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.
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)
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
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 | 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) |
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.
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.
(ω₀/Q) · s
H(s) = ─────────────────────────────
s² + (ω₀/Q)·s + ω₀²
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.
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
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.
| 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 |
| 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
This project is shared for educational and reference purposes. Feel free to study, fork, and build upon it — attribution appreciated.
- 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