Focusrite ISA One ANALOGUE

Focusrite ISA One – Transformer-Based Professional Microphone Preamplifier

Focusrite ISA One is built around the classic ISA series microphone preamplifier featuring the renowned Lundahl input transformer. The circuit is derived from the original ISA110 design and has been used in successive Focusrite products for over four decades.

The Lundahl input transformer delivers a distinctive sonic character that cannot be authentically replicated by transformerless designs. ISA One expands upon the original concept with four carefully selected input impedances, allowing precise microphone matching — from enhancing the personality of vintage microphones to achieving a transparent, modern response.

A fully independent front-panel DI channel increases flexibility for artists, engineers, and producers. The DI section offers High (2.4 MΩ) and Low (470 kΩ) impedance switching, independent gain control, an amplifier output, a dedicated rear-panel XLR output, and routing to the optional A-D converter.

Features

• ISA series transformer-based microphone preamplifier
• Rugged, portable chassis design
• Lundahl LL1538 input transformer
• Four selectable input impedances (including original ISA110 setting)
• Independent front-panel DI channel
• DI High (2.4 MΩ) / Low (470 kΩ) impedance switching
• Independent DI gain control and amplifier output
• Separate rear-panel XLR output
• Headphone output with level control
• Two rear TRS inputs for external stereo cue mix
• Dedicated insert point for external processing
• Optional stereo 192kHz A-D converter (118 dB dynamic range)
• Backlit moving-coil VU meter with variable calibration
• Two six-LED peak meters (pre- or post-insert metering)

Technical Specifications

Mic Input Response

• Gain Range (Mic):
• 0dB to 60dB in 10dB steps 20 dB of variable gain
• EIN (Mic):
• -126dB measured at 60dB of gain with 150 Ohm terminating impedance and 22Hz/22kHz band-pass filter
• Noise (Mic):
• Noise at output with unity gain (0 dB) and 22 Hz-22 kHz band pass filter -97 dBu
• Signal-To-Noise Ratio (Mic):
• 106 dB relative to max headroom (9dBu)
• Frequency Response (Mic):
• At minimum gain (0 dB) -0.5dB down at 10Hz and -3dB down at 125kHz
• CMRR (Mic):
• 98dB (Channel 1, 1kHz, maximum gain with 24 dBu input)
• Crosstalk Channel To Channel (Mic):
• With 10dB@1kHz input to chA, chB output =104dBrA
• With 10dB@10kHz input to chA, chB output = 84dBrA

Line Input Response

• Gain Range (Line):
• -20dB to 10dB in 10dB steps 20 dB of variable gain
• Input Impedance (Line):
• 10 kO from 10 Hz to 200 kHz
• Total Harmonic Distortion Noise (Line):
• Measured with a 0 dBu input signal, and a 22 Hz-22 kHz band pass filter 0.0001
• Frequency Response (Line):
• At unity gain (0 dB) -0.3dB down at 10Hz and -3dB down at 200kHz

Instrument Input Response

• Gain Range (Instrument):
• 10dB to 40dB continuously variable
• Input Impedance (Instrument):
• High = greater than 1M
• Low = greater than 300k
• THD (Instrument):
• At minimum gain (10)

High-pass Filter

• Roll-Off:
• 18 dB per octave (3 pole filter)
• Frequency:
• Fixed 75Hz measured at the 3dB down point