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Description

The very first thing you will notice is that I have broken with tradition with this amp, and there are no component values shown. Given the performance of the circuit, and the fact that I have already sold a couple as completed, finished amplifiers, I am not about to give away all my secrets for the design. If you want the component values, you must purchase the PCB. There are no exceptions, so don't ask.

The schematic of the amp is shown in Fig. 1, and it is about as simple as a high power MOSFET amplifier can get - it is considerably simpler than most, but lacks nothing in performance. The circuit diagram belies the ability of the amplifier though, so do not be tempted to think that it cannot perform as well as more complex designs - it does, and exceeds the performance of many (if not most) of them. It will be seen that I elected to use a bootstrap current source rather than an active version - there is negligible cost difference, but I was unwilling to make such a radical change after testing the prototype and being so impressed with the results. (If it ain't broke, don't fix it!)

The front end is a conventional long-tailed pair (LTP) using a current mirror load and an active current sink in the "tail". Interestingly, adding the current mirror made no difference to distortion, but reduced the DC offset to less than 25mV. The improvement was such that I elected to retain the mirror.

In tests thus far (both measurement and listening), I have been unable to detect even a hint of what is commonly referred to as the "MOSFET sound". The relatively high levels of low order distortion and suceptibility to crossover (or "notch" distortion that plague most MOSFET designs are completely missing - indeed, even with zero bias on the MOSFETs, crossover distortion below 10kHz is barely measurable, let alone audible!

Note Carefully:
The most critical aspect of the design is the PCB layout, and it is very doubtful that if you make your own board, that you will get performance even approaching mine. Power output is essentially unchanged, but distortion and stability are achieved by a compact and carefully designed layout for the front end and driver circuits, which minimises any adverse PCB track coupling that causes much higher distortion levels, and may cause oscillation.

This is not a ploy on my part to get people to purchase my PCBs - that has already been taken care of by leaving out the component values. The simple fact is that unless the PCB layout is done with the utmost care, any amplifier can be made to have far greater distortion levels and reduced stability margins than the published figures suggest.

Low Power Version
As shown in the schematics below (figures 1 and 2), the amplifier can be made in high or low power version, and although there is a bit of vacant PCB real estate in the low power design, it is significantly cheaper to make and will be more than sufficient for most constructors. If this version is built (using only 1 pair of MOSFETs), it is essential to limit the supply voltage to +/-56V so that it can drive both 4 and 8 ohm loads without excess dissipation. With this voltage, expect about 100W continuous into 8 ohms, and around 150W into 4 ohms. Naturally, dual MOSFET pairs may be used at this voltage as well, providing much better thermal performance (and therefore cooler operation), far greater peak current capability and slightly higher power. This version may be used at any voltage from +/-25V to +/-56V.

Fig 1
Figure 1 - Standard (Low Power) Version

The MOSFETs used are Hitachi lateral devices, 2SK1058 (N-Channel) and 2SJ162 (P-Channel). These are designed specifically for audio, and are far more linear than the (currently) more common switching devices that many MOSFET amps use. Unfortunately, they are not especially cheap, but their performance in an audio circuit is so much better than vertical MOSFETs, HEXFETs, etc., that there is no comparison. Note that using HEXFETs or any other vertical MOSFET type is not an option. They will fail in this circuit, as it was not designed to use them.

An alternative (and possibly marginally better than the 2S series) is the Exicon ECX10N16 and ECX10P16 (available from Profusion PLC in the UK). These have been used in most of the amps I have built, and they work very well. So potential constructors can verify that the semiconductors are available before purchasing a PCB, this information has now been included. All other parts are quite standard.

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.

Fig 2
Figure 2 - High Power Version

Although not shown, the transistors and MOSFETs are the same in this version as for the low power variant.

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)
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