Focusrite ISA 428 Pre Pack User Guide

Introduction

Thank you for purchasing the ISA 428 Pre Pack from the Focusrite team. Focusrite prides itself on designing and delivering high-quality audio units. We hope your new unit provides many years of productive recording. For feedback on your recording experiences, please email sales@focusrite.com.

The Focusrite Team: Martin, Crispin, Helen, Tim, Tom, Mick, Dave, Simon, Paul, Phil, Micky, Pauline, Jo, Chris, Giles, Chris, Rob, and Simon.

Important Safety Instructions

Please read and save these instructions. Follow all warnings marked on the unit.

• Do not obstruct air vents or insert objects through apertures.
• Do not use a damaged or frayed power cord.
• Unplug the unit before cleaning. Clean with a damp cloth only. Do not spill liquid on the unit.
• Refer servicing to qualified personnel if the power cord is damaged, liquid has entered the unit, the case is damaged, or the unit does not operate normally. Adjust only controls covered by the operating instructions.
• Do not defeat the purpose of polarized or grounding-type plugs. Consult an electrician for outlet replacement if the plug does not fit.

WARNING: THIS UNIT MUST BE EARTHED BY THE POWER CORD. NEVER DISCONNECT THE MAINS EARTH.

Ensure the correct mains voltage setting and fuse are used. Do not change mains voltage settings while the mains supply is connected. Replace fuses only with the correct value. The internal power supply unit contains no user-serviceable parts. Refer all servicing to a qualified service engineer.

Power Connections

An IEC mains lead with the correct moulded plug for your country is supplied. The wiring colour code is:

• USA, Canada, Taiwan, Japan: Live - Black, Neutral - White, Earth - Green
• Other countries: Live - Brown, Neutral - Blue, Earth - Green and Yellow

Signal Connections

(Optional ADC shown fitted)

XLR (Audio) Inputs and Outputs: All 3-pin XLR balanced audio connectors (Output, Mic IP, and EXT A/D IP) are wired as follows: Pin 1: Screen/Chassis, Pin 2: Audio 0°, Pin 3: Audio 180°.

1/4" Line Input and Insert Sends and Returns: Wired as follows: Tip: Audio 0°, Ring: Audio 180°, Sleeve: Screen/Chassis.

Inst. Hi Z IP: Wired as follows: Tip: Audio 0°, Sleeve: Screen/Chassis.

Mic IP/Line IP/Inst. Hi Z IP (front panel): These inputs can be used as the input to the ISA 428 channels 1-4, referred to as 'internal' channels.

Outputs - Channels 1-4: These are the main analogue signal outputs, fed by Mic, Line, or Instrument Inputs. They also connect to the ADC 'internal' channels 1-4 via the Soft Limiter circuit.

Insert Send and Return: Allows an external unit (e.g., Red 3 Compressor or Red 2 EQ) to be placed within the signal chain, prior to the output and post the high pass filter.

ADC Inputs 5-8: Act as line level inputs for routing 'external' signals to the optional ADC channels 5-8 via the Soft Limiter. Using these inputs with a single optional ADC card allows up to eight analogue inputs to be routed to eight digital outputs. With a second ISA 428 unit and a single optional ADC card, eight pre-amps can be routed to all eight ADC channels.

Retrofitting the Optional ADC: The optional ADC can be retrofitted to a standard ISA 428 by the user. Full fitting instructions are included with the ADC.

Getting to know the ISA 428

Power: Applies power to the unit. Turn on the ISA 428 before powering up connected devices.

Instrument Inputs: Four unbalanced Instrument input connectors (1-4) are located on the front panel for connecting low-level unbalanced signals from passive guitars, basses, keyboards, or electro-acoustic guitars.

Metering:

Peak Meter and O/L LED: This moving coil meter has a fast reaction time and slow decay, ideal for displaying and holding peak signal levels. The meter is fed from after the high pass filter and before the insert send. It indicates signal level after pre-amp gain and the signal sent to the insert send output. The meter is calibrated relative to the overload point of the optional ADC card (0dBfs = max digital conversion level) and the analogue overload point (0dBfs = +22dBu, 6dB below max analogue level). The O/L (overload) indicator lights red when the signal exceeds 0dBfs (+22dBu), serving as a safety feature.

Analogue/ADC dBfs Output meters: These vertical LED columns indicate peak signal levels for channels 1-8. There are two modes:

• Mode 1 (Analogue only): Meters 1-4 indicate analogue level at the ISA 428 XLR outputs. 0dBfs (red LED) indicates +22dBu, meaning a +4dBu signal is present at the output (-18dBfs). Meters 5-8 have no function.
• Mode 2 (Digital, optional ADC card fitted): Meters 1-8 indicate the signal level after the Soft Limiter and before ADC conversion. 0dBfs (red LED) indicates the maximum signal level for the optional ADC card and should only be lit briefly to ensure good quality recording without digital overload. The Soft Limiter's effect on peak signal levels will be indicated by a reduction in peak levels on the LED meter.

Input Stage

Three input options are provided for compatibility with microphone, line, or instrument level sources.

Input: Pressing the INPUT switch cycles through the three inputs (Mic, Line, Instrument), indicated by LEDs. A mix of input types can be selected across the four channels.

Mic Input Gain: With Mic input selected, gain ranges from 0dB to +60dB in 10dB steps. The 30-60 switch selects between two gain modes:

• Mode 1 (0-30dB): 30-60 switch OFF. Gain knob operates from 0dB to +30dB.
• Mode 2 (30-60dB): 30-60 switch ON (illuminated). Gain knob operates from 30dB to +60dB.

An additional 20dB of gain can be applied using the Trim knob.

Line Input Gain: With Line input selected, gain ranges from -20dB to +10dB in 10dB steps. The 30-60 switch is inactive. An additional 20dB of gain can be applied using the Trim knob.

Instrument Input Gain: Gain is applied using the Trim control only, offering a +10dB to +40dB gain range. This input is suitable for high impedance sources like guitar/bass pickups or vintage synthesizers.

Trim: Provides additional variable gain of 0dB to +20dB when Mic or Line inputs are selected. This is useful for achieving higher gain levels (up to 80dB total with Mic gain) or for making small adjustments during recording without intrusive stepped changes.

+48V: Provides +48V phantom power for condenser microphones via the rear panel XLR connector. This switch does not affect other inputs. Consult your microphone's manual to avoid potential damage to ribbon microphones.

Phase

Pressing PHASE inverts the phase of the selected input to correct phase problems with multiple microphones or incorrect wiring polarity.

Insert In

Pressing the INSERT switch breaks the signal path, sending the channel input signal to the rear panel Insert Send connector and returning it via the Insert Return connector.

Filter In

Pressing the FILTER IN switch activates the High Pass Filters. The filter provides an 18dB/octave roll-off, with a variable control for the roll-off frequency from 16Hz to 420Hz.

Diagram showing High Pass Filter curve with -18dB/octave slope.

Impedance

Pressing IMPEDANCE cycles through four transformer pre-amp input impedance values. Selecting different values tailors the performance of the ISA 428 pre-amp and the connected microphone for desired level and frequency response.

Analogue to Digital converter (ADC) Soft Limiter

Pressing ADC SOFT LIMITER activates the Soft Limiter, providing total protection for all 8 channels on the ADC.

ADC Soft Limiter Operation: The Soft Limiter clamps the audio instantaneously, preventing the signal from exceeding the maximum level the ADC can accurately convert (0dBfs). This protects the optional ADC card from overload. This function is only active when an ADC card is fitted.

8 Channel Digital Output Option

The ISA 428 can function as a high-quality 8-channel ADC for analogue-to-digital transfer with the optional ISA 428 digital output board. This allows eight super-clean, protected, high-quality paths to digital. A single ISA 428 unit can act as an 8-channel digital input expansion unit to any DAW. Channels 1-4 route to ADC output channels 1-4, and channels 5-8 route to ADC output channels 5-8.

Alternatively, two ISA 428 units with a single A/D option can create an 8-channel mic pre to A/D system. Digital formats available include AES/EBU, SPDIF, and ADAT™ optical. ADAT™ outputs can operate in high-speed SMUX mode for 96kHz transfer speeds (muted during 192kHz operation).

24-bit/96kHz ADAT™ interface operation: The card provides digital outputs for all eight ISA 428 channels, operating at sample frequencies of 44.1, 48, 88.2, and 96kHz, with dithering to 16-, 20-, or 24-bit depths. It features two ADAT™ 'lightpipe' output connectors. At speeds up to 48kHz, both connectors transmit all 8 channels simultaneously. Above 48kHz, due to bandwidth limitations, each audio channel uses two ADAT™ digital channels, requiring two connectors for 8 channels of conversion at high speed.

• 44.1/48kHz: Connector 1 = channels 1-8; Connector 2 = channels 1-8 (identical).
• 88.2/96kHz: Connector 1 = channels 1-4; Connector 2 = channels 5-8.

ADAT™ lightpipe cables are available from dealers or Studiospares (UK: +44 (0)20 7482 1692, stock number 585-510).

24-bit/192kHz AES/SPDIF operation: The card also provides AES and SPDIF format outputs via two 9-pin D-type connectors on the rear panel, supporting the full range of sample rates (up to 192kHz) and bit depths.

Digital Output Front Panel Controls

Clock Select: Pressing this switch allows selection of sample frequencies: 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz, and 192kHz.

Bit Depth Select: Selectable between 24, 20, and 16 bits.

Ext Select: Pressing EXT allows the ISA 428 to be slaved to an external word clock source. Selecting 256X allows slaving to an external clock running at 256 times the sample frequency (e.g., Digidesign 'Superclock').

Lock LED: Indicates when the unit is synchronized to an external clock.

Image of the ISA 428 front panel showing digital output controls.

Applications

Mic Pre-amp Input Impedance: The interaction between the microphone and the pre-amp interface affects the microphone's level and frequency response.

• Level: Professional microphones with low output impedances benefit from higher impedance settings on the ISA 428 for more level.
• Frequency response: Microphones with defined presence peaks can be enhanced by lower impedance settings. Higher input impedance settings can emphasize high frequencies, improving clarity and ambient information.

Experimenting with different impedance settings can achieve desired 'colouration' for instruments or voices.

Switchable Impedance: In Depth Explanation:

Dynamic moving coil and condenser microphones: These typically have low nominal output impedance (150-300Ω at 1kHz) for noise immunity and driving long cables. Low output impedance loads the microphone's output voltage, emphasizing frequency-related variations. Matching impedance (e.g., 200Ω pre-amp to 200Ω microphone) reduces output and signal-to-noise ratio by 6dB.

To minimize loading and maximize signal-to-noise ratio, pre-amps traditionally have input impedances about ten times greater than the microphone's (1.2kΩ to 2kΩ). Higher input impedance settings tend to flatten low and mid-frequency response and boost high frequencies compared to low impedance settings.

Ribbon microphones: Ribbon microphones have extremely low impedance (around 0.2Ω) and require an output transformer. The output transformer's ratio (around 1:30) increases the ribbon voltage and also the output impedance to around 200Ω at 1kHz. This impedance varies with frequency, potentially doubling at resonance points and rolling off at low/high frequencies. Therefore, pre-amp input impedance significantly affects ribbon microphone signal levels and frequency response. It is recommended that the pre-amp input impedance is at least 5 times the nominal microphone impedance.

• For 300-120Ω ribbon mics, 600Ω (Low) impedance is suitable.
• For 120-200Ω ribbon mics, 1.4kΩ (ISA 110) impedance is recommended.

Impedance Setting Quick Guide:

• High impedance settings: Generate more overall level, flatten low/mid-frequency response, improve high-frequency response.
• Low impedance settings: Reduce microphone output level, emphasize low/mid-frequency presence peaks and resonant points.

Signal Connections

Recording from analogue outputs to DAW:

Diagram showing connections for Microphone and Line inputs to a DAW via balanced XLR and TRS jacks. Also shows connection to an outboard processor.

Digital connections:

Diagram showing digital connections from a DAW and Word Clock source to a Digital Mixing Console via lightpipe and BNC connectors. Also shows AES format connection via a 9-pin D-type to 4-off XLR conversion cable.

Two ISA 428 units used as single 8-channel ADC

Diagram illustrating the connection of two ISA 428 units for 8-channel ADC operation.

Using the ISA 428 with a Digidesign 192 HD™ interface:

Analogue mode: Connect the ISA 428's analogue XLR outputs to the Digidesign 192 HD™ interface's DB25 'Analog Input' connector using a DB25 to 8 female XLR cable. Cables are available from Digidesign or Hosa (part no. DTF 805).

Digidesign Cable Information:

• DB25-XLR M+F AES/EBU Snake, 12' (MH080)
• DB25-XLRM Snake, 12' (MH081)
• DB25-XLRF Snake, 12' (MH082)
• DB25-TRS Snake, 12' (MH083)
• DB25-XLR M+F AES/EBU Snake, 4' (MH097)
• DB25-XLRM Snake, 4' (MH098)
• DB25-XLRF Snake, 4' (MH099)
• DB25-TRS Snake, 4' (MH100)

The ISA 428 ADC level meters are calibrated to show 0dBfs at +22dBu. The HD interface can be calibrated to indicate 0dBfs at +22dBu for easier level checking. Refer to the Digidesign 192 HD™ user guide for calibration instructions.

Digital Mode: Connect the ISA 428 AES digital output 9-pin connector to the HD unit's 25-way 'AES/EBU I/O' connector using two cables: a Focusrite 9-way to 4 XLR cable and a Digidesign 25-way to AES I/O XLR cable (called 'DB25-XLR M+F AES/EBU DigiSnake™').

Note on HD AES channel restriction: The HD unit's DB25 AES/EBU connector can accept 8 channels of AES/EBU data up to 96kHz. However, at speeds over 96kHz (up to 192kHz), it can only accept 4 channels of audio data. When the ISA 428 operates at speeds up to 96kHz, a single 9-way to AES XLR cable can route all 8 channels into the HD interface. At 176.4 to 192kHz, a single cable can only route 4 channels (either channels 1-4 or 5-8, depending on the connector used).

Other Compatible Focusrite Products

• Red 2: Dual equaliser with 4-band EQ (shelving HF/LF, parametric High/Low Mids), High-pass and Low-pass filters.
• Red 3: Class A VCA-based dual mono/stereo compressor/limiter for high-class compression and limiting.
• Red 7: Direct recording processor with compressor, de-esser, exciter, Red Range mic pre, and dynamics from Red 3.
• ISA 430 Producer Pack: Includes a wide range of classic Focusrite modules in a single frame, with new processing technology, routing flexibility, and digital connectivity.
• ISA 220 Session Pack: Provides legendary audio processing tools, featuring original ISA 430 circuits and new features.
• Platinum Compounder: High-performance dual-channel dynamics processor with high-quality compression and a powerful Bass Expander.
• Platinum MixMaster: Analogue stereo audio processor for project studio mastering.
• Platinum Penta: 2U rack-mountable unit featuring a Focusrite Class A pre-amp, editable stereo preset compressor with 'Tube Sound' technology.
• Platinum Trak Master: Affordable tracking device with high-quality mic pre, compression, 3-band EQ, and 'Tube Sound' control.
• Platinum VoiceMaster Pro: Voice recording equipment with Class A pre-amp, latency-free monitoring, voice-optimised EQ, Vintage Harmonics, and Tube Sound.

Visit www.focusrite.com for more information on these and other compatible products.

FAQS

Q: What are the basic features of the ISA 428?
A: Four Focusrite mic pre's, eight line inputs, four instrument inputs, optional 8-channel 192kHz A/D conversion.

Q: Which applications is the ISA 428 suitable for?
A: Multi-channel front end for DAWs, interface/A/D converter for synths, additional mic pre's for digital consoles.

Q: Which Focusrite pre is featured on the ISA 428?
A: The original transformer-balanced mic pre from classic 1980s Focusrite consoles, also featured in the ISA 430 Producer Pack.

Q: Do the pre's have the usual phantom power and High Pass Filter controls?
A: Yes, plus switchable mic impedance, inserts per channel, phase reverse, and phantom power.

Q: What's significant about the impedance switching for each pre?
A: Each pre can be matched perfectly or offset for creative 'response colours'. Impedance is switchable between 4 settings: original ISA 110 (Zobal network influence), Low (600 Ohms), High (2.4k), and Higher (6.8k).

Q: Are insert points featured?
A: Yes, switchable in- or out-of-circuit on channels 1-4.

Q: What are the four extra inputs on the left hand side of the front fascia for?
A: Unbalanced inputs for easily connecting sources like guitars/basses without an external DI box.

Q: Why are there eight line inputs but only 4 mic pre's?
A: Allows routing of 8 line sources to the 8-channel 192k converter with a single unit. Also allows conversion of a 4-pre system to an 8-pre system with 2 units.

Q: Why is it called "428"?
A: Because it's easy to convert a 4-pre-to-digital system to an 8-pre-to-digital system.

Q: Can I use 2x ISA 428s with 1x A/D card? If so, how does the routing work?
A: Yes. Mic pre's 1-4 on the first unit route directly to digital outputs 1-4. Mic pre's 5-8 on the second unit loop back to line inputs 5-8 on the first unit, then route to digital outputs 5-8.

Q: But if I run an 8-channel system how can I monitor levels?
A: Each 428 includes 8 output meters and 4 channel peak-reading input meters.

Q: What's the specification of the A/D option?
A: AES, SPDIF, ADAT™ formats; sample rates 44.1-192kHz (ADAT max 96kHz); 16, 20, 24 bit with dithering; internal/external word clock, 256X clock; S/N Ratio better than 120dBfs 'A-weighted'. Connections via 2 x 9-pin D-type and standard lightpipes; word clock via BNC in/out.

Q: Does the ISA 428's A/D option feature word clock as standard?
A: Yes, word clock can be fed in via a BNC connector for synchronization to any word clock master source.

Q: Why do I need word clock anyway?
A: To ensure all digital equipment bit-streams are synchronized. A word clock master dictates the word clock for slaves. Failure to sync results in audible clicks and glitches. The 428 regenerates word clock at its BNC output.

Q: What if my system only operates at 16 bit, 44.1 kHz?
A: The ISA 428 features adaptive dithering and supports most sample rates. You can use it with 16, 20, or 24 bit systems operating at 44.1, 48, 88.2, 96, 176.4, or 192kHz.

Q: How many rack spaces does ISA 428 take up?
A: The ISA 428 is a 2U device.

Q: What rear panel connections are featured?
A: 4 XLR mic inputs, 8 XLR line inputs, 4 balanced XLR analogue outputs, balanced 1/4" send/return inserts per channel, digital connections (if A/D fitted), and a voltage-switching power socket.

Specifications

Mic Input Response:

• Gain range: 0dB to 60dB in 10dB steps.
• Input Impedance (IKHz): Low (600Ω), ISA110 (1400Ω), Med (2400Ω), High (6800Ω).
• EIN (equivalent input noise): -128dB (at 60dB gain, 150 Ohm termination, 20Hz/22kHz bandpass filter).
• Noise at main output (unity gain): -97dBu (20Hz/22kHz bandpass filter).
• Signal to noise ratio (relative to max headroom, 28dBu): 125dB.
• Signal to noise ratio (relative to OdBfs, +22dBu): 119dB.
• THD (at medium gain, 30dB): 0.001% (1kHz, -20dBu input, 20Hz/22kHz bandpass filter).
• Frequency response (min gain, 0dB): -0.25dB at 20Hz, -3dB at 120kHz.
• Frequency response (max gain, 60dB): -2.5dB at 20Hz, -3dB at 120kHz.
• CMRR (at full gain, 60dB): 80dB.

Line Input Response:

• Gain range: -20dB to +10dB in 10dB steps.
• Input Impedance: 10kΩ (10Hz to 200kHz).
• Noise at main output (unity gain): -91dBu (20Hz/22kHz bandpass filter).
• Signal to noise ratio (relative to max headroom, 28dBu): 119dB.
• Signal to noise ratio (relative to OdBfs, +22dBu): 113dB.
• THD (at unity gain, 0dB): 0.002% (+4dBu input, 20Hz/22kHz bandpass filter).
• Frequency Response (at unity gain, 0dB): 0.25dB down at 20Hz, -3dB down at 140kHz.

Instrument Input Response:

• Gain range: 10dB to 40dB continuously variable.
• Input Impedance: > 1 Meg Ohm.
• Noise (minimum gain, 0dB): -90dBu (20Hz/22kHz bandpass filter).
• Noise (maximum gain, 40dB): -78dBu (20Hz/22kHz bandpass filter).
• THD (minimum gain, 0dB): 0.006% (-10dBu input, 20Hz/22kHz bandpass filter).
• Frequency Response (at 10dB gain): 0.2dB down at 26Hz, 0dB at 32kHz.
• Frequency Response (at 40dB gain): -3dB down at 26Hz, -3dB down at 32kHz.

High Pass Filter:

• Roll off: 18dB per octave, 3-pole filter.
• Frequency Range: Continuously variable from 16Hz to 420Hz (measured at the 3dB down point).

Input Meter:

• Calibrated for 0dBfs = +22dBu (max level for ADC conversion). Indicates level after High Pass Filter and before Insert Send.
• O/L LED: Lights red when signal level exceeds 0dBfs.

Soft Limiter:

• Threshold: -6dBfs (+16dBu).
• Limiter ratio is level dependent: -6dBfs to -4dBfs (Infinite:1), -4dBfs to 0dBfs (2:1), 0dBfs to +6dBfs (1.5:1).
• Attack time: Instant.
• Release time: Instant.
• Noise: -95dBu (20Hz/22kHz bandpass filter).

ADC Meter:

6 LED meter calibrated to 0dBfs = +22dBu (maximum level into the converter). Meter calibration points:

(-42dBfs represents the average level to allow 20dB of headroom for EQ and dynamics processing).

Warranty

All Focusrite products are covered by a one-year warranty against manufacturing defects. Focusrite or its authorized distributors will remedy any faults as quickly as possible. This warranty is in addition to statutory rights.

The warranty does not cover:

• Carriage to and from the dealer or factory.
• Labour charges for repairs not performed by an authorized distributor or Focusrite UK.
• Consequential loss or damage.
• Damage caused by abuse, negligence, improper operation, storage, or maintenance.

If a product is faulty, contact the dealer from which it was purchased. Pack the product correctly for return, preferably in original packaging. Focusrite will endeavor to remedy the fault quickly.

Complete and return the Warranty Registration Card or register online at http://www.focusrite.com.

Accuracy

Focusrite Audio Engineering Ltd. has made every effort to ensure the accuracy of this manual but makes no representations or warranties regarding its contents.

Copyright

© 2002-2003 Focusrite Audio Engineering Ltd. All rights reserved. No part of this manual may be reproduced or transmitted without the express prior consent of Focusrite Audio Engineering Ltd.

ADAT™ is a registered trademark of Alesis Corporation Inc.

192 HD™ is a registered trademark of Digidesign Inc.