This article aims to guide you in building a simple 20 Watt Amplifier circuit. The amplifier circuits detailed below will deliver a full output power of 20 watts into an 8 Ohm loudspeaker.
Contents
Why a Single Ended Class-A Amplifier
A single-ended Class-A amplifier is arguably one of the best examples of solid-state single-ended output. In this design, the passive load can be a transformer, resistor, amplifier, or a current sink. Here, we use an inexpensive current sink with high linearity, making it suitable for this project.
Many electrical engineers often recommend using 1:1 transformers or inductors. However, we will avoid this because both components are quite expensive and require high precision. Otherwise, they may negatively impact sound quality due to their non-linear and frequency-dependent nature.
In this experiment, we have utilized a basic circuit—a 60-watt power amplifier—with the capability to modify it for optimal Class-A operation. Many have attempted this approach and achieved positive results.
Using +/- Dual Power Supply
Additionally, we have employed a +/- 20 volts power supply. This can be regulated, conventional, or involve a capacitance multiplier. Before clipping, it should be capable of delivering around 22 watts. It is advisable to use a larger heat sink, as the amplifier is likely to generate significant heat.
In our previous experiment constructing the amplifier, we used a quiescent current of 3A. For this project, we have reduced it to 2.6A to decrease power dissipation. However, each amplifier will still dissipate at least 110W.
Using either large plastic case devices or TO-3 transistors is highly recommended, as heat transfer is one of the biggest challenges in building this amplifier. We also recommend using separate dissipation for each transistor to achieve low thermal resistance.
You can opt for a larger transistor for this development, but it can be costly. Therefore, for a more budget-friendly approach, it is better to use two parallel transistors. They are cheaper than large transistors while still maintaining quality.
Below is the schematic diagram of the simple 20-watt amplifier circuit to assist in building the system.
20W Class-A Amplifier Circuit
The sink shown in the diagram is built on a concept similar to the output stages. Four 1 ohm, 1W resistors [0.25 ohm] are placed in parallel. Some experimentation may be necessary as the current is determined by the base-emitter voltage of the BC549. In this circuit, the BC549 draws base current that is in excess from the resistors. When the voltage across the resistors exceeds 0.65V, the transistor activates and adjusts the balance. Additionally, you can set the DC offset using a 1K trimpot to manage the long-tailed pair (LTP).
Optimum Current
Ideally, a Class-A amplifier should maintain an operating current 110% higher than the peak current of the speaker. For an 8 ohm loudspeaker with a +/- 22V supply, the speaker’s maximum current will be:
This calculation does not account for current loss during output. There will be a loss of 3 volts in the output circuit, due to losses in the emitter or driver resistors and the output device.
The maximum voltage is thus 2.375A @ 8 ohms = 19V peak. Adding a fudge factor to 110%, the operating current is 2.6125A (approximately 2.6A), resulting in an output power of 22.5W.
However, note that while the negative supply is constant, the positive supply varies with the available steady current. With high signals, the current doubles as the upper transistor turns on, or for negative peaks, it drops to zero. This is common in single-ended Class-A amplifiers and complicates the power-supply design.
Adjust Quiescent Current
If the current sense resistor is more than optimal, use a trimpot and wiper to the base of BC549 for accurate current flow. Keep the sense resistor distant from high source generators, like power resistors, to prevent current drop as the amplifier heats up.
Exercise caution with the trimpot, as the wiper connects to the -35V supply line. Incorrect handling may damage the trimpot. Start with the wiper at the collector of the output devices and gradually increase the current to the desired setting. Using a multi-turn pot as an alternative is recommended for precision.
The following diagram shows the construction of a current sink variable for the proposed 20-watt amplifier circuit.
Variable Current Source
The use of 1K resistors, as shown in the figure, ensures that the current will not reach infinite levels even if the pot becomes an open circuit. It is also necessary to allow sufficient time (10 minutes or more) for the temperature across the heat sink to stabilize. The time required to reach operating temperature can vary based on the size of the heat sink, as larger heat sinks have higher thermal mass and thus take longer to stabilize.
The heat sink is one of the most critical components in a Class-A design. Therefore, it is essential to use a heat sink with a thermal rating of less than 0.5°C/Watt. For instance, if the dissipation is around 110W quiescent, a heat sink with this specification will result in a 55°C temperature rise, causing the transistors to reach 80°C and become very hot. Using a heat sink with a thermal rating of 0.25°C/Watt can be considered, but it will not significantly affect the heat generated.
This 20 watt amplifier circuit is easy to construct, and most of the components can be sourced from your ‘junk’ box. The design includes a Darlington pre-driver (Q1 and Q2), a VBE multiplier transistor (Q3), and a quasi-complementary output stage comprising transistors Q4-7.
An all-around shunt feedback loop is implemented from the Q7 collector to Q1’s base via R3. This resistor, along with R2, provides DC feedback and input bias. The voltage gain, and thus the amplifier’s sensitivity, is set at 33 and 370mV through the ratio of the resistive divider R3 to R1.
The quiescent current through transistors Q5 and Q7 should be adjusted to 30mA using the preset PR1. R4 and R5 form the collector load of the Darlington transistor, which is bootstrapped by capacitor C2 to supply a current drive for the output stage.
Despite its simplicity, the 20 watt amplifier can produce high-quality audio and works well with 4, 8, or 16 ohm loads.
Using MOSFETs
The circuit presented in the following figure is for those who want to experiment with a straightforward amplifier using power MOSFETs in the output stage.
The design employs a simple setup similar to those using a common emitter input transistor (Tr1) directly driving a common source MOSFET driver device (Tr2), which then directly drives the complementary common source output transistors (Tr3 and Tr4).
R5 provides 100% negative feedback in the amplifier on DC, ensuring that R1 to R3 bias the output with the appropriate potential. C6 and R4 partially decouple the feedback at audio frequencies, delivering a voltage gain of approximately 20 times (26dB).
This design gives the circuit an input sensitivity of roughly 625mV RMS into 70k for an output power of 20 watts RMS. R8 is used to set the appropriate quiescent current through the output transistors, which is around 80 to 100mA.
Power MOSFETs operate in the negative coefficient mode, meaning temperature compensation circuitry is unnecessary for them. R9 and C4 are configured as a low pass filter at the input of the circuit, helping to eliminate RF interference issues. C5 shifts the circuit slightly towards higher frequencies, assisting stability and reducing radio frequency vulnerability.
C3 and C8 serve as the input and output DC blocking capacitors, respectively. With a 50-volt DC supply and an 8-ohm speaker load, this amplifier circuit can effortlessly deliver an output power of 20 watts RMS.
An output power of approximately 15 watts RMS can be achieved with a supply voltage of roughly 40 volts, and around 30 watts RMS can be reached using a 60-volt DC input with an 8-ohm speaker load.
Although the circuit might not qualify as super Hi-Fi by current standards, it can produce an impressive performance level for a design of such simplicity (it uses only 4 transistors).
The overall harmonic distortion is usually well below 0.1% at most output powers and frequencies, although it can increase somewhat at high and low output powers and with high frequencies (as one might expect).
While the BC177 transistor used for Tr1 has a maximum emitter-to-collector voltage rating of only 45 volts, it is safe to use this device in the circuit with a 50-volt supply.
