🎛️ 5-Band Analog Parametric Audio Equalizer

A fully analog, State Variable Filter (SVF) based parametric equalizer covering the full audio spectrum (20 Hz – 20 kHz), designed as Minor Project II at NIT Andhra Pradesh.

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📌 Project Overview

This project implements a 5-band analog parametric equalizer using the KHN (Kerwin-Huelsman-Newcomb) State Variable Filter topology. It features independent control of center frequency, gain (±9 dB), and Q (bandwidth) for each band — without any cross-coupling between parameters.

The complete design flow covers:

• Transfer function derivation and circuit analysis
• LTspice behavioral and SPICE-level simulation
• Proteus verification with exact TL074CDR models
• KiCad schematic capture (EESchema) and 4-layer PCB layout (PCBnew)

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🗂️ Repository Structure

analog-parametric-equalizer/
│
├── ltspice/                  # LTspice simulation files (.asc)
│   ├── full_circuit.asc      # Complete cascaded equalizer
│   ├── low_shelf.asc
│   ├── band1_250Hz.asc
│   ├── band2_800Hz.asc
│   ├── band3_2500Hz.asc
│   └── high_shelf.asc
│
├── kicad/                    # KiCad project files
│   ├── Mini_project.kicad_pro
│   ├── Mini_project.kicad_sch
│   ├── Mini_project.kicad_pcb
│   └── Mini_project.kicad_prl
│
├── docs/                     # Documentation
│   ├── Project_report.pdf    # Full 27-page project report
│   └── ppt_EQ.pptx           # Presentation slides
│
├── images/                   # Screenshots and renders
│   ├── ltspice_full_circuit.png
│   ├── kicad_schematic.png
│   ├── pcb_top_view.png
│   ├── pcb_3d_isometric.png
│   └── sim_smile_curve.png
│
└── README.md

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⚙️ Filter Architecture

The equalizer uses a cascaded (series) signal chain — each filter block feeds the next, which simplifies gain staging and inter-band tuning.

Audio In (3.5mm TRRS)
    │
    ▼
[DC Block + Impedance Match]  →  [ESD Protection — TVS1400DRV]
    │
    ▼
[Low Shelf Filter]        fc: 120–500 Hz,    Gain: ±9 dB
    │
    ▼
[SVF Band 1 — Bass]       f₀: 120–500 Hz,   Gain: ±9 dB,  Q: adjustable
    │
    ▼
[SVF Band 2 — Midrange]   f₀: 400–1600 Hz,  Gain: ±9 dB,  Q: adjustable
    │
    ▼
[SVF Band 3 — Upper Mid]  f₀: 1.2–5 kHz,    Gain: ±9 dB,  Q: adjustable
    │
    ▼
[High Shelf Filter]        fc: 1.2–5 kHz,   Gain: ±9 dB
    │
    ▼
[Output Buffer + ESD]  →  Audio Out (3.5mm TRRS)

Band Summary

Band	Center Freq	Tuning Range	Integrator Caps	Frequency Pot
Low Shelf	120–500 Hz	Adjustable	C11 = 80 nF	RV2 (100kΩ dual)
Band 1 Bass	250 Hz	120–500 Hz	C3, C4 = 33 nF	RV3/RV4 (10kΩ dual)
Band 2 Mid	800 Hz	400–1600 Hz	C7, C8 = 10 nF	RV7/RV9 (10kΩ dual)
Band 3 Hi-Mid	2.5 kHz	1.2–5 kHz	C9, C10 = 3.3 nF	RV10/RV12 (10kΩ dual)
High Shelf	1.2–5 kHz	Adjustable	C2 = 6.8 nF	RV15 (100kΩ dual)

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🔬 Key Design Achievements

Q–Gain Decoupling

In a standard SVF, bandpass peak = Q × Vin, meaning tuning Q inadvertently changes the boost/cut magnitude. This was solved by relocating the Q-control resistor to the BP output feedback path of the summing amplifier — peak amplitude is now constant at Vin, fully independent of Q.

Dual-Gang Frequency Control

Center frequency ω₀ = 1/(RC) requires both integrators to track the same R simultaneously. Dual-gang potentiometers ensure both integrators always see equal resistance, maintaining SVF symmetry at any frequency setting.

SVF Bandpass Transfer Function

              (ω₀/Q) · s
H(s) = ─────────────────────────────
         s² + (ω₀/Q)·s + ω₀²

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🧰 Components

Core ICs:

• TL074CDR — Quad JFET-input op-amp, SOIC-14 (×5, giving 20 op-amp stages)
• TVS1400DRV — TVS ESD protection array (×2, at input and output)

Op-Amp Key Specs (TL074CDR):

Parameter	Value
Supply voltage	±15 V
Slew rate	13 V/µs
GBW product	3 MHz
Input bias	65 pA
Noise voltage	18 nV/√Hz

Total BOM: 89 components, 56 fixed resistors, 11 capacitors, 16 potentiometers, 2 audio connectors.

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📊 Simulation Results

All five filter blocks were simulated in LTspice XVII (AC sweep, 20 Hz – 20 kHz, 1000 pts/decade) and verified in Proteus using exact TL074CDR SPICE models.

Block	Target f₀/fc	Result	Gain Range	Status
Low Shelf	250 Hz nominal	~250 Hz, tunable 120–500 Hz	±9 dB	✅ Pass
Band 1 (Bass)	250 Hz	~250 Hz, tunable 120–500 Hz	±9 dB	✅ Pass
Band 2 (Mid)	800 Hz	~800 Hz, tunable 400–1600 Hz	±9 dB	✅ Pass
Band 3 (Hi-Mid)	2.5 kHz	~2.5 kHz, tunable 1.2–5 kHz	±9 dB	✅ Pass
High Shelf	2.3 kHz nominal	~2.3 kHz, tunable 1.2–5 kHz	±9 dB	✅ Pass

Preset EQ responses verified: V-Shape (Smile Curve), Vocal Boost

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🖥️ PCB Design

Designed in KiCad PCBnew v9.0 with a 4-layer FR-4 stackup:

Layer	Role
F.Cu (Top)	Signal routing, component placement
In1.Cu	Solid GND plane
In2.Cu	±15 V power distribution
B.Cu (Bot)	Secondary signal routing

PCB Stats: 1,468 track segments · 150 vias · A3 drawing sheet

Layout follows signal-flow order (left → right), with decoupling caps adjacent to op-amp supply pins and all potentiometers along one edge for front-panel mounting.

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🛠️ Tools Used

Tool	Version	Purpose
LTspice XVII	—	AC sweep simulation
Proteus Design Suite	—	SPICE verification
KiCad EESchema	9.0	Schematic capture
KiCad PCBnew	9.0	PCB layout

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👥 Team

Name	Roll No.
Mohit Kumar Gupta	623148
Satyam Kumar	623172
Vishal Ray	622271

Guide: Dr. M. C. Raju, Assistant Professor, DECE, NIT Andhra Pradesh Course: Minor Project II (EC399) · Academic Year 2025–26

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📄 License

This project is shared for educational and reference purposes. Feel free to study, fork, and build upon it — attribution appreciated.

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🔮 Future Work

• PCB fabrication and hardware assembly
• Audio characterisation using REW (THD+N, noise floor, dynamic range)
• Enclosure and front-panel design for all 16 potentiometers
• Expand to 7 or 10 bands with additional SVF stages
• PCB noise optimisation based on measured results