An incredible Hi-Fi Amp that's virtually bomb-proof — like the best valve amps

Power MOSFETs are a relatively new addition to the range of semiconductor devices available. Small signal MOSFETs have been around for some years, mostly finding uses in high frequency applications, but it was found difficult to make MOSFETs with gate-to-drain voltages greater than 30V (most are rated at 20V), and with high current capabilities, such as would be required in power amps. The high voltages possible with power MOSFETs are achieved by separating the gate and drain layers with a layer of ion implanted silicon dioxide. In addition, a field plate is provided on the source, near the gate. These two measures prevent electric field concentration, which at high voltages would otherwise destroy the gate. The high current capability is achieved by using a comb-shaped structure for the drain and source regions.

Figure 1 shows the connection configuration for the MOSFETs used in this amplifier. Although a T03 package is used to give excellent heat dissipation, MOSFETs are far superior to bipolar transistors in their response to high temperatures. As a bipolar transistor heats up for a given voltage, the current through it becomes greater; i.e. it has a positive temperature characteristic. If the temperature were allowed to continue to rise thermal runaway would ensue and the transistor would be destroyed. A MOSFET, however, has a negative thermal characteristic. As the transistor becomes hotter, the current tends to decrease, so power MOSFETs are most unlikely to be destroyed due to high temperatures.

Power MOSFETs also have a far wider frequency response than bipolar power transistors, so that a very wide and extremely flat frequency response can be obtained, without any complicated circuitry. Figure 3(a) shows the typical output characteristic of a power MOSFET for gate-to-source voltages (V_gs) from 1V to 10V in 1V steps. Figure 3(b) shows the remarkably low total harmonic distortion generated by this amplifier. It is scarcely measurable, even with the best test equipment available and certainly far below the minimum audible level.

Circuit Description

TR1 and TR2 form a stable, differential input buffer amplifier, the bias current for each transistor being set to 0.5mA. The 2SA872 transistor is used because it has a very low noise output but can handle high voltages. TR3 and TR4 form a 'current mirror' to give a high open-loop voltage gain. TR5 acts as a constant-current load and this low-noise, high-gain, class A amplifier stage is all that is required to drive the power MOSFETs TR6 and TR7. The transistors in the driver stage need to have a high voltage durability, high F_T and low C_ob. They also have to supply sufficient power to charge and discharge the gate-to-source capacitance of the power MOSFETs. In this case a bias current of around 50mA is sufficient to ensure adequate power is available at all frequencies and power levels.

The input impedance of the amplifier is set, by R2, to 47k, and C2 bypasses any RF signals present at the input. The amplifier has a gain of 33, and this is set by R7 and R6, via decoupling capacitor C3. R13 and R14 improve the stability at high frequencies by reducing the effective gate load capacitance. C7 and R15 are a Zobel network which, in conjunction with R16 and L1, ensures excellent stability into reactive loads at high frequencies.

Construction

Fit the five Veropins, labelled 1 to 5, to the PCB and solder. Fit and solder diode D1 taking care that it is the right way round. Fit and solder all the resistors except R16, and all the capacitors, taking care with the polarity of the electrolytic ones, C1, C3, C9 and C11 (refer to Figure 5). Scrape or burn the enamel off one end of the piece of enamelled copper wire and solder it to one lead of R16, close to the body of the resistor. Now wind the wire tightly around the resistor ten times to form L1, as shown in Figure 6. Do not cut the wire, but hold it tightly and scrape off the enamel where it will touch the other lead-out wire of the resistor, then wrap it around the lead and solder. Fit this composite component to the PCB and solder. Fit and solder the preset to the PCB, then the transistors (TR1-5).

Make the heatsink bracket shown in Figure 7. (Note that this is available ready-made, and is included in the kit supplied by Maplin Electronic Supplies Ltd.) The mounting bracket fits to the component side of the PCB as shown in the photograph. Align it with the holes in the PCB and put one bolt through the centre hole from underneath using a 6BA nut, bolt and shakeproof washer. Referring to Figure 8, place a nylon bush in each of the four large holes in the bracket, smear both faces of both mica washers with Thermpath silicone grease and place these in position. Mount the two power MOSFETs, ensuring that TR6 (2SK133) is fitted closest to the coil L1. Put in the 6BA bolts to hold the transistors from underneath and secure them using nuts and shakeproof washers. Solder the bolt heads to the track on the PCB. Finally solder the drain and gate pins to the PCB and re-check all component positions, polarisations and solder joints.

Power Supply

The PSU (T1, BR1, C12 and 13 and FS1 and 2) will produce approximately 44-0-44V DC. For a stereo pair use a 4A transformer at 32-0-32V instead of a 2A type. Alternatively, toroidal transformers rated at 35-0-35V could be used, 160VA for a single amp and 300VA for a stereo pair. If the transformer voltage is increased to 40-0-40V and TR6 and 7 replaced by a 2SK134 and 2SJ48 [Errata: Should read '2SJ49'] respectively, output powers in excess of 75W RMS into 8 ohms are possible.

Figure 9 shows how simple it is to parallel the output transistors to achieve even higher powers. Using the higher voltage and transistor types just mentioned power levels in excess of 125W RMS into 4 ohms are possible with a 1V RMS input signal if this circuit is used.

Setting Up

With no speaker connected and fuses not inserted, check that the voltage across C12 is approximately 45V (± 5V) and that the voltage across C13 is the same. Switch off and short C12 and C13 in turn with a resistor (e.g. one of the test resistors). Now connect FS1 and FS2, via 100R 5W resistors, to pins 2 and 5 respectively. Connect 0V to pin 4. Check with a multimeter set to the highest resistance range, that there is no connection between the MOSFET cases and the mounting bracket. Turn RV1 fully clockwise.

Insert 250mA fuses for test purposes as FS1 and FS2 and switch on again. If either fuse blows or any component gets excessively hot switch off immediately. If all is well, connect a DC voltmeter between pin 1 and pin 4. The meter should read about 0V (not more than ±100mV). Switch off and remove the two 100R resistors. Connect FS2 directly to pin 5 and connect a multimeter switched to about 100mA DC between FS1 and pin 2 (+ve lead to fuse and -ve lead to pin 2). Switch on again and rotate RV1 slowly until the meter reads 50mA. Leave for 10 minutes and re-adjust.

Switch off, disconnect the meter and connect FS1 direct to pin 2. The mounting bracket must now be bolted to a good-sized heatsink or a substantial chassis. Finally, connect a loudspeaker to pin 1. Note that the speaker negative terminal must be returned to the 0V in the power supply and not to pin 4 of the amplifier. We recommend making the negative tag of C12 the only 0V point with more than one wire attached. Replace the 250mA fuses with 2A types and connect an input between pins 3 and 4. Note that if you use a toroidal transformer you will have to use antisurge fuses and the test fuses used should be 500mA rating.