Promitheus Hexfet Power Amplifier

Here you can find the whole article of the Elektor magazine of the new upgraded Hexfet amp. The new amplifier uses bigger output devices, IGBTs in T0-247 Package. It has more power and uses new technology transistors. These transistors are insulated gate bipolar transistors (IGBT). They have an input like a FET so the work with voltage and they handle current like a bipolar.
Look here for The IGBT or HexFet amp.

I used the same board for my own upgrade. I used the higher power supply voltage of the second edition for IGBTs but since I didn't like to use IGBTs (I am a great fan of MOSFETs) I went on and used bigger Mosfets instead in TO-247 P case. I used the IRFP250 and IRFP9250 from Intersil/Harris. They are very cheap and reliable. The IR are the same too. IGBTs are a lot more expensive. This amplifier is great with very good sound and a lot of power for all kinds of speakers.

Modification

The original design already allowed for a higher-output version, whence the duplicated holes for the output transistors on the printed-circuit board. At that time, advance information on the IGBTs was already available, but samples were not.

Although IGBTs are quite different from HEXFETs, the board for the original design can be used without any modification. In fact, the circuit has hardly changed. The most noticeable alteration is the replacement of the fuses in the source lines of the power FETs by emitter resistors for the IGBTs. The only other changes are in the value of two resistors in the compensating circuit of the input stage, of one in the quiescent-current circuit, and of one resistor and two capacitors in the protection circuit.

This means that anyone who has built the original HEXFET amplifier can quickly modify it to the upgraded version. One item needs to be replaced, however:
the mains transformer. After all, more power can not be obtained from the same supply voltage/current. The original transformer with 2×25 V secondaries must be replaced by one that provides 2×30 V at 3.75 A. This will result in a direct voltage of ±43 V.