Figure 1 - Low Power Version High Power Version
The same PCB is used, but has an extra pair of MOSFETs. Since the devices are running in parallel, source resistors are used to force current sharing. Although these may be replaced by wire links, I do not recommend this. This version may be operated at a maximum supply voltage of +/-70V, and will give up to 180W RMS into 8 ohms, and 250W into 4 ohms. Short term (peak) power is around 240W into 8 ohms and 380W into 4 ohms. These figures are very much dependent on your power supply regulation, determined by the VA rating of the transformer, size of filter caps, etc.




                                                     Figure 2 - High Power Version

Although not shown, the transistors and MOSFETs are the same in this version as for the low power variant. The additional capacitors (C11 and C12) shown are to balance the gate capacitance. The P-Channel MOSFETs have significantly higher gate capacitance than their N-Channel counterparts, and the caps ensure that the two sides of the amp are roughly equal. Without these caps, the amp will almost always be unstable.

As noted above, the PCB is the same for both versions, but for Fig. 2 it is fully populated with 2 pairs of power MOSFETs. The high power version may also be used at lower supply voltages, with a slight increase in power, but considerably lower operating temperatures even at maximum output, and potentially greater reliability.

With both versions, the constructors' page gives additional information, and the schematics there include an enhanced Zobel network at the output for greater stability even with the most difficult load. This is provided for on the PCB, and allows the amp to remain stable under almost any conditions.

The entire circuit has been optimised for minimum current in the Class-A driver, while still providing sufficient drive to ensure full power capability up to 25kHz. The slew rate is double that required for full power at 20kHz, at 15V/us, and while it is quite easy to increase it further, this amp already outperforms a great many other amps in this respect, and faster operation is neither required nor desirable.

    Note - There are actually two caps marked C5, and two marked C6. This is what is on the PCB overlay, and naturally was not found until it was too late. Since these caps cannot be mixed up, it will not cause a problem.

In both versions of the amp, R7 and R8 are selected to provide 5mA current through the voltage amplifier stage. You will need to change the value to use a different supply voltage ...

    R7 = R8 = Vs / 10 (k)   (Where Vs is one supply voltage only)

For example, to set the correct current for ±42V supplies ...

    R7 = R8 = 42 / 10 = 4.2k (use the next lower standard value - 3.9k)

Construction
The suggested power supply is completely conventional. Although a small amount of additional power can be obtained by using an auxiliary supply (to boost the rail voltage for the MOSFET drive stage), this is at the expense of greater complexity and more things to go wrong. The transformer for the supply should be matched to the expected power you wish to obtain from the amp. The following table shows the recommended transformer voltage and VA rating for a single channel - either use two transformers or a single unit with twice the VA rating shown for stereo.

    AC Volts    DC Volts    VA    Power (8Ω)
    20-0-20    +/-28V        100        40
    25-0-25    +/-35V        100        50
    30-0-30    +/-42           160        80
    40-0-40    +/-56V         200       150    (Recommended Supply Voltage)
    50-0-50    +/-70V         300       240

Note that all powers shown are "short term" or peak - continuous power will always be less as the supply collapses under load. Peak power levels are usually achieved (or approached) with most music because its transients are generally between 6dB and 10dB greater than the average power output. Transformer VA ratings shown are a guide only - larger or smaller units may be used, with a marginal increase or reduction of peak power. Always use at least the size shown for subwoofer use! Values in bold are preferred, and will give enough power for most systems along with optimum reliability and low operating temperature.

                                              Figure 3 - Power Supply Circuit Diagram

Figure 3 shows the power supply circuit diagram for a ±56V supply, and there is nothing new about it. As I always recommend, the bridge rectifier should be a 400V/35A chassis mount type, and should be properly chassis mounted using heatsink compound.

Filter capacitors must be rated to at least the nominal supply voltage, and preferably higher. If possible, use 105°C rated caps, and join the earthed terminals very solidly to form the star earthing point.

    Note - The fuse should be selected according to the size of the power transformer. For any toroidal transformer over 300VA, a soft start circuit is highly recommended. Use the transformer manufacturer's suggested fuse - if this information is not available, ask the supplier - not me!

The DC supply must be taken from the capacitor terminals - never from the bridge rectifier. Using several small capacitors will give better performance than a single large one, and is usually cheaper as well. For example, the performance of 10 x 1,000uF capacitors is a great deal better (in all respects) than a single 10,000uF cap, at between 50% to 70% of the cost of the large unit. This lunch is not free, but it is heavily discounted