compressing a bass guitar. Though it can be a bit confusing, the complex signal flexibility is one of the box’s strengths."

"In the Focusrite tradition, the ISA 430 MKII sounds wonderful. The box has tremendous clarity and detail and is extremely clean and quiet. The EQ is both musical and powerful and the compressor and de-esser are quick and easy to use, and they sound extraordinary."

Key Features

• ISA-series transformer-based pre-amp
  Featuring mic impedance switching and unique ‘mic air’ effect (a wire-wound inductor for increased spaciousness).
• Dual topology compressor
  The ISA 430 MkII is the first ISA to feature both the classic transparent VCA circuit, and avintage optical circuit with both soft compression and crunch settings.
• Unmatched flexibility
  The ISA 430 Mk II features an incredible variety of insert points and variable signal path arrangements. It can operate as four independent modules!
• Precision VU metering at multiple points
  Including input level, Insert 1 or 2 Return level, Compressor gain reduction or Sidechain listen level (switched).
• Classic Focusrite EQ design
  an expanded version of the original ISA 110 with two additional shelving positions.
• Gate and Expander
  Proprietary Focusrite discrete Class A VCA Gate and Expander designs, now both featuring a 'Listen' option for precision control.
• De-esser featuring 'De-ess Listen'
  A combination of EQ and phase cancellation are used to create a smoother and less obtrusive result.
• Optional Strereo 192kHz ADC
  Embodies cutting-edge conversion technology, incorporating Focusrite analogue circuitry to deliver A/D conversion of the highest quality (120dB Dynamic range).