Focusrite ISA220 Session Pack Channel Strip

The Focusrite ISA220 Session Pack Channel Strip provides all the audio precision tools required to infuse your session with Focusrite’s renowned sonic performance. It features many of the original circuits of the flagship ISA 430 MKII Producer Pack, and also includes some new facilities of its own. It also features the same digital output option as the original ISA 430, providing you with a high quality digital route direct from the ISA220 into your digital recording system.

 Left Front

Focusrite ISA220 Session Pack Channel Strip Features:
-ISA-series transformer-based pre-amp
-The benefits of Focusrite’s classic pre-amp topology include superb common-mode rejection, an excellent overload margin, an extremely
  low noise floor and super-wide bandwidth.
-Focusrite EQ:  A four-band EQ, based on the EQ circuits featured in the ISA 430 and ISA 110.
-VCA-based Compressor with 'Blend':  Achieve smoother compression at extreme settings by mixing the uncompressed source signal along
  with the compressed.
-De-esser featuring 'De-ess Listen':  A combination of EQ and phase cancellation are used to create a smoother and less obtrusive result.
-Precision VU metering:  a classic analogue VU style meter which can be switched to display input or compressor gain-reduction levels.
-Frequency adaptive, 3-band limiter:  Ensure distortion-free limiting and helps to prevent critical digital overloads.
-Optional 24-bit/96 kHz stereo digital output:  This AES/EBU, S/PDIF & TOSLINK card - Delta Sigma converter running at 128kHz
  oversampling and dithers down to 20- or 16-bit

 Right Front

The ISA220 starts with the classic Focusrite transformer-coupled microphone pre amp, with phantom power and phase reverse. A four-band Focusrite EQ section follows, based on the EQ circuits featured in the ISA 430 and ISA 110. The compressor features a proprietary Class A VCA with side-chain circuitry, and can be placed in the signal flow before the EQ at the press of a button, if desired. Also included is a unique feature labelled “Blend”, which provides a means of achieving smoother compression at extreme settings by mixing uncompressed source signal along with the compressed portion.

 Left Rear

The Focusrite ISA220 Session Pack Channel Strip also features an opto-based de-esser which uses a combination of EQ and phase cancellation to create a smoother and less obtrusive result. Last but not least, a frequency adaptive, 3-band limiter is provided to ensure distortion-free limiting.

 Right Rear

For visual reference, ISA220 provides a classic analogue VU style meter which can be switched to display input or compressor gain-reduction levels. A pair of 16 segment LED meters are included to display output levels, both analogue and digital in conjunction with the optional 24-bit/96 kHz stereo digital output (AES/EBU, S/PDIF & TOSLINK) card. This card offers 44.1 - 96 kHz operation and can be used as a high-quality mono / stereo tracking converter, or at mix down as a stereo converter for final mastering.

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.

Focusrite ISA220 Session Pack Channel Strip Specifications:
-Inst. Hi Z I/P
  -Noise: -96dBu
  -THD: 0.003% with 0dBu
-Mic I/P
  -Noise: 123dB EIN with 150 Ohm input resistance at 60dB of gain
  -THD: 0.0008%
-Compressor
  -Threshold Range: -28dB to +12dB
  -Ratio: 1.5:1 to 10:1
  -Slope: Soft knee
  -Attack: 500ms to 25ms
  -Release: 100ms to 4s, variable or auto (program dependent)
-De-Esser
  -Threshold Range: 22dB
  -Frequency Range: 2k2 to 9k2
  -Ratio at Centre Frequency: 2:1
-Limiter
  -Threshold Range: 20dBu
  -Ratio: infinite (Brick Wall)
  -Attack: Frequency dependent
-Dimensions
  -484mm (W) x 85mm (H) x 250mm (D)
  -19" (W) x 3.5" (H) x 9.8" (D)