Audio Amplifiers
Single-channel ultra-low-noise inverting audio preamplifier based on the Texas Instruments NE5532 op-amp, featuring +33.4 dB closed-loop gain, input DC blocking, and high-frequency phase compensation.
Circuit overview
Click any component in the schematic above to inspect its electrical ratings, function, and connections.
This preamplifier implements an ultra-low-noise audio gain stage optimized for low-impedance dynamic microphones (150 Ω to 600 Ω) using the Texas Instruments NE5532 operational amplifier (datasheet SLOS075K). Powered from symmetrical ±15 V rails, the circuit operates in an inverting configuration with closed-loop voltage gain Av = -RF / RI = -47 kΩ / 1 kΩ = -47 V/V (+33.44 dB, 180° phase inversion). Setting input resistor R1 to 1 kΩ matches dynamic microphone capsules while suppressing the op-amp input noise current contribution (0.7 pA/√Hz). Input DC blocking capacitor C1 (10 µF) forms a 15.9 Hz high-pass corner, eliminating phantom bias or transducer DC currents from shifting the internal summing node. Phase compensation capacitor C2 (47 pF C0G) shunts feedback resistor R2 above 72 kHz, guaranteeing high-frequency stability and blocking RF interference without compromising audio fidelity. The non-inverting input (pin 3) is referenced to ground via DC bias resistor R3 (1 kΩ) to equalize input bias current voltage drops, while capacitor C3 (10 µF) provides an AC noise ground. Series resistor R4 (47 Ω) isolates the op-amp output from capacitive cable loading, and output capacitor C6 (10 µF) ensures clean DC-isolated line-level delivery.
| Closed-Loop AC Gain (Av) | -47 V/V (+33.44 dB, 180° phase inversion) |
|---|---|
| Input Resistor (RI / R1) | 1 kΩ, 1%, 0.125 W metal film |
| Feedback Resistor (RF / R2) | 47 kΩ, 1%, 0.125 W metal film |
| Frequency Response (-3 dB) | 15.9 Hz to 72.0 kHz |
| Equivalent Input Noise Voltage | ≈ 6.5 nV/√Hz total (5.0 nV/√Hz op-amp broadband) |
| Power Supply Rails (V+ / V-) | ±15 V symmetrical split supply (±3 V to ±20 V) |
| Input Impedance (ZIN) | 1 kΩ (set by R1 at virtual ground node) |
| Output Drive Capability | Up to ±38 mA short-circuit current, drives 600 Ω loads |
The NE5532 operational amplifier is energized from symmetrical ±15 V regulated rails (+15 V on pin 8, -15 V on pin 4). These split rails permit the internal Class-AB output stage to swing symmetrical AC audio waveforms up to ±13.5 V peak-to-peak into standard loads without DC rail biasing. Symmetrical supplies eliminate crossover distortion artifacts and maintain wide dynamic range exceeding 110 dB.
The non-inverting input (pin 3) is held at 0 V DC ground potential. Because the NE5532 exhibits high open-loop DC voltage gain (A_OL ≈ 100 dB) and wide 10 MHz unity-gain bandwidth, negative feedback forces the inverting terminal (pin 2) to match pin 3, creating an ideal virtual ground node at 0 V. Signal current generated by the microphone flows across input resistor R1 (1 kΩ) directly into the feedback network. The closed-loop AC voltage gain is governed strictly by the resistor ratio:
AV = -(RF) / (RI) = -(47 kΩ) / (1 kΩ) = -47 V/VIn decibels, this corresponds to 20 \log10(47) ≈ +33.44 dB of clean voltage gain with 180° phase inversion.
Direct current is blocked from the input transducer by series capacitor C1 (10 µF). Together with input resistor R1 (1 kΩ), capacitor C1 establishes a single-pole high-pass transfer characteristic with a -3 dB low-frequency cutoff frequency:
fL = (1) / (2\pi × RI × C1) = (1) / (2\pi × 1000\,Ω × 10 × 10^(-6) F) ≈ 15.9 HzThis cutoff passes the full audible bass spectrum down to 20 Hz with less than 0.8 dB attenuation and prevents low-frequency subsonic handling rumble from driving the amplifier into saturation.
In parallel with feedback resistor R2, ceramic capacitor C2 (47 pF C0G) introduces a feedback zero that rolls off circuit gain above the audible range:
fH = (1) / (2\pi × RF × CF) = (1) / (2\pi × 47000\,Ω × 47 × 10^(-12) F) ≈ 72.0 kHzRestricting high-frequency gain prevents RF interference from mobile transmitters and switch-mode power supplies from entering the feedback loop, preserving wide phase margin and ensuring unconditional stability under all operating conditions.
The NE5532 bipolar input stage draws input bias currents (I_B ≈ 200 nA typical) through both differential bases. Non-inverting input resistor R3 (1 kΩ) matches the DC source resistance seen by the inverting node (R1 || R2 ≈ 980 Ω), cancelling input offset voltage drift. Capacitor C3 (10 µF) shorts R3 thermal noise to ground at audio frequencies (> 16 Hz). At the output, isolation resistor R4 (47 Ω) de-Q's any capacitive reactance from external interconnect cables, preventing high-frequency output ringing while output capacitor C6 (10 µF) delivers a pristine DC-isolated audio signal.
Calculates the closed-loop voltage gain magnitude and phase inversion determined by the feedback-to-input resistance ratio.
| Symbol | Parameter | Typical value |
|---|---|---|
| AV | Closed-loop voltage gain | -47V/V |
| RF | Feedback resistor | 47kΩ |
| RI | Input series resistor | 1kΩ |
Calculates the low-frequency high-pass corner frequency established by input DC blocking capacitor C1 and input resistance R1.
| Symbol | Parameter | Typical value |
|---|---|---|
| fL | Low-frequency -3 dB cutoff | 15.9Hz |
| RI | Input series resistance | 1kΩ |
| C1 | Input coupling capacitor | 10µF |
Calculates the high-frequency roll-off pole set by feedback capacitor C2 in parallel with feedback resistor R2.
| Symbol | Parameter | Typical value |
|---|---|---|
| fH | High-frequency -3 dB cutoff | 72.0kHz |
| RF | Feedback resistance | 47kΩ |
| CF | Feedback compensation capacitor | 47pF |
Calculates total input-referred equivalent noise voltage density including amplifier noise voltage, resistor Johnson noise, and noise current.
| Symbol | Parameter | Typical value |
|---|---|---|
| en(total) | Total input noise density | 6.5nV/√Hz |
| en(amp) | NE5532 broadband voltage noise | 5.0nV/√Hz |
| RI | Input source resistance | 1.0kΩ |
| in | NE5532 input noise current | 0.7pA/√Hz |
Every part needed to build this circuit. Select a reference designator to inspect it on the schematic.
| RefDes | Component | Value / rating | Package | Role in circuit | Reference |
|---|---|---|---|---|---|
| Dual Low-Noise High-Speed Audio Operational Amplifier Texas Instruments NE5532P, SLOS075K Rev. K; Pin 1 OUT1, Pin 2 IN1-, Pin 3 IN1+, Pin 4 V- (-15V), Pin 8 V+ (+15V). | NE5532P | PDIP-8 | Inverting audio microphone preamplifier stage (Channel A used) | Dual Low-Noise High-Speed Audio Operational Amplifier pinout & datasheet | |
| Input Series Resistor 1 kΩ, 1%, 0.125 W metal film resistor. Sets input impedance to 1 kΩ to minimize Johnson thermal noise. | 1kΩ | Axial / 0805 | Input signal scaling and impedance definition | — | |
| Closed-Loop Feedback Resistor 47 kΩ, 1%, 0.125 W metal film resistor. Establishes closed-loop AC voltage gain of Av = -47 V/V (+33.4 dB). | 47kΩ | Axial / 0805 | Closed-loop voltage gain setting | — | |
| Non-Inverting Input DC Bias Resistor 1 kΩ, 1%, 0.125 W metal film resistor. Balances DC source resistance seen by differential input terminals. | 1kΩ | Axial / 0805 | Input bias current DC offset compensation | — | |
| Output Cable Isolation Resistor 47 Ω, 1%, 0.125 W metal film resistor. Prevents op-amp phase erosion and parasitic oscillation under cable capacitance. | 47Ω | Axial / 0805 | Capacitive cable load isolation | — | |
| Input AC Coupling Capacitor 10 µF, 50 V aluminum electrolytic / bipolar capacitor. Establishes 15.9 Hz low-frequency high-pass corner with R1. | 10µF | Radial / 1206 | Input DC blocking | — | |
| Feedback Phase Compensation Capacitor 47 pF, 50 V C0G/NP0 ceramic capacitor. Forms 72 kHz high-frequency low-pass roll-off with R2. | 47pF | 0805 / Radial | High-frequency RF suppression and loop stability | — | |
| Non-Inverting Bias AC Bypass Capacitor 10 µF, 50 V aluminum electrolytic capacitor. Shorts thermal Johnson noise of R3 to ground across audio frequencies. | 10µF | Radial / 1206 | AC grounding of non-inverting bias node | — | |
| Output AC Coupling Capacitor 10 µF, 50 V aluminum electrolytic / bipolar capacitor. Blocks residual DC output offset from subsequent audio equipment. | 10µF | Radial / 1206 | Output DC blocking | — |
Traps found in real builds, with the fix for each.
Because the NE5532 has a bipolar input stage with an input noise current density of 0.7 pA/√Hz, using source or input resistances above 10 kΩ causes current-induced noise voltage (i_n · R) and resistor Johnson noise to overpower the low 5 nV/√Hz voltage noise.
Keep input resistor R1 at or below 1 kΩ and ensure driving transducer impedance is low (150 Ω to 600 Ω). For high-impedance sources like guitar pickups (> 100 kΩ), use a JFET-input op-amp such as the OPA2134 or TL072.
Driving long shielded audio cables or capacitive loads exceeding 100 pF directly from the NE5532 output creates an additional pole with the op-amp open-loop output impedance (Zo ≈ 50 Ω to 100 Ω), eroding phase margin and triggering ultrasonic parasitic oscillation between 5 MHz and 15 MHz.
Install series isolation resistor R4 (47 Ω, 1% metal film) directly adjacent to the op-amp output pin ahead of coupling capacitor C6 and the output cable terminal.
The NE5532 input stage features internal back-to-back protective diodes across the inverting and non-inverting terminals. In an inverting configuration with large input transients or DC power interruptions, differential voltage exceeding ±0.6 V forces heavy conduction through these diodes, creating sudden input distortion and excessive current draw.
Ensure input coupling capacitor C1 is rated for full supply voltage (50 V) and verify that input signal transients do not exceed the linear operating range. Input resistor R1 (1 kΩ) provides safe current limiting during large overdrive conditions.
At frequencies above 100 kHz, the NE5532 power supply rejection ratio (PSRR) degrades significantly. Operating from symmetrical ±15 V rails without local ceramic bypass capacitors leads to high-frequency supply ripple feedthrough and inter-channel crosstalk.
Mount 100 nF X7R ceramic bypass capacitors directly between pin 8 (V+) and GND, and between pin 4 (V-) and GND, within 5 mm of the IC package terminals with low-inductance ground plane connections.
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