Overview: The Focusrite ISA One is a high-quality preamplifer for
recording microphone, line-level, or instrument-level sources. The ISA One is
based on the Focusrite Heritage sound that dates back to the company's founding
in the mid-1980s. This portable device is housed in an all-metal enclosure with
a handy carrying handle on the top.
Transformer Based: The ISA One is equipped with a Lundahl LL1538 input
transformer, which is based on the original ISA110 preamp. This input
transformer provides a unique character to the sound.
Input Connections: Microphones and line-level sources are connected on
the rear panel, while instruments are plugged into the front panel. The
microphone input has an XLR connector, and the line-level input has separate XLR
and 1/4" TRS connectors. The instrument input features two 1/4" jacks: an
unbalanced (TS) input marked "DI" for connecting the instrument and another
labeled "AMP" to send the signal back to an amplifier. An external input
connector (1/4" TRS), marked "EXT", allows an external mono signal to be
connected for monitoring or converting to digital format. Separate "Send" and
"Return" 1/4-inch TRS jacks let you transmit and receive the main mic/line/instrument
input for additional signal processing.
Output Connections: The ISA One has separate XLR outputs for main mic/line/inst
input and the DI input. You can connect an external stereo signal to the left
and right "Cue Mix" 1/4-inch TRS jacks to provide a monitor mix for a recording
artist. On the front panel, a 1/4-inch TRS jack is provided for headphones. The
headphone output is variable, and can be connected to the cue mix, when active,
by pressing the Cue Mix button.
Level Meters: The ISA One offers several metering options to allow you
to view the levels of any of the inputs. A traditional moving-coil VU meter
gives you an average level reading for the main input signal. Two vertical LED
meters show the peak level of the same signal on the left meter and either an
instrument connected to the DI input or the signal received at the External
input on the right meter. The VU meter and the left LED meter have two modes:
default mode (metering the main input signal) and Post Insert mode (metering the
signal received at the insert return, when the insert is active). Both the VU
meter and the LED meters can be calibrated using the two corresponding dials on
the rear panel.
Input Gain Controls: The front panel has three rotary controls for
setting the input gain:
- Stepped Gain Control: When the Mic Input is selected, you can
vary the gain from 0dB to +60dB in 10dB steps. The gain range is split
between two gain modes: 0dB to +30dB and +30 to +60dB (with the 30-60 switch
"on"). When the Line Input is selected, you have access to gain settings
ranging from -20dB to +10dB. The 30-60 switch is inactive.
- Trim: An additional 20dB of gain can be applied to the Mic or
Line Input signal using the smooth rotation Trim knob. The additional 20dB
of gain is useful for recording sources with very low output dynamic and
ribbon microphones, and when adjusting the gain during recording. The fine
tuning action of the trim control makes it much easier to apply small
amounts of gain adjustment, compared to the 10dB-stepped gain knob.
- Instrument Input Gain: When the instrument input is selected,
gain is applied to the input signal using the separate Instrument Gain dial.
This smooth rotary control allows +10dB to +40dB of gain range. The trim
control is inactive when the instrument input is active.
Phantom Power: A single 2-way switch provides +48V of phantom power
when using condenser microphones that require it. Phantom power is provided to
the rear panel XLR microphone connector. You should refer to the microphone's
user manual to determine if phantom power is needed. Possible damage can occur
when phantom power is applied to some microphones, such as ribbon microphones.
Phase: When recording with multiple microphones, placement of the mics
may cause phase issues. The Phase button on the ISA One inverts the phase of the
selected input, and may correct phase problems. It can also correct phase
problems caused by wiring polarity errors.
High Pass Filter: You can engage a high-pass filter in the audio path
to remove any unwanted bass caused by proximity effect or rumble. The filter is
fixed at 75Hz with an 18dB per octave slope.
Input Impedance: The Input Impedance or Z In Switch lets you choose
from four transformer preamp input impedance values: Low (600Ω), ISA 110
(1400Ω), Med (2400Ω) and High (6800Ω). Switching the input impedance can have an
effect on level and frequency response. A separate Instrument Input Impedance
(Instrument Z In) toggles between High (1MΩ) and Low (300Ω) impedance settings
for the Instrument input.
Digital Option: An optional digital output board is
available from Focusrite that can be retrofitted into the ISA One. This ADC
circuit board allows you to convert analog signals into various digital formats,
including AES, S/PDIF, and ADAT. Output resolutions of 44.1, 48, 88.2, 96,
176.4, and 192 kHz are available through this optional circuit. LED indicators
are provided on the front panel for digital operations.