An Ultra-Low-Noise Pre-Amp For Audio Applications
This compact preamplifier module is designed to amplify small signals from high-source-impedance sensors with minimal added noise. It delivers a midband gain of 60 dB and a flat frequency response from approximately 16 Hz to 43 kHz. The clean roll-off at the corner frequencies produces a natural-sounding filter response well suited to audio applications.
The board operates from a single 12 V DC supply by default, with an option for dual-supply operation via component substitution. Screw terminals are provided for signal input (CN2) and output (CN3). A 3-pin header (CN1) accepts the power supply connections.
Specs and Features
- Power supply: 12 VDC (single supply, default)
- Dual-supply operation supported via optional component population
- Voltage gain: 1000 V/V (60 dB)
- Frequency response: ~16 Hz to 43 kHz (−3 dB)
- Input stage IC: JFE150 JFET, SOT23-5 package
- Output stage IC: OPA202 precision op-amp, SOIC8 package
- Screw terminals (5.08 mm pitch) for signal input and output
- 3-pin male header (2.54 mm pitch) for power input
- On-board power LED (D1)
- Mounting holes: 4 × M3
- PCB dimensions: 54.93 × 29.05 mm
The input signal is AC-coupled through C8 (10 µF) to the gate of U1 (JFE150). Resistor R7 (1 MΩ) sets the DC gate bias.
The JFE150's drain current is proportional to the gate voltage: IDS = gm × VGS. Therefore, it is an effective current-controlled transconductance element. The DC drain bias is set by R5 (4.02 kΩ), which is connected to the +VS rail.
The OPA202 (U2) is configured as a transimpedance amplifier. Its inverting input is driven by the JFET drain current, and feedback resistor R13 (1 MΩ) sets the transimpedance gain.
The non-inverting input is biased by a resistor divider (R2 and R4, both 110 kΩ) that sits at mid-supply (~6 V) in single-supply mode. This VBIAS shifts the OPA202 output DC level to approximately 6 V, keeping it within its linear range on a single rail.
Output isolation resistor R6 (49.9 Ω) improves stability when driving capacitive loads above 1 nF. It can be removed if no capacitive load drive is required.
Power-supply filtering is handled by a ferrite bead (R16, 600 Ω at 100 MHz) in series with the supply rail, supplemented by bulk (C1, 47 µF tantalum) and bypass capacitors (C2, C3, 100 nF each).
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This compact preamplifier module is designed to amplify small signals from high-source-impedance sensors with minimal added noise. It delivers a midband gain of 60 dB and a flat frequency response from approximately 16 Hz to 43 kHz. The clean roll-off at the corner frequencies produces a natural-sounding filter response well suited to audio applications.
The board operates from a single 12 V DC supply by default, with an option for dual-supply operation via component substitution. Screw terminals are provided for signal input (CN2) and output (CN3). A 3-pin header (CN1) accepts the power supply connections.
Specs and Features
- Power supply: 12 VDC (single supply, default)
- Dual-supply operation supported via optional component population
- Voltage gain: 1000 V/V (60 dB)
- Frequency response: ~16 Hz to 43 kHz (−3 dB)
- Input stage IC: JFE150 JFET, SOT23-5 package
- Output stage IC: OPA202 precision op-amp, SOIC8 package
- Screw terminals (5.08 mm pitch) for signal input and output
- 3-pin male header (2.54 mm pitch) for power input
- On-board power LED (D1)
- Mounting holes: 4 × M3
- PCB dimensions: 54.93 × 29.05 mm
The input signal is AC-coupled through C8 (10 µF) to the gate of U1 (JFE150). Resistor R7 (1 MΩ) sets the DC gate bias.
The JFE150's drain current is proportional to the gate voltage: IDS = gm × VGS. Therefore, it is an effective current-controlled transconductance element. The DC drain bias is set by R5 (4.02 kΩ), which is connected to the +VS rail.
The OPA202 (U2) is configured as a transimpedance amplifier. Its inverting input is driven by the JFET drain current, and feedback resistor R13 (1 MΩ) sets the transimpedance gain.
The non-inverting input is biased by a resistor divider (R2 and R4, both 110 kΩ) that sits at mid-supply (~6 V) in single-supply mode. This VBIAS shifts the OPA202 output DC level to approximately 6 V, keeping it within its linear range on a single rail.
Output isolation resistor R6 (49.9 Ω) improves stability when driving capacitive loads above 1 nF. It can be removed if no capacitive load drive is required.
Power-supply filtering is handled by a ferrite bead (R16, 600 Ω at 100 MHz) in series with the supply rail, supplemented by bulk (C1, 47 µF tantalum) and bypass capacitors (C2, C3, 100 nF each).
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