LM1875 — 20W Monolithic Hi-Fi Audio Power Amplifier

Overview

The LM1875 (LM1875T) is a 20W monolithic high-fidelity audio power amplifier IC manufactured by Texas Instruments (originally National Semiconductor). Enclosed in a 5-lead TO-220 package, it is famous in audiophile communities as the basis of the legendary "Gainclone" DIY hi-fi amplifier builds.

Delivering up to 20 Watts RMS into $4\ \Omega$ or $8\ \Omega$ speakers with ultra-low Total Harmonic Distortion ( $\text{THD} = 0.015\%$ at 1 kHz), the LM1875 operates on split power supplies from $\pm 8\text{V}$ to $\pm 30\text{V}$ (or single supply $16\text{V} \dots 60\text{V}$ ). It includes internal thermal shutdown protection and output short-circuit protection to ground and power rails.

Quick reference


Amplifier Type	Monolithic Hi-Fi Audio Power Amplifier
Output Power	$20\text{ W}_{\text{RMS}}$ into $4\ \Omega$ or $8\ \Omega$ load at $V_{CC} = \pm 25\text{V}$
THD + Noise	$0.015\%$ typical at $1\text{ kHz}, 20\text{W}, 8\ \Omega$
Supply Voltage (Split Rail)	$\pm 8.0\text{ V}$ to $\pm 30.0\text{ V}$ DC
Supply Voltage (Single Rail)	$16.0\text{ V}$ to $60.0\text{ V}$ DC
Gain Bandwidth Product	$5.5\text{ MHz}$
Slew Rate	$8.0\text{ V}/\mu\text{s}$
PSRR (Power Supply Rejection)	$94\text{ dB}$
Package	5-lead TO-220 (TO-220-5 Pentawatt)

Pinout (5-Lead TO-220 Package)

Looking at the front labeled face of the 5-lead package with leads pointing down:

text

        ┌─────────────┐
        │   O   [TAB] │  (Metal Mounting Tab = V- Supply Rail!)
        ├─────────────┤
        │   LM1875T   │  (Front Package Face)
        └─┬─┬─┬─┬─┬───┘
          1 2 3 4 5

Pin	Name	Type	Description
1	`+IN`	Input	Non-Inverting Audio Signal Input
2	`-IN`	Input	Inverting Feedback Input
3	`V-`	Power	Negative Power Supply Rail (Connect to $-V_{CC}$ or GND; Connected to Metal Tab!)
4	`OUT`	Output	Speaker Output Terminal
5	`V+`	Power	Positive Power Supply Rail ( $+V_{CC}$ )

Warning

Metal Tab Warning: Metal Tab is connected to V-! The TO-220 metal tab is internally connected to Pin 3 (V-). In a split-supply system ( $\pm 25\text{V}$ ), the tab is at $-25\text{V}$ , NOT ground. Always use a mica or ceramic insulator when bolting the LM1875 to a grounded metal heatsink.

Classic Split-Supply Audio Amplifier Circuit (Gain = 21)

text

        Audio Input ───[1µF Film Cap]───┬─── [Pin 1: +IN]
                                        │
                                  [22kΩ to GND]
                                                              ┌─── R_feedback (22kΩ) ───┐
                                                              │                         │
                                    GND ───[10µF]───[1kΩ]─────┴─── [Pin 2: -IN]         │
                                                                       LM1875           │
  +25V DC Supply Rail ──────────────────────────────────────────── [Pin 5: V+]          │
                                                                                        ├─── Speaker Out (Pin 4) ───► ( + ) Speaker
  -25V DC Supply Rail ──────────────────────────────────────────── [Pin 3: V-] ─────────┤
                                                                                        └─── [Boucherot Cell: 2.2Ω + 0.1µF to GND]

Common mistakes

Omitting the output Zobel / Boucherot network ( $2.2\ \Omega + 0.1\ \mu\text{F}$ ): High-gain power op-amps oscillate at RF frequencies into complex speaker wire inductance. Always connect a $2.2\ \Omega$ resistor in series with a $0.1\ \mu\text{F}$ ceramic capacitor from Output (Pin 4) to GND directly at the chip pins.
Insufficient heatsinking: Dissipating $20\text{W}$ RMS output generates up to $15\text{W}$ of internal heat. Run the chip on a heavy aluminum heatsink ( $R_{\theta JA} < 3^\circ\text{C/W}$ ).

Notes

LM1875 vs TDA2030 / TDA2050: LM1875 offers lower THD ( $0.015\%$ vs $0.5\%$ ) and higher supply voltage capability ( $60\text{V}$ vs $36\text{V}$ ), making it superior for hi-fi audio.