Focusrite ISA 428 Pre Pack User Manual

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1

Introduction

Thank you for purchasing the ISA 428 Pre Pack brought to
you by the Focusrite team – Martin, Crispin, Helen, Tim,
Tom, Mick, Dave, Simon, Paul, Phil, Micky, Pauline, Jo, Chris,
Giles, Chris, Rob and Simon.

The chaps at Focusrite are a jolly hard working bunch and
take a great deal of pride in designing, building and delivering
the best audio units around. We hope your new Focusrite
unit lives up to that reputation, and that you enjoy many
years of productive recording. If you would like to tell us
about your recording experiences then please email us at:
[e-mail]

The Focusrite Team

Contents

Introduction..............................................................................................1

Contents.....................................................................................................1

Important Safety Instructions.............................................................1

Power Connections...............................................................................1

Signal Connections.................................................................................2

Getting to know the ISA 428.............................................................3

Metering.....................................................................................................3

Input Stage.................................................................................................4

Analogue to Digital converter (ADC) Soft Limiter...................5

8 Channel Digital Output Option....................................................5

Digital Output Front Panel Controls..............................................6

Applications...............................................................................................7

Other Compatible Focusrite Products.......................................10

FAQs.........................................................................................................11

Specifications.........................................................................................13

Warranty.................................................................................................14

Accuracy..................................................................................................14

Copyright................................................................................................14

Important Safety Instructions

Please read all of these instructions and save them for
future reference. Follow all warnings and instructions
marked on the unit.

• Do not obstruct air vents in the rear panel. Do not
insert objects through any 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.

• Unplug the unit and refer servicing to qualified service
personnel under the following conditions:
If the power cord or plug is damaged; if liquid has
entered the unit; if the unit has been dropped or the
case damaged; if the unit does not operate normally or
exhibits a distinct change in performance. Adjust only
those controls that are covered by the operating
instructions.

• Do not defeat the safety purpose of the polarised or
grounding-type plug. A polarised plug has two blades
with one wider than the other. A grounding type plug
has two blades and a third grounding prong. The wider
blade or the third prong are provided for your safety.
When the plug provided does not fit into your outlet,
consult an electrician for replacement of the obsolete
outlet.

WARNING:

THIS UNIT MUST BE EARTHED BY THE POWER

CORD.

UNDER NO CIRCUMSTANCES SHOULD THE

MAINS EARTH BE DISCONNECTED FROM THE

MAINS LEAD

This unit is capable of operating over a range of mains
voltages as marked on the rear panel. Ensure correct mains
voltage setting and correct fuse before connecting mains
supply. Do not change mains voltage settings while mains
supply is connected. To avoid the risk of fire, replace the
mains fuse only with the correct value fuse, as marked on
the rear panel. The internal power supply unit contains no
user serviceable parts. Refer all servicing to a qualified
service engineer, through the appropriate Focusrite dealer.

Power Connections

There is an IEC mains lead supplied with the unit, which
should have the correct moulded plug for your country.
The wiring colour code used is:

For units shipped to the USA, Canada, Taiwan and Japan:

Live - Black Neutral - White Earth - Green

For units shipped to any other country:

Live - Brown Neutral - Blue Earth - Green and Yellow

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2

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°

Line IP and Insert Sends and Returns

1/4” balanced jack wired as follows:
Tip Audio 0°
Ring Audio 180°
Sleeve Screen/Chassis

Inst. Hi Z IP

1/4” unbalanced jack wired as follows:
Tip Audio 0°
Sleeve Screen/Chassis

Mic IP/Line IP/Inst. Hi Z IP (front panel)

Any one of these inputs may be used as the input to the ISA
428 channels 1-4. Signals routed to these inputs are
referred to as the ‘internal’ channels or signal paths.

Outputs - Channels 1-4

These outputs are used as the main analogue signal outputs,
and are fed by whatever is connected to the Mic Inputs,
Line Inputs, or Instrument Inputs. These outputs also
connect to the ADC ‘internal’ channels 1-4 via the Soft
Limiter circuit.

Insert Send and Return

Allows an external unit, such as a 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

The ADC inputs 5-8 act as line level inputs and are used to
route ‘external’ signals to the optional ADC channels 5-8
via the Soft Limiter. Using these inputs in conjunction with a
single optional ADC card, up to eight analogue inputs can
be routed to the eight digital outputs. Using these inputs in
conjunction with a second ISA 428 unit and a single optional
ADC card, eight pre amps can be routed to all eight ADC
channels. (See ‘Signal Connections’ on page 8.)

Retrofitting the Optional ADC

The optional ADC can be retrofitted to a standard ISA 428
at any time. The card can be fitted easily by the user – no
engineering experience is required. Full fitting instructions
for this option are included along with the ADC.

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Getting to know the ISA 428

Power

Applies power to the unit. Turn on the ISA 428 before
powering up devices to which the outputs are connected.

Instrument Inputs

Instrument sources may only be connected via the front
panel. Four unbalanced Instrument input connectors are
located to the far left of the front panel and are numbered
1-4, correlating to each of the four channels. These
connectors are used primarily for connecting low level
unbalanced signals such as those from passive guitars and
basses, or from active instruments such as keyboards and
electro-acoustic guitars.

Metering

Peak Meter and O/L LED

This moving coil meter has been designed to have a very
fast reaction time and a slow decay time, and so is perfect

for displaying and holding the peak level of the channel
signal during recording. The meter is fed with a signal taken
from the point in the channel path after the high pass filter
and before the insert send connector. The moving coil peak
meter reading therefore informs the user of the signal level
in the channel after gain has been applied by the pre-amp,
and also shows the level of the signal being sent to any
external equipment connected to the insert send output.

The meter is calibrated relative to the overload point of the
optional ADC card, (0dBfs on the meter refers to the
maximum level that can be converted by the ADC), and
relative to the analogue overload point, (where then 0dBfs
on the meter refers to an analogue level of +22dBu, 6dB
below the maximum analogue level available from the unit).

Within the meter is a hidden overload indicator marked
O/L that lights (red) when the signal in the channel exceeds
0dBfs (+22dBu), and acts as an additional safety feature for
monitoring peak signal levels.

Analogue/ADC dBfs Output meters

These vertical columns of LEDs indicate the peak signal
levels of channels 1-8 in one of two modes, the modes being
defined by the fitting of the optional ADC card as follows:-

Mode 1. Analogue only unit (no optional ADC card
fitted).

Meters 1 to 4 indicate the analogue level at the ISA 428
XLR output connectors for channels 1 to 4. 0dBfs (reached
when the red LED is lit) indicates that a signal level of
+22dbu is present at the output (–18dBfs) therefore

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indicating that there is a signal level of +4dBu at the output.
Meters 5 to 8 have no function in this mode.

Mode 2. Digital (optional ADC card fitted).

Meters 1 to 8 indicate the signal level that exists in the
signal path after the Soft Limiter and just before the point of
conversion on the optional ADC card. 0dBfs (reached when
the red LED is lit) indicates the maximum signal level that
can be converted by the optional ADC card and should only
be lit for very short durations to ensure a good quality
recording with no digital overload. The meter signal is taken
from a point after the Soft Limiter, so if the Soft Limiter has
been selected to protect the ADC from overload, the effect
of the Soft Limiter on the peak signal levels will be indicated
by a reduction in peak levels on the LED meter.

Input Stage

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

Input

Pressing INPUT steps through each of the three inputs, as
indicated by the corresponding LEDs. When the Mic LED is
lit, the microphone input is active etc. Hence a mixture of
microphone, line and instrument inputs may be selected
across the four channels simultaneously.

Mic Input Gain

With the Mic input selected, the user has access to the full
gain range in 10dB steps from 0dB to +60dB (yellow
legend). The gain range is split between two gain modes
depending upon the status of the 30-60 switch.

Mode 1 Mic Gain Range 0-30

With the 30-60 switch off, the rotary gain knob operates
over a gain range of 0dB to +30dB, the level of gain chosen
being indicated on the front panel by the outer arc of yellow
numbers around the gain knob.

Mode 2 Mic Gain Range 30-60

With the 30-60 switch on (illuminated), the rotary gain
knob operates over a gain range of 30dB to 60dB, the level
of gain chosen being indicated on the front panel by the
outer arc of yellow numbers around the gain knob.

An additional 20dB of gain can be applied to the signal after
the Mic/line gain knob using the Trim knob. See ‘Trim’
control text below for full explanation.

Line Input Gain

With the line input selected, the user has access to gain
settings ranging from –20dB to +10dB, indicated on the
front panel by the arc of white numbers around the gain
knob. The 30-60 switch is inactive when the line input is
selected, as the gain range for Line level inputs is restricted
to –20dB to +10dB in 10dB steps.

An additional 20dB of gain can be applied to the signal after
the Mic/line gain knob using the Trim knob. See ‘Trim’
control text below for full explanation.

Instrument Input Gain

With the instrument input selected, gain is applied to the
input signal by using the trim control only, which allows
+10dB to +40dB of gain range. The level of gain chosen is
indicated on the front panel by the outer arc of yellow
numbers around the gain knob. This input is suitable for
high impedance sources such as guitar or bass pickups
(which may be connected directly without the need for an
external DI box) or vintage synthesizers with high
impedance outputs.

Trim

The Trim control provides additional variable gain of 0dB to
+20dB when Mic or line inputs are selected. The level of
gain chosen is indicated on the front panel by the inner arc
of white numbers around the gain knob.

The additional 20dB of gain that can be applied to the Mic
or Line signal is very useful for two reasons:

When high gain is required

The trim used in conjunction with the Mic gain of 60dB will
give a total of up to 80dB of pre-amp gain, making it very
useful for getting good digital recording levels from very low
output dynamic and ribbon microphones.

Gain adjustment during recording

When small amounts of gain adjustment are needed to
correct for performance level variations during recording,
use the trim knob rather than the stepped Mic/Line gain
knob, as switching the 10dB gain steps would be much too
intrusive. It is therefore good practice to apply some Trim
gain before using the 10dB stepped gain knob to find the
optimum recording level so that the Trim control can be
used to gently add or take away gain later, if so required.

+48V

Pressing the +48V switch provides +48V phantom power,
suitable for condenser microphones, to the rear panel XLR
microphone connector. This switch does not affect the
other inputs. If you are unsure whether your microphone
requires phantom power, refer to its handbook, as it is
possible to damage some microphones (most notably
Ribbon Microphones) by providing phantom power.

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5

Phase

Pressing PHASE inverts the phase of the selected input, to
correct phase problems when using multiple microphones,
or when incorrect wiring polarity has occurred.

Insert In

Pressing the INSERT switch (illuminated) breaks the signal
path of the channel, so that the channel input signal is sent
out of the unit from the rear panel Insert Send connector
and returned to the same point in the signal chain via the
rear panel Insert Return connector.

Filter In

Pressing the FILTER IN switch makes the Hi Pass Filters
active in the audio path. The filter provides 18dB/octave
roll-off. A variable control allows the roll-off frequency to
be set within the range of 16Hz to 420Hz.

Impedance

Pressing IMPEDANCE steps through each of the four
transformer pre-amp input impedance values, as indicated
by the corresponding LEDs. By selecting different values for
the impedance of the ISA 428 transformer input, the
performance of both the ISA 428 pre-amp and the
microphone connected can be tailored to set the desired
level and frequency response.

Analogue to Digital converter (ADC)
Soft Limiter

Pressing ADC SOFT LIMITER activates the ADC Soft
Limiter - giving total protection of all 8 channels on the
ADC.

ADC Soft Limiter Operation

The Soft Limiter circuit acts instantaneously, clamping down
the audio so that the signal can never go above the
maximum level that the ADC can accurately convert
(0dBfs). Consequently, it is impossible to overload the
connected optional ADC card. This function is a part of the
ADC signal path, and so only operates when an ADC card
is fitted. If no card is fitted then the switch is inactive.

8 Channel Digital Output Option

The ISA 428 can be used as a high quality 8 channel ADC
for analogue transfer to digital with the addition of the
optional ISA 428 digital output board. The 4 external ADC
inputs and the main channel inputs can all be fed to the
ADC, via the Soft Limiter, ensuring 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 always route respectively to ADC
output channels 1-4.

Alternatively, two ISA 428 units with a single A/D option
can be used to create an 8-channel mic pre to A/D system
(see ‘Signal Connections’ on page 8). Digital formats
available on the ADC are AES/EBU, SPDIF and ADAT™
optical format (the ADAT™ outputs can also operate in
high speed SMUX mode for 96kHz transfer speeds, but are
muted during 192kHz operation).

24-bit/96kHz ADAT™ interface operation

The card provides digital outputs for all eight ISA 428
channels, which operate over the sample frequency ranges

44.1, 48, 88.2 and 96kHz, and can be dithered to 16-, 20-,

or 24-bit depths depending upon the destination.
The card features two ADAT™-type ‘lightpipe’ output

connectors. For speeds up to 48kHz both connectors
transmit all 8 channels simultaneously. However, ADAT™­type connectors are bandwidth-limited at sample rates
above 48kHz - each audio channel uses two ADAT™ digital
channels to accommodate the increased quantity of data,
hence the need for two ADAT™ connectors to allow 8
channels of conversion at high speed.

The ADAT™ output connectors operate as follows:

44.1/48Khz sample rates:

Connector 1 = channels 1 to 8 in parallel.
Connector 2 = channels 1 to 8 in parallel (identical to
connector 1)

88.2/96Khz sample rates:

Connector 1 = channels 1 to 4.
Connector 2 = channels 5 to 8.

ADAT™ lightpipe cables are available from your local
dealer, or in the UK from Studiospares (tel [phone]

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 connector on the rear panel. The full
range of sample rates (up to 192kHz) and bit depths are
available.