Focusrite ISA TWO User manual

User Guide
focusrite.com
FA0540-03
Two channel classic transformer mic pre

2
CONTENTS
About this User Guide .....................................................................3
INTRODUCTION ..............................................................................3
ISA TWO CONTROLS AND FEATURES ........................................................4
Front Panel ................................................................................4
Channel Controls ..........................................................................4
Input Selection..........................................................................4
Phase ..................................................................................4
Mic Input Gain ..........................................................................4
+48V ...................................................................................5
Line Input Gain .........................................................................5
Instrument Input........................................................................5
Z In (Input Impedance)...................................................................5
Filter...................................................................................5
Insert ..................................................................................5
Meter Calibration .......................................................................6
Rear Panel ................................................................................7
AC Mains Inlet ..........................................................................7
Channel Mic Inputs......................................................................7
Channel Line Inputs .....................................................................7
Channel Outputs ........................................................................7
Channel Insert Sends & Returns .........................................................7
Meter Calibration Pot....................................................................7
PHYSICAL CHARACTERISTICS ...............................................................8
Power Requirements ......................................................................8
APPENDICES .................................................................................9
1. Connector Pinouts.......................................................................9
2. Preamp Input Impedance ..............................................................10
3. Pro Tools Interfacing ...................................................................12
PERFORMANCE AND SPECIFICATIONS .....................................................13
Focusrite Pro Warranty and Service ......................................................15

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About this User Guide
This user guide applies to the ISA Two mic pre. It provides information about installing and using the
unit, and how it can be connected into your system.
If you feel that additional information might be of assistance, be sure to consult the site:
pro.focusrite.com/technical-support, which contains a comprehensive collection of common
technical support queries.
Pro Tools® and Pro Tools | HDTM are trademarks or registered trademarks of Avid Technology, Inc. or its subsidiaries
in the United States and/or other countries.
Dante® and Audinate® are registered trademark of Audinate Pty Ltd.
Box Contents
• ISA Two unit
• AC power lead
• Safety information cut sheet
INTRODUCTION
Thank you for purchasing the Focusrite ISA Two.
The ISA Two provides two of Focusrite’s prestigious transformer-based microphone preamps, and
can be used to record microphone, line-level or instrument sources. Microphones and line-level
sources are connected via the rear panel, whilst instrument inputs can be plugged directly into the
front panel jack sockets.
The front panel also features gain, adjustable lter frequency and other settings such as phantom
power, phase and input impedance for both inputs. Peak level LED metering in dBFS is provided for
each channel to indicate when the output is reaching the digital clipping point; a meter calibration
control is provided on the rear panel.
First introduced in 1985, the ISA microphone preamp holds a reputation for outstanding transparency
along with subtle warmth contributed by transformer core saturation. The variable impedance circuit
allows ISA users to match the preamp with a wide trange of microphones.

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ISA TWO CONTROLS AND FEATURES
Front Panel
Channel Controls
Input Selection
Each press of the Input button steps through the three available input sources: Mic/Line/Instrument.
Phase
Pressing Phase inverts the polarity of the selected input. This can be useful when multiple mics are
used in close proximity (ie., on a drum kit).
Mic Input Gain
The Gain switch adjusts the mic gain in three 10 dB steps. Its range is either 0–30 dB or 30–60 dB
when the 30–60 switch is pressed. An additional 0–20 dB of continuous gain adjustment is available
using the Trim control.
To avoid an excessive jump in level, It is recommended that the stepped Gain switch is turned to minimum before
pressing the 30-60 switch.
Before starting a recording, or if used for PA/live work, set theTrim control to near its centre position.This will allow
for some gradual gain adjustment up or down without the use of the stepped control.
Channel 1 Channel 2 Power
switch
Instrument
inputs
LED
meters
Applies +48V
phantom power to
the mic input XLR
Inverts polarity
(Phase) of the
selected input
Input Trim pot
Mic/Line: 0 +20 dB
Instrument: +10 +40 dB
Applies the
variable-frequency
high-pass Filter to
the selected input
Filter roll-off
frequency
pot
Mic and
Instrument input
impedance (Z In)
selection
Switches the external
Insert return into the
channel path
Input
source
selection
Selects the high
(30-60 dB) mic
gain range on the
Gain switch
10 dB stepped Mic/
Line Gain switch
Mic: 0-30 / 30-60
Line: -20 +10

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Channel Controls...
+48V
Pressing the +48V button applies phantom power to the Mic input XLR. This switch does not affect
the Line or Instrument inputs.
If you are unsure whether your microphone requires phantom power please refer to its handbook. Certain
microphones (most notably ribbon and unbalanced mics) could be damaged by applying phantom power.
Note that the +48V phantom buttons will always be in the off state after power up.
Line Input Gain
The Gain switch adjusts the gain between -20 dB and +10 dB in 10 dB steps. Continuous gain
adjustment of up to 20 dB can be added using the Trim control.
Instrument Input
The Instrument inputs are accessed via standard 1/4” mono jacks on the front panel. The level is set
using the Trim control only and is continuously adjustable from +10 dB to +40 dB.
Z In (Input Impedance)
With the Mic input selected, pressing the Z In button steps through the four
transformer preamp input impedance options. The values are shown in the
table.
For additional information on impedance selection see Appendix 2, “Preamp Input
Impedance” on page 10.
With the Instrument input selected, pressing the switch toggles between High
(guitar pickups) and Low (vintage/high Z-out equipment) settings, as shown
in the lower table.
The Line input impedance is xed at 10 kΩ and is not affected by the Z In
switch.
Filter
Pressing the Filter button inserts the 18 dB/octave high-pass lter into channel path; it is applied
to whichever input is selected. The High Pass Filter control allows the roll-off frequency to be set
within the range 16 Hz to 420 Hz.
The lter is useful for removing any unwanted low frequencies, eg., rumble transmitted through oor mounted
mic stands, etc.
Insert
Pressing Insert places the Insert Return signal into the channel path before the Output connector,
allowing the inclusion of external effects units.
The Insert Send output is always available and is post the input Gain, Phase and Filter controls.
Low 600 Ω
ISA 110 1.4 kΩ
Med 2.4 kΩ
High 6.8 kΩ
Low 470 kΩ
High 2.4 MΩ
Mic Impedance
Instrument Impedance

6
Channel Meters
The LED meters indicate level at the channel output connectors. The scale is in dBFS,
ie., the level in dB, relative to maximum output (reached when the ‘0’ LED illuminates).
The default calibration is for ‘0’ to indicate a signal level of 22 dBu.
Meter Calibration
The level at which the ‘0’ LED illuminates can be adjusted using the Peak Meter Calibration
pot on the rear panel. The default setting of 0 dBFS = 22 dBu occurs when the knob is in
its central, detent position.
Rotating the pot sets the value between 0 dBFS = 16 dBu (fully anticlockwise) and 0 dBFS
= 24 dBu (fully clockwise).

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AC Mains Inlet
Standard IEC receptacle for AC mains. ISA Two features a ‘Universal’ PSU, enabling it to operate on
any supply voltage between 100 V and 240 V AC.
Channel Mic Inputs
On latching XLR-3 female connectors, with switchable phantom power per channel.
Channel Line Inputs
On balanced 1/4” TRS jack sockets.
Channel Outputs
On XLR-3 male connectors.
Channel Insert Sends & Returns
Analogue sends and returns on balanced 1/4” TRS jack connectors.
The insert signal is added into a channel path by pressing the front panel Insert switch.
Meter Calibration Pot
Adjusts the level at which the channel meter ‘0’ LED illuminates. See page 6.
See Appendix 1 on page 9 for connector pinouts.
Rear Panel
Mic input
Meter calibration
trim pot Line input
Insert Send
& Return
Channel
Output
Mains
inlet

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Case dimensions are illustrated in the diagram above.
ISA Two requires 1U of vertical rack space. Allow an additional 75mm of rack depth behind the unit
for cable access.
ISA Two weighs 3.7 kg and for installations in a xed environment (eg., a studio rack), the front-
panel rack mountings* will provide adequate support. However, if the unit is to be used in a mobile
situation (eg., ight-cased for touring, etc.), it is recommended that side support rails or shelves are
used within the rack.
*Always use M6 bolts and cage nuts specically designed for 19” equipment racks. An Internet search using the
phrase “M6 cage nuts“ will reveal suitable components.
Cooling vents are provided at each side; ensure that when mounted in a rack these vents are
not obstructed. Do not mount the unit immediately above any other equipment which generates
signicant heat, for example, a power amplier.
Note.The maximum operating environmental temperature is 40°C / 104°F.
Power Requirements
ISA Two is mains powered and incorporates a ‘Universal’ power supply which can operate on any AC
mains voltage from 100 V to 240 V. The AC connection is via a standard 3-pin IEC connector on the
rear panel.
A mating IEC cable is supplied with each unit – this should be terminated with a mains plug of the
correct type for your country.
Power consumption for ISA Two is 35 W.
Please note that there are no fuses or other user-replaceable components of any type in any unit.
Please refer all servicing issues to the Customer Support Team (see “Customer Support and Unit Servicing”
on page 15).
PHYSICAL CHARACTERISTICS
465.0mm / 18.3”
325.0mm / 12.8”
31.8mm / 1.25”

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APPENDICES
1. Connector Pinouts
Mic Input
Connector: XLR-3 female
Output
Connector: XLR-3 male
Pin Signal
1 Screen
2 Hot (+ve)
3 Cold (–ve)
Pin Signal
1 Screen
2 Hot (+ve)
3 Cold (–ve)
Instrument Input
Connector: Unbalanced (TS) 1/4” Jack socket
Line Input / Insert Send / Insert Return
Connector: Balanced (TRS) 1/4” Jack socket
Pin Signal
Tip Hot (+ve)
Ring Cold (–ve)
Sleeve Ground
Pin Signal
Tip Hot (+ve)
Sleeve Ground
Tip Ring Sleeve
Tip Sleeve

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Appendices...
2. Preamp Input Impedance
A major element of the sound of a mic pre is related to the interaction between the specic microphone
being used and the type of mic preamp interface technology it is connected to. The main area in which
this interaction has an effect is the level and frequency response of the microphone, as follows:
Level
Professional microphones tend to have low output impedances and so more level can be achieved by
selecting the higher impedance positions of the ISA Two mic preamp.
Frequency response
Microphones with dened presence peaks and tailored frequency responses can be further enhanced
by choosing lower impedance settings. Choosing higher input impedance values will tend to
emphasise the high frequency response of the microphone connected, allowing you to get improved
ambient information and high end clarity – even from average-performance microphones. Various
microphone/ISA Two preamp impedance combinations can be tried to achieve the desired amount
of colouration for the instrument or voice being recorded. To understand how to use the impedance
selection creatively, it may be useful to read the following section on how the microphone output
impedance and the mic preamp input impedance interact.
Switchable Impedance – In Depth Explanation
Dynamic Moving Coil and Condenser Microphones
Almost all professional dynamic and condenser microphones are designed to have a relatively low
nominal output impedance of between 150 Ω and 300 Ω when measured at 1 kHz. Microphones are
designed to have such low output impedance because the following advantages result:
• They are less susceptible to noise pickup
• They can drive long cables without high frequency roll-off due to cable capacitance
The side-effect of having such low output impedance is that the mic preamp input impedance
has a major effect on the output level of the microphone. Low preamp impedance loads down the
microphone output voltage, and emphasizes any frequency-related variation in microphone output
impedance. Matching the mic preamp resistance to the microphone output impedance (eg., making
a preamp input impedance 200 Ω to match a 200 Ω microphone) still reduces the microphone output
and signal to noise ratio by 6 dB, which is undesirable.
Impedance Setting – Quick Guide
In general the following selections will yield the following results:
High mic preamp impedance settings:
• Will generate more overall level
• Will tend to make low- and mid-frequency response of the microphone atter
• Will improve high-frequency response of the microphone.
Low preamp impedance settings:
• Will reduce the microphone output level
• Will tend to emphasise the low- and mid-frequency presence peaks and
resonant points of the microphone

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2. Pre Amp Impedance...
To minimise microphone loading, and to maximise signal to noise ratio, preamps have traditionally
been designed to have an input impedance about ten times greater than the average microphone,
around 1.2 kΩ to 2 kΩ. (The original ISA 110 preamp design followed this convention and has an
input impedance of 1.4 kΩ at 1 kHz.) Input impedance settings greater than 2 kΩ tend to make the
frequency-related variations of microphone outputs less signicant than at low impedance settings.
Therefore high input impedance settings yield a microphone performance that is atter in the low
and mid frequency areas and boosted in the high frequency area when compared to low impedance
settings.
Ribbon Microphones
The impedance of a ribbon microphone is worthy of special mention, as this type of microphone is
affected enormously by preamp impedance.
The ribbon impedance within this type of microphone is very low, around 0.2 Ω, and requires an
output transformer to convert the low voltage it generates into a signal capable of being amplied by
a preamp. The transformer uses a ratio of around 1:30 (primary:secondary) to increase the ribbon
voltage to a useful level. This transformer ratio has the effect of increasing the output impedance of
the mic to around 200 Ω at 1 kHz.
The transformer impedance, however, is very dependent upon frequency – it can almost double at
some frequencies (known as the resonance point) and tends to roll off to very small values at low
and high frequencies. Therefore, in common with dynamic and condenser microphones, the mic
preamp input impedance has a signicant effect on the signal level and frequency response of the
ribbon microphone output transformer, and the associated ‘sound quality’ of the microphone. It is
recommended that a mic preamp connected to a ribbon microphone should have an input impedance
of at least 5 times the nominal microphone impedance.
For a ribbon microphone impedance of 30 Ω to 120 Ω, the input impedance of 600 Ω (Low) will work
ne. For 120 Ω to 200 Ω ribbon microphones, the input impedance setting of 1.4 kΩ (ISA 110) is
recommended.

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Appendices...
3. Pro Tools Interfacing
• Analogue out to Pro Tools | HD
ISA Two
Red 16Line
2 x XLR-3 female – DB25 male analogue
Pro Tools interface

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PERFORMANCE AND SPECIFICATIONS
Microphone Inputs
All measurements taken at minimum gain, Z In: medium, unless otherwise stated. Measurements taken at the analogue outputs
Gain Range 0 to 30 dB, or 30 to 60 dB with ‘30-60’ switch enabled, in 10 dB steps,
plus 0 to 20 dB of continuous trim
Maximum Input Level +7 dBu
Input Impedance Transformer balanced, Low: 600 Ω, ISA 110: 1.4 kΩ, Medium: 2.4 kΩ, High: 6.8 kΩ
Signal-to-Noise Ratio 122 dB ‘A’-Weighted (typical), maximum gain
Frequency Response 20 Hz – 20 kHz ± 0.2 dB | 10 Hz – 110 kHz ± 1.5 dB
THD + N -92 dB (0.0025%) @ -1 dBr
High-Pass Filter 18 dB/octave, switchable, variable frequency: 16 – 420Hz
EIN <-123 dBu ‘A’-Weighted (typical), maximum gain
Common Mode Rejection
Ratio
-93 dB @ 1kHz
Line Inputs
All measurements taken at minimum gain, Z In: Low, unless otherwise stated, RS= 50 Ω. Measurements taken at the analogue outputs
Gain Range -20 to +10 dB in 10 dB steps, plus 0 to 20 dB continuous trim
Maximum Input Level +25 dBu
Input Impedance Electronically balanced 10 kΩ
Signal-to-Noise Ratio 122 dB ‘A’-Weighted (typical), maximum gain
Frequency Response 20 Hz – 20 kHz ± 0.1 dB | 10 Hz – 122 kHz ± 3 dB unity gain
THD + N -91 dB (0.0028%) @ -1 dBr
High-Pass Filter 18 dB/octave, switchable, variable frequency: 16 – 420Hz
Common Mode Rejection
Ratio
-65 dB @ 1 kHz
Instrument Inputs
All measurements taken at minimum gain, Z In: Low, unless otherwise stated, RS= 600 Ω. Measurements taken at the analogue outputs
Gain Range +10 to +40 dB continuous, using Trim pot
Maximum Input Level +18 dBu
Input Impedance Low: 470 kHz, High: 2.4 MΩ
Signal-to-Noise Ratio 100 dB ‘A’-Weighted
Frequency Response 20 Hz – 20 kHz ±0.1 dB | 10 Hz – 110 kHz ± 1.2 dB
THD + N -83 dB (0.0071%) @ -1 dBFS
High-Pass Filter 18 dB/octave, switchable, variable frequency: 16 – 420Hz

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Performance and Specications . . .
Crosstalk
All measurements taken at minimum gain, Z In: Medium
Microphone Inputs -60 dB, 20 Hz – 20 kHz
Line Inputs -80 dB, 20 Hz – 20 kHz
Instrument Inputs -80 dB, 20 Hz – 20 kHz
Power
PSU 1 x Internal, 100 – 240 V, 50 / 60 Hz
Consumption 35 W.
Dimensions
Height 88mm / 3.46”
Width 482mm / 18.98”
Depth 325mm / 12.8”
Weight
Weight 3.70 kg / 8.12 lbs
Output Level
Maximum output level +24 dBu
Environmental
Operating Temperature 40°C / 104°F Maximum ambient operating temperature
Connectivity
Front panel
Instrument inputs 2 x 1/4” mono jack
Rear Panel
Microphone inputs 2 x XLR-3 female
Line level inputs 2 x 1/4” balanced jack
Insert sends
Insert returns
2 x 1/4” balanced jack
2 x 1/4” balanced jack
Outputs 2 x XLR-3 male

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Focusrite Pro Warranty and Service
All Focusrite products are built to the highest standards and should provide reliable performance for
many years, subject to reasonable care, use, transportation and storage.
Very many of the products returned under warranty are found not to exhibit any fault at all. To
avoid unnecessary inconvenience to you in terms of returning the product please contact Focusrite
support.
In the event of a Manufacturing Defect becoming evident in a product within 36 months from the date
of the original purchase Focusrite will ensure that the product is repaired or replaced free of charge.
A Manufacturing Defect is dened as a defect in the performance of the product as described and
published by Focusrite. A Manufacturing Defect does not include damage caused by post-purchase
transportation, storage or careless handling, nor damage caused by misuse.
Whilst this warranty is provided by Focusrite the warranty obligations are fullled by the distributor
responsible for the country in which you purchased the product.
In the event that you need to contact the distributor regarding a warranty issue, or an out-of-warranty
chargeable repair, please visit: pro.focusrite.com/rest-of-the-world
The distributor will then advise you of the appropriate procedure for resolving the warranty issue.
In every case it will be necessary to provide a copy of the original invoice or store receipt to the
distributor. In the event that you are unable to provide proof of purchase directly then you should
contact the reseller from whom you purchased the product and attempt to obtain proof of purchase
from them.
Please do note that if you purchase a Focusrite product outside your country of residence or business
you will not be entitled to ask your local Focusrite distributor to honour this limited warranty,
although you may request an out-of-warranty chargeable repair.
This limited warranty is offered solely to products purchased from an Authorised Focusrite Reseller
(dened as a reseller which has purchased the product directly from Focusrite Audio Engineering
Limited in the UK, or one of its Authorised Distributors outside the UK). This Warranty is in addition
to your statutory rights in the country of purchase.
Registering Your Product
To access optional bundled software, please register your product at: focusrite.com/register
Customer Support and Unit Servicing
You can contact our Customer Support team free of charge:
Email: [e-mail]
Phone (UK): [phone]
Phone (USA): [phone]
Troubleshooting
If you are experiencing problems with your ISA Two, we recommend that in the rst instance, you visit
our Support Help Centre at: pro.focusrite.com/help-centre