In this post we learn how to make a simple 150 watt power amplifier circuit using a typical OCL design which ensures cheap layout and use of minimum components, with high reliability.
Referring to the figure a perfectly symmetrical OCL based amplifier can be seen , using discrete components suitable for all electronic enthusiasts and hobbyists for going through an in-depth practical study with its topology.
This OCL amplifier circuit is a mid-range power amplifier capable of delivering a good 150 watts of power due to its symmetrical structure, wide frequency response, simple layout and so on. The sound quality will be quite satisfactory, and comparable to other equivalent high-fidelity amplifiers normally preferred by the users for home use.
How the amplifier circuit works
The first stage of the circuit can be seen built with a complementary symmetrical differential configuration, each of the BJT channels using 2SC1815, 2SA1015 consume about 1mA, while in the quiescent state
The next stage is designed for handling the voltage amplification and this also makes use of a complementary push-pull design, through a set of high power complementary pair of BJTs namely A180, C180, which runs using a current of about 5mA.
The two 1N4148 ensure a drop of 1.6V required for biasing the relevant bases of the complementary BJTs.
The next two complementary power BJTs involving TIP41C, TIP42C create the driver stage or the intermediate buffer stage fo the last power transistors.
The inclusion of this high efficiency buffer/driver stage becomes one of the main features of the modern OCL amplifier design, which helps to offer a high load impedance, and thereby ensures a very stable Higher gain amplifier output stage.
Additionally this type of capacitor less topology also ensures a lower output resistance across the output power transistor stage, which in turn helps the output junction capacitance Cbe charging rate to become faster, thus improving the overall transient characteristics and frequency stability of the circuit.
However the operating current of this stage can be slightly higher, at around (10-20) mA, for each of the channels which may sometimes go as high as 100mA under higher full volume, this happens because the specified quiescent current may be capable of saturating the output stage to the most optimal levels.
As can be witnessed in the given 150 watt amplifier circuit diagram, the emitter resistances of the driver stage employs a floating termination, and these are not connected with the earth line, and this causes the amplifier to operate typically in the Class A range, and ensure a maximum bias voltage for the output stage.
The power output stage is wired using the traditional complementary capacitor less design and features an FT (frequency transition) level of as high as 60 Mhz, across the BJTs C2922, A1216, through a quiescent current consumption of around 100mA.
The amplifier also employs a negative feedback loop across the output stage and input inverting stage, which sets the amplifier to a gain level of approximately 31.
If you find it difficult to get the parts mentioned in the diagram, you could replace them with the following equivalents.
VT1, VT2 = BC546
VT3, VT4 = BC556
VT6 = MJE340
VT5 = MJE350
VT9 = TIP3055
VT10 = TIP2955
How to Convert into Higher wattage Power Amplifier
The title of the article suggests that the mentioned design is intended for delivering 150 watts of power, but in reality the specs are actually never restricted for such designs. You can easily upgrade the circuit to produce much higher outputs simply by increasing the voltage upto 90V.
The power devices mentioned in the above parts list are specifically selected to handle higher voltages and to enable the required upgrades.
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