This is a nice final audio amplifier to build it and have fun. The amplifier is able to drive up to 100W to a 4 Ohm load or up to 70W to an 8 Ohm load. The total distortion is less than 0.1% (even in high frequencies). The circuit uses ordinary electronic components and the amplifier has DC coupled output, for best performance even at low frequencies.
The final stage of the amplifier uses a pair of complementary Darlington transistors (T7 and T8). Darlington transistors offer high current and hence high power amplification.
The input stage uses T1 and T2. Then, there is a driving stage, which uses T3 and T4. T3 and T4 are connected in series. The driving stage is used for amplification but also to bias the final stage with a specific idle current.
The final stage of the amplifier uses a symmetrical topology. A key advantage of the symmetrical topology is that the idle voltage on the load is zero. Therefore, there is no need for a DC blocking capacitor to isolate the load from any DC. Thus, by avoiding the use of a DC blocking capacitor, we also diminish any undesirable high pass filtering from an RC filter at the output (formed by the DC blocking capacitor and the load resistor).
The amplifier has high input impedance which is about 100K due a bootstrap biasing technique based on C4 and R2 and also, because T1, by its nature, has a quite high input impedance. T1 and T2 form a differential amplifier. A differential amplifier has two inputs: The first one is at the base of T1 and at the other one is at the base of T2. The first input receives the input signal directly, while the other input is used to add negative feedback from the output through R6. The feedback is DC coupled and the DC component of the feedback is used to stabilize the output to zero potential at rest.
The voltage gain (voltage amplification) of the differential input stage, which can be regarded as a non-inverting amplifier, is determined by the ratio of the sum R3 plus R6, to R3. That is:
Uo / Ui = (R3 + R6) / R3 = 1 + R6 / R3 = 3420/120 = 28,5
The base of T4 receives signal from the collector of T1. T4 and T3 amplify the input signal in order to provide enough power to drive the output stage. Due to the use of Darlington type transistors at the output, there is no need for extreme intermediate amplification. Due to this in fact, T4 and T3 do not use high currents and they do not get very hot, so they do not require do be cooled.
T3 biases the output stage and resistors R18 and R19 stabilize its operation. The idle DC voltage at the connection point of R18 and R19 (and moreover at the load) is adjusted from P1 potentiometer which actually adjusts the collector-emitter voltage of T3. R11 is "Bootstrapped" through C5, and its impedance is increased, thereby increasing the gain of the driver stage.
The output stage uses T7 and T8. T7 and T8 are complementary Darlington type transistors and they have the following characteristics (at room temperature):
- maximum collector-emitter voltage = 100 V
- maximum collector current = 16 A
- maximum thermal loss = 150 W
No matter how efficient are the output transistors, we should protect them from overloading. For this reason we use T5 and T6.
The voltage drop across R18 or R19, is proportional to the output current. Thus, we use these voltages to sense a potential overload. In case of an overload (short circuit at the load), a high output current results in high voltage drops across R18 and R19. These voltages make T5 and T6 conductive via voltage dividers R16 / R14 and R17 / R15. At conductivity, T5 and T6 draw current from T7 and T8 bases, through D2 and D3, thereby limiting the driving current in T7 and T8.
All capacitors in the circuit have important roles:
C1 limits the frequency response at high frequencies above the audible range in order to limit the noise level. C3 is also used to cause attenuation at high frequencies. C6, C7 and C8, as well as R20 and C9, stabilize the amplifier. C10 and R21, C12, C11 and R22, serve to suppress any high frequency noise in the supply voltage.
At peak output on a 4 Ohm load, the amplifier requires a current of about 2.25A. For an 8 Ohm load, the peak current is about 1.1 A. There is a common truth that the quality of an audio amplifier is strongly determined from its power supply unit. Indeed, that is true but there is no need for exaggerations. A simple power supply unit, able to provide symmetrical voltages of ± 40 V, is sufficient.
For low cost and simplicity, we use a simple non- stabilized power supply unit. Given the absence of stabilization, output voltages of ± 40 V correspond to a low limit, during peak power consumption. At less than peak power, the power consumption is reduced, and the output voltage of the non stabilized power supply unit is expected to be higher than 40V.
However, the transistors of the amplifier withstand up to a maximum of ± 50 V. Therefore, to ensure a safety margin, we prefer to build a power supply unit at ± 46 V. Thus, we expect to have a small difference of about 6 V, between resting and full load. This is a quite small voltage drop which requires a very low internal resistance of the power supply unit. The internal resistance of a non stabilized power supply unit is mostly determined by the transformer. That's why we prefer to use a high quality toroidal transformer.
The power supply unit has only a transformer, a diodes bridge rectifier and some electrolytic capacitors for voltage smoothing. Fuses are also used for protection in case of a permanent short circuit.
The 100W audio amplifier circuit can be used for a single audio channel. Therefore, if you want to create a stereo version of the amplifier, you may use two identical circuits and two identical power supply units.
Some construction details
The amplifier and the power supply can be easily built in the printed circuit boards provided herein:
It is a good practice for the power resistors R18 and R19, to be placed at a distance of at least 5 mm from the circuit board surface. This way, we ensure good air flow for sufficient heat diffuse.
Sufficient heat sinks are required for T7 and T8. It is better to place T7 and T8 on separate heat sinks of 1.2 ° C / W. If you put the two transistors on the same heat sink, then you need to divide the thermal resistance of the heat sink with the total number of the transistors. Therefore, for the two transistors (T7 and T8) on the same heat sink, a thermal resistance less than or equal to 0.6 ° C / W would be sufficient.
There should be no direct contact between any transistor and the heat sink. Unexpected short-circuits may arise from direct contact (for example, if the heat sink is mounted to a grounded chassis, or both transistors are placed on the same heat sink) and that is because the metallic case of each transistor is internally connected to the collector). Thus, for mounting the transistors on their heat sink(s), you may use appropriate insulators, e.g. of mica.
You must use a suitable shielded cable for the input signal and cable’s shield should be grounded.
The boards of both the amplifier and the power supply have exactly the same dimensions. This is done intentionally, in order to allow the mounting of the power supply board directly below the amplifier’s board by using screws and spacers. This "intelligent" arrangement reduces the required space and allows the construction of an autonomous module.
After construction, the amplifier requires some calibration. The calibration is a simple adjustment of P1 trimmer.
The calibration is done with the aim of a DC ampere meter – millimeter while setting a zero input signal. During calibration, the load (speaker) should be also disconnected..
Turn off power supply. Short-circuit the input of the amplifier to ensure the absence of an input signal. Make sure that the output is not connected anywhere (disconnect any speaker).
Remove F2 fuse and place the terminals of an ammeter at its terminals. The ammeter should be set at a 1A DC scale.
Turn P1 potentiometer fully anticlockwise. Check all connections and connect power supply. The ammeter should show approximately 0A. If the reading is greater than zero, immediately disconnect power supply and check for any circuit error.
If everything is as it should be, change the scale of the ammeter to 100mA and adjust P1 so that the milli-ammeter to show approximately 80mA. At that time, the quiescent current in the output transistor will be about 50mA.
Calibration completed. Place F2 fuse at its place and enjoy.
If someone goes wrong, don’t be upset. Just find the error by taking voltage at current measurements at some circuit nodes and compare these measurements with reference values indicated at the schematic. Reference values have been measured while the load (speaker) was connected, and there was a zero input signal.
List of components (100W amplifier)
Resistors (1/4W or 1/2W unless otherwise noted):
R1 = 120K
R2, R5, R6 = 3K3, R3 = 120Ω
R4, R8 = 680Ω, R7 = 1K5
R9 = 5K6, R10 = 1K2, R11 = 2K7
R12, R13 = 270Ω, R14, R15 = 15Ω, R16, R17 = 220Ω, R18, R19 = 1Ω/9W, R20 = 10Ω
R21, R22 = 1Ω, P1=trimmer pot 1K
C1 = 470pF
C2 = 10µF/63V, C3 = 150pF
C4 = 1000µF/10V, C5 = 220µF/50V, C6 = 47pF
C7, C8 = 560pF, C9 = 47nF
C10, C11 = 680nF, C12, C13 = 100nF
T1, T2 = BC 556A, T3, T5 = BC 547B, T4 = BC 639
T6 = BC 557B
T7 = BDX67B or BDX67C, T8 = BDX66B or BDX66C, D1 =zener 9V1/1.3W
D2, D3 = 1N4148 or 1N914
List of components (2x45V power supply unit)
R1, R2 = 3K3/1W
C1 = 100nF
C2, C3 = 4700µF/63V
Semiconductors : D1, D2 = LED
B1 = B80 C3200/5000 diodes rectifier bridge
F1 = fuse 1,5A
F2, F3 = fuses 2,5A
Tr1 = Transformer 2x30V, 2x3.75A or more, 225VA or more