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Audio Amplifiers

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Audio Power Amplifiers

Attribution

Many chips and modules are available. This makes it easy to build an audio power amplifier subsystem.

LM386 Chip

Data Sheet

This chip is ideal for portable low voltage circuits like small radios. No heatsink is needed.

4 to 18 Volts DC supply, depending on the chip version.
0.325 to 1.0 Watts output, depending on the chip version.
4 to 8 mA quiescent (silence) current.
0.2% distortion
Voltage gain is 20.
Frequency response 20 to 20 000 Hz with carefully chosen coupling capacitors.

LM386 Pinout

LM386 Circuit and Layout

Connecting all the zero volt wires as close together as possible is called single point earthing or grounding. This reduces mains hum and the chances that the circuit will oscillate unexpectedly. With this low power amplifier, it's not really necessary but it's essential with higher powered circuits.

LM386 Details

Q1 is the LM386 audio power amplifier chip.
C1 is the input coupling capacitor. It passes the AC audio signals and blocks any DC signal at the input.
VR1 is a potentiometer used as a potential divider used as a volume control.
D1 is a diode. This passes current in one direction only. It protects against accidental connection of the power supply the wrong way round.
C2 is a decoupling capacitor.
- It reduces the performance of the amplifier at high frequencies.
- At these higher frequencies, there is an increased risk of unwanted oscillations.
- C2 prevents these.
R2 reduces the current through C2 to a suitable level and controls high frequency the filtering.
C3 is the output coupling capacitor.
- There is a DC voltage on the output pin 5 of the LM386 chip.
- This DC voltage would destroy the loudspeaker but the capacitor passes the audio signal and blocks the DC voltage.
C4 decouples the power supply. Any AC signals arriving via the power supply line are grounded.

Chip Selection Criteria

Output Power

Measured in RMS Watts.
0.3 Watts - small radio, intercom
2 Watts - small to medium computer speakers
20 Watts - typical domestic HiFi.
200 Watts - small stage amplifier for concerts
2000 Watts - large venue, huge speaker systems

Output Power = V_rms² / R

V_rms = 0.7 V_peak

Supply Voltage

Battery, low voltage energy efficient portable audio equipment.
Battery, 0 and 12 Volt lines for use in vehicles.
Mains powered with positive and negative supply lines.

Frequency Response

The Voltage BANDWIDTH of an amplifier is defined as the range of frequencies where the output voltage is at least 0.7 or 70% of the maximum output.
The Power BANDWIDTH of an amplifier is defined as the range of frequencies where the output power is at least 0.5 or 50% of the maximum output.

This image shows a bandwidth going from 10Hz up to about 20 kHz giving a 19990 Hz bandwidth which rounds off to 20 kHz.

Audio Bandwidth

300 to 3000 Hz, telephone quality.
100 to 4000 Hz, AM radio quality.
30 to 15000 Hz, FM radio quality.
30 to 22000 Hz, CD audio HiFi quality.
20 to 20000 Hz, Typical human hearing (as people age, the highest frequency they can hear drops to 12kHz or below).

Distortion

The output should be identical to the input but at a higher voltage or current. Most circuits come close to this ideal but are not perfect. The damage to the output signal is called distortion. If is often expressed in percent.

10% - The sound will be clear but harsh - walkie talkies
1% - A nicer sound. It's not hard to achieve this quality.
Better than 0.1% - A very nice sound. It's hard to achieve. This can get expensive.

Input Resistance

This is important if the signal source has an output resistance greater than the chip's input resistance. In this case a buffer or pre-amplifier would be needed.

LM380

Not as easy to build this circuit as the lower power LM386 above.

10 to 22 Volts - Not ideal for small portable equipment.
0.13 Watts quiescent (silence) power drain.
Voltage gain is 50.
Peak current 1.3 Amps.
0.2% distortion
2.5 Watts output.
To remove waste heat, solder device pins 3, 4, 5, 10, 11, 12 into a 1/16 inch epoxy glass board with 2 ounce copper foil with a minimum surface of 6 square inches. Use single point earthing.
The ground wires are all connected to the same point or as close together as possible. This minimises the risk of parasitic oscillations.

TDA2030

Another Audio Power Amplifier IC

For safe operation and full output power, this chip needs to be bolted onto a fairly large heat sink.

This is an extract from an S T Microelectronics data sheet. The device provides 8 to 14 Watts of audio depending on the power supply voltage and the load resistance.

Higher supply voltages give larger power outputs. This is a square law so doubling the supply gives four times the power.

Keeping the device cool is critical. It has thermal shutdown which will limit the output power if it becomes too hot. A good heat-sink needs to be used.

R1	is a potentiometer used as a volume control. It also determines the input resistance of the sub-system.
R2 and R3	form a voltage divider providing half the power supply.
C2	is a decoupling capacitor. It ensures that the halved power supply voltage is steady and free from any AC signal.
R4	holds the non inverting input at a DC level equal to half the power supply voltage.
C1	is a coupling capacitor. It couples the audio input to the non inverting pin of the amplifier. It also blocks the DC voltage between the audio input and the chip.
C4 and C5	are decoupling capacitors. Two are needed because the C4 performs well at low frequencies but badly at high frequencies. C5 performs well at high frequencies but not at low ones. Between them they decouple the full range of frequencies.
TDA2030	This is the audio amplifier Integrated Circuit (IC). It is similar to an ideal op-amp but it provides a much higher power output.
R5 and R6	control the gain of the amplifier. It is a non inverting amplifier and the gain is found from 1 + R6 / R5.
C3	is a decoupling capacitor. It grounds AC signals so the chip works as a normal non inverting amplifier for AC signals. It leaves a DC feedback path so the chip's DC levels are well controlled.
D1 and D2	prevent the output going above the DC + supply or below the DC - supply. This protects the chip from back emf voltage spikes caused by driving an inductive load. (Some speaker systems contain cross-over filter circuitry containing inductors.)
C7	is a coupling capacitor. It couples the audio output to the loudspeaker and blocks the DC level which would otherwise destroy the speaker.
C6 and R7	This is an equalisation circuit. Without this the amplifier would probably oscillate at a high frequency, perhaps inaudible, but likely to damage the speaker or tweeter if there is one.