project

Audio Amplifier Circuit

EE210 Fall 2024

Introduction

Enable audio to hear music.

This system processes audio sources and adjusts them with volume, bass, and treble controls. Bright LED indicators show the audio levels, while a connected speaker plays back your changes instantly.

To achieve this, I designed and implemented a five-stage audio processing chain: a summing op-amp mixer, a tone control filter, a volume control stage, an LED-based volume indicator, and a fixed-gain power amplifier. The system accepts a stereo input, converts it to mono, adjusts the tone and volume, shows a real-time visual indication of signal amplitude, and drives a speaker output. Each stage was built using individual components such as op-amps, resistors, capacitors, and transistors, and was individually tested with an oscilloscope before integration into the complete system.

Op‑amp mixing
Tone filtering
Frequency analysis
Potentiometer
Signal visualization
Mono conversion
Oscilloscope testing
Schematic drafting
Breadboarding

System Objectives

High‑level signal flow with post‑stage approximate amplitude envelopes.

Block	Function	Key Range / Thresholds	Core Parts
1 Mixer	Stereo → mono summing, per‑channel trim	In: 0.025–1.0 Vpp ea. Out: ~0.01–0.04 Vpp nominal	Op‑amp, 2× 20 kΩ pots
2 Tone	Adjustable bass / treble tilt	Gain LF 1/3×→3× (HF inversely 3×→1/3×), 20 Hz–20 kHz	RC network + pot
3 Volume	Post‑EQ amplitude control	0 → ~1.2 Vpp	20 kΩ pot divider
4 LED VU	Amplitude visualization	Comparators at 20 / 60 / 80 / 120 mV	4 refs + LEDs
5 Power Amp	Current drive to speaker	Fixed gain, drives low‑Z load	TIP31 / TIP32 pair

Block 1 – Summing Mixer. Two AC‑coupled stereo channels feed an inverting summing node. Independent 20 kΩ potentiometers set per‑channel contribution without clipping the combined headroom (target mono output ~0.01–0.04 Vpp for nominal sources).
Block 2 – Tone Filter. A single potentiometer sweeps a tilt‑style RC network: low frequencies can be boosted up to 3× while highs attenuate to 1/3× (or vice‑versa), covering the 20 Hz–20 kHz band with smooth spectral shaping.
Block 3 – Volume Control. A 20 kΩ divider implements user gain from mute (≈0 Vpp) to ≈1.2 Vpp while presenting a low source impedance to downstream comparator and driver stages.
Block 4 – LED Level Indicator. Four comparators reference a 9 V ladder (≈20, 60, 80, 120 mV). Progressive LED illumination forms a simple pseudo‑VU display without loading the audio path.
Block 5 – Power Amplifier. Complementary emitter follower (TIP31/TIP32) supplies current gain; the op‑amp front end biases the pair to limit crossover distortion and cleanly drive the speaker at listening levels.

Theory & Experimental Methods

Complete schematic: summing mixer, tone filter, volume divider, LED comparator ladder, complementary emitter follower power stage — Complete schematic – unified view of the five functional blocks (source: complete schem.png).

Theoretical Overview

Block 1 (Summing Mixer): Implements an inverting summer. For equal input resistors R_in and feedback resistor R_f, each channel contribution ≈ −(R_f/R_in)·V_CH. Potentiometers precede the summing node providing variable effective R_in (amplitude trim) to keep summed V_out within the 0.01–0.04 V_pp design envelope.
Block 2 (Tone Filter): A single‑pot “tilt” network whose impedance division versus frequency alters relative LF/HF gains. At one extreme G_LF ≈ 3×, G_HF ≈ 1/3×; reversed at the other. Capacitor reactances chosen so pivot occurs inside the audio band (order of a few kHz) while still spanning 20 Hz–20 kHz.
Block 3 (Volume Divider): Passive 20 kΩ potentiometer forming a variable attenuator with low output impedance ( ≪ comparator input impedance ), preserving tone shaping while setting 0–1.2 V_pp user level.
Block 4 (LED VU Ladder): Resistor ladder establishes monotonic DC thresholds (≈20/60/80/120 mV). Each comparator outputs a logic‑level drive through its series LED resistor (~810 Ω) giving stepped amplitude indication. Instantaneous (no envelope) response intentionally chosen for simplicity.
Block 5 (Power Stage): Complementary emitter follower provides current gain (β aggregation) without additional voltage gain; op‑amp supplies drive and bias to minimize crossover distortion while keeping the driver within linear output swing for the target speaker load.

Full breadboard build — neat wiring and short returns help keep noise low.

Design Process

Breadboard overview of the full audio chain — Complete schematic used as reference during block‑by‑block bring‑up.

Blocks 1–5 Implementation

Compact gallery of the five functional stages. Click any card for details.

Summing amplifier stage on breadboard — Summing mixer schematic / breadboard stage.

Details

A classic inverting summing amplifier mixes left/right into mono. Input potentiometers trim each channel to preserve headroom and minimize noise at the summing node.

Tone control RC network with potentiometer — Tilt EQ network schematic.

Details

Potentiometer‑controlled RC tilt network shapes the spectrum (≈20 Hz–20 kHz) providing complementary bass boost vs. treble cut (and vice‑versa) without clipping.

Volume control potentiometer wiring — Output level control potentiometer.

Details

20 kΩ potentiometer as a divider provides smooth attenuation from mute to ≈1.2 Vpp while presenting low source impedance to comparators and driver.

Comparator-based LED volume indicator — Comparator ladder + LEDs.

Details

Four comparators reference a resistor ladder (~20/60/80/120 mV) lighting LEDs progressively for instantaneous amplitude visualization.

Power amplifier stage with TIP31/TIP32 — Complementary emitter follower stage.

Details

Complementary TIP31/TIP32 emitter follower furnishes current gain; op‑amp front‑end biases the pair to limit crossover distortion and cleanly drive the load.

Results

Stereo signals were mixed into a stable mono channel.
Tone control produced clear bass and treble adjustments.
The volume control stage provided smooth, reliable output scaling.
The LED indicator responded dynamically to signal amplitude, creating a real-time visual representation of the music.
The amplifier stage drove the speaker effectively at normal listening levels.

Oscilloscope Results

Each block was verified by capturing input (reference) and output waveforms. Use consistent vertical scaling so amplitude changes and threshold activations are visually comparable. Replace the src attributes below with your final PNGs/JPGs (recommended width 1000–1400 px for clarity). Keep captions concise and emphasize what changed between traces (gain, attenuation, threshold crossings).

Block 1 – Summing Mixer (Independent Channel Trim)

Left channel input (top) vs mixed output (bottom) at maximum trim — Left channel max trim: output ≈2.0 Vpp (matches input) → full gain path engaged.

Left channel input (top) vs mixed output (bottom) at minimum trim — Left channel min trim: output ~80 mVpp residual while input unchanged — confirms attenuation range & isolation.

Right channel input (top) vs mixed output (bottom) at maximum trim — Right channel max trim: symmetric ≈2.05 Vpp contribution shows balanced summing network.

Right channel input (top) vs mixed output (bottom) at minimum trim — Right channel min trim: output suppressed to noise floor (~80 mVpp) with negligible bleed.

Block 2 – Tone Control (Tilt Extremes at 20 kHz)

20 kHz input vs boosted output – potentiometer clockwise — Clockwise (boost): output ≈8.4 Vpp vs ≈4.1 Vpp input — high‑frequency emphasis.

20 kHz input vs attenuated output – potentiometer counter‑clockwise — Counter‑clockwise (cut): output reduced to ≈1.2 Vpp — full attenuation range demonstrated.

Block 3 – Volume Stage (Max vs Near‑Mute)

1.2 Vpp input vs full-scale output — Pot at max: output ≈1.29 Vpp (≈unity gain) — minimal insertion loss.

1.2 Vpp input vs muted output — Pot near min: output ~80 mVpp residual noise — effective mute without loading source.

Block 4 – LED Indicator (Comparator Threshold Region)

Real-time LED ladder response while sweeping volume control — successive thresholds light as amplitude crosses ≈20/60/80/120 mV.

Input and comparator node near LED thresholds — Comparator node traverses ≈20/60/80/120 mV thresholds — drives stepped LED illumination (not visible on scope but correlated).

Block 5 – Power Stage (Drive Integrity)

Power amplifier input vs output across load — Output mirrors input symmetrically; absence of crossover notch indicates proper bias of complementary emitter follower.