Noise Filtering Circuit for Audio: Build with Op-Amp

Audio noise filtering with op-amps is a fundamental skill for any electronics engineer working on sound systems, sensor interfaces, or communication equipment. Whether you need to eliminate 50Hz hum from India’s mains power, remove high-frequency switching noise from SMPS, or build a precision low-pass filter for microphone preamplifiers, op-amp filter circuits provide the solution. This guide covers Butterworth, Sallen-Key, and active notch filter designs with practical calculations and component values for Indian electronics projects.

Common Audio Noise Types in India

Indian electronics projects face specific noise challenges:

• 50Hz mains hum: India’s 230V/50Hz AC power induces 50Hz (and harmonics at 100Hz, 150Hz…) into audio through ground loops, transformer coupling, or inadequate shielding. This is the most common audio noise complaint.
• Switching noise (SMPS): USB chargers, LED drivers, and DC-DC converters produce 50-500kHz switching transients that appear as high-pitched whine in audio
• RF interference: Mobile phone GSM bursts (217Hz characteristic buzz), WiFi (2.4GHz), and Bluetooth cause intermittent audio artifacts
• Thermal noise: Johnson-Nyquist noise from resistors — unavoidable but minimizable through component selection
• Microphone preamp noise: Input-referred noise of op-amps amplified to audible levels at high gain settings

Op-Amp Filter Fundamentals

Op-amp active filters use resistors and capacitors with an op-amp to achieve precise frequency response without inductors (which are bulky and have parasitic resistance). The key parameters:

/* Filter Design Fundamentals
 *
 * Cutoff Frequency (fc):
 * For RC filter: fc = 1 / (2π × R × C)
 *
 * Filter order:
 * 1st order: -20dB/decade rolloff beyond fc
 * 2nd order: -40dB/decade rolloff beyond fc
 * 4th order: -80dB/decade rolloff (Butterworth cascade)
 *
 * Butterworth response: maximally flat passband
 * Chebyshev: steeper rolloff but ripple in passband
 * Bessel: best time-domain (phase) response, audio preferred
 *
 * Quality factor Q:
 * Q = 0.707 (Butterworth, no peaking)
 * Q > 1 (resonance peak at fc)
 *
 * Component selection for India:
 * Use 1% metal film resistors (not 5% carbon)
 * Use NPO/C0G ceramic or polypropylene film capacitors
 * Avoid X7R/Y5V ceramics in filter circuits (capacitance varies with voltage)
 */

Low-Pass Filter: Remove High-Frequency Noise

A low-pass filter (LPF) passes frequencies below fc and attenuates above. Essential for anti-aliasing before ADC, removing SMPS whine, and limiting audio bandwidth.

2nd Order Sallen-Key Low-Pass Filter at 4kHz

/* Sallen-Key LPF Design
 * Topology: Voltage-controlled voltage-source (VCVS)
 * Response: Butterworth (Q = 0.707)
 * Cutoff: 4kHz (voice frequency limit)
 * Op-amp: LM358, TL072, or NE5532
 *
 * Component values for 4kHz Butterworth:
 * R1 = R2 = R = 10kΩ (standard E24 value)
 * C1 = 2C, C2 = C
 * C = 1 / (2π × fc × R × √2)
 *   = 1 / (2π × 4000 × 10000 × 1.414)
 *   = 2.82nF → use 2.7nF (nearest standard)
 * C1 = 5.6nF, C2 = 2.7nF
 *
 * Circuit:
 *  Vin ---R1--- Node A ---R2--- (+) op-amp
 *                |                     |
 *               C1                     | (Output)
 *                |                     |
 *               GND           C2 between Node A and Output
 *               
 * Op-amp feedback: Output → (-) directly (unity gain buffer)
 * For gain > 1: Add Rf/Rg voltage divider at (-) input
 */

// LM358 Pinout reminder:
// Pin 3: (+) IN_A
// Pin 2: (-) IN_A
// Pin 1: OUT_A
// Pin 8: VCC (3.3V-32V single supply)
// Pin 4: GND

// Mid-supply biasing for single supply op-amp:
// Create VCC/2 reference: two 10kΩ resistors from VCC to GND
// Connect midpoint to input signal via 1µF cap (AC coupling)
// Connect midpoint to (+) input via 100kΩ (DC bias)

Simplified First-Order RC LPF

/* Passive RC Low-Pass Filter (no op-amp)
 * Simple, effective for gentle rolloff
 * fc = 1 / (2π × R × C)
 *
 * For 10kHz cutoff (remove high-frequency hash, keep audio):
 * Choose C = 10nF (standard)
 * R = 1 / (2π × 10000 × 0.00000001) = 1592Ω → use 1.5kΩ
 *
 * For 20kHz cutoff (audio bandwidth limit):
 * C = 10nF, R = 800Ω → use 820Ω
 *
 * Disadvantage: source loading — op-amp buffer recommended after filter
 */

High-Pass Filter: Eliminate DC and Low-Frequency Rumble

High-pass filters (HPF) remove DC offset and low-frequency noise like mechanical vibration and HVAC rumble below a cutoff frequency. Essential for microphone preamplifiers and audio interfaces.

/* 2nd Order Sallen-Key High-Pass Filter
 * Cutoff: 80Hz (remove mechanical rumble, keep voice bass)
 * Response: Butterworth
 *
 * Component values for 80Hz HPF:
 * C1 = C2 = C = 100nF (standard)
 * R = 1 / (2π × fc × C × √2)
 *   = 1 / (2π × 80 × 0.0000001 × 1.414)
 *   = 14,095Ω → use two 7.5kΩ in series = 15kΩ (closest standard)
 *   Or use 15kΩ directly
 *
 * Circuit (mirrors LPF with R and C swapped):
 *  Vin ---C1--- Node A ---C2--- (+) op-amp
 *                |                     |
 *               R1                     | (Output)
 *                |                     |
 *               GND           R2 between Node A and GND
 *               
 * DC blocking: This filter inherently blocks DC (cuts below 80Hz)
 */

/* Simple voice frequency HPF at 300Hz:
 * Removes low-frequency rumble while preserving speech clarity
 * C = 100nF
 * R = 1 / (2π × 300 × 0.0000001) = 5.3kΩ → use 5.1kΩ (E24)
 */

50Hz Notch Filter: Kill Mains Hum

The Twin-T notch filter is the most effective passive circuit for eliminating a specific frequency — critical for removing India’s 50Hz mains hum from audio signals. The notch can achieve 40-60dB attenuation at exactly 50Hz with proper component matching.

/* Twin-T Notch Filter Design for 50Hz
 * Achieves deep notch at single frequency
 * India mains frequency: 50Hz (±2Hz per DISCOMS)
 *
 * Component values:
 * Choose C = 100nF (2× precision required)
 * R = 1 / (2π × f_notch × C)
 *   = 1 / (2π × 50 × 0.0000001)
 *   = 31,831Ω → use 33kΩ (E24, close enough)
 *
 * If exact 50Hz needed:
 * R = 31.83kΩ → parallel 33kΩ and 1MΩ → ≈ 32.7kΩ
 * Or use 10-turn trimpot for fine adjustment
 *
 * TWIN-T Circuit (3R and 3C arrangement):
 *
 *          R           R
 * IN ---[33k]--- N1 ---[33k]--- OUT
 *          |              |
 *         C(100nF)    C(100nF)
 *          |              |
 *         GND            N2
 *          |              |
 *     C(100nF)      R/2(16.5kΩ)
 *          |              |
 *    IN (junction)----GND
 *
 * Simplified: Use EasyEDA or KICAD to draw properly
 * Online calculator: www.electronics2000.co.uk/calc/twin-t.php
 *
 * ACTIVE version (with op-amp) — sharper notch:
 * Connect op-amp as unity gain buffer at output
 * Feedback partial output back to center node (R/2, C×2 junction)
 * This increases Q factor: notch depth improves to 60dB+
 */

void testNotchFilter() {
  // Arduino ADC-based verification
  // Generate 50Hz sine wave on DAC, measure filtered output
  // Compare amplitude with and without filter
  // Expected: >30dB reduction at 50Hz, <3dB at 100Hz and 20Hz
}

Bandpass Filter for Voice Frequencies

/* Voice Bandpass Filter: 300Hz - 3400Hz
 * Passes telephone-quality voice band
 * Cascade HPF + LPF for bandpass response
 *
 * Stage 1: HPF at 300Hz (removes rumble + hum below voice)
 * Stage 2: LPF at 3400Hz (removes noise above voice)
 *
 * Combined: 4th order bandpass, -80dB/decade outside band
 *
 * Op-amp cascade (using quad op-amp LM324 or NE5532 dual):
 *
 *        [LM324 Op-Amp A]     [LM324 Op-Amp B]
 * Mic → | HPF 300Hz | → | LPF 3400Hz | → ADC/Recording
 *        ¯¯¯¯¯¯¯¯¯¯¯     ¯¯¯¯¯¯¯¯¯¯¯¯
 *
 * Total gain: Set to unity (0dB) at 1kHz for clean signal
 * Add gain stage (Op-Amp C) before filter: 20-26dB for mic preamp
 */

// For Arduino noise reduction via digital filtering:
// Apply Butterworth IIR filter in software if hardware filter not practical
void softwareLowPass(float input, float &output) {
  // 2nd order Butterworth LPF, fc = 1kHz, fs = 8kHz
  // Coefficients from: mkfilter.sourceforge.net
  static float x1 = 0, x2 = 0, y1 = 0, y2 = 0;
  float b0 = 0.2929, b1 = 0.5858, b2 = 0.2929;
  float a1 = -0.0000, a2 = 0.1716; // Denominator coefficients
  
  float y = b0*input + b1*x1 + b2*x2 - a1*y1 - a2*y2;
  x2 = x1; x1 = input;
  y2 = y1; y1 = y;
  output = y;
}

Practical Build and PCB Tips

Op-Amp Selection for India

Layout Guidelines

• Keep bypass capacitors (100nF + 10µF) as close as possible to op-amp power pins
• Use a ground plane on PCB — eliminates loop antennas that pick up 50Hz hum
• Separate analog and digital grounds — connect only at single star point
• Route audio signal traces away from power traces and clock lines
• Shield audio input with surrounding ground pour on PCB
• For microphone preamplifiers, use shielded cable between microphone and PCB

Frequently Asked Questions