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25W Class-A Hi-Fi Power Amplifier

25W Class-A Hi-Fi Power Amplifier

The P3A amplifier has proven extremely popular, and the DoZ continues to provide enthusiasts with a simple, reliable and easy to build Class-A amplifier.
The basis for this amplifier has been around now for several years as Project 3A, and requires only relatively small modifications to be able to operate in Class-A. The biggest change is in output power (reduced dramatically from the 60-100W of the Class-AB version), but at 25W is still more than enough for many people.

The power supply is where you will see major changes – it has to be able to supply a continuous current of 1.5A, and needs very low ripple and noise. The design shown below will be seriously expensive to build, but this is the case with any Class-A amplifier, and is to be expected.

The first thing that needs to be examined with a Class-A amplifier is power dissipation of the output transistors. At the recommended supply voltage of ±25V DC (nominal) and a quiescent current of 1.5A, each power transistor will dissipate 37.5W, or 75W for the pair in a single channel. The thermal resistances that need to be considered are listed below, along with typical values …


Rth Typical Value
Junction – Case 0.7°C / W
Case – Heatsink 1.0°C / W
Heatsink – Ambient 0.5°C / W
Total (Junction – Ambient) 2.2°C / W

Table 1 – Thermal Resistances (Each Transistor)

The typical derating is a linear curve, starting at 25°C junction temperature and allowing zero dissipation at 150°C. OnSemi often use a derating factor of 1.43W/°C, starting from 25°C – not an unreasonable value, but this assumes a maximum junction temperature of up to 200°C. For maximum reliability I will use a figure of 1.6W/°C, which derates a 200W transistor to zero Watts at 150°C – much safer all ’round.

Based on Table 1, each transistor will operate at …

Tj = Rth * PowerSo …

Tj = 2.2 * 37.5 = 82.5°C Above Ambient!

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