HEXFET AMPLIFIER UPGRADE
The Medium Power HEXFET amplifierpublished in this magazine in December1993 has one small drawback: it delivers‘only’ 60 W into 8 � (or 120 W into4 ). Otherwise, it is a first class amplifierthat provides excellent music reproduction,which is evidenced not so much by measurementas by audition. To some listeners,it has a quality not unlike that of a valveamplifier. Because of its popularity and themany requests for a version with higheroutput power, it has been upgraded to providearound 90 W into 8 � (about 160 Winto 4 ).By a stroke of good fortune, a pair ofIGBTs (Insulated Gate Bipolar Transistors –see our June 1995 issue) proved ideal replacementsfor the HEXFETs used in theoriginal design. Apart from the figures forpower output, the technical specification remainsvirtually the same (see box).
ModificationThe original design already allowed for ahigher-output version, whence the duplicatedholes for the output transistors on theprinted-circuit board. At that time, advanceinformation on the IGBTs was already available,but samples were not.Although IGBTs are quite different fromHEXFETs, the board for the original designcan be used without any modification. Infact, the circuit has hardly changed. Themost noticeable alteration is the replacementof the fuses in the source lines of thepower FETs by emitter resistors for theIGBTs. The only other changes are in thevalue of two resistors in the compensatingcircuit of the input stage, of one in the quiescent-current circuit, and of one resistorand two capacitors in the protection circuit.This means that anyone who has built theoriginal HEXFET amplifier can quicklymodify it to the upgraded version.One item needs to be replaced, however:the mains transformer. After all, morepower can not be obtained from the samesupply voltage/current. The original transformerwith 225 V secondaries must be replacedby one that provides 230 V at3.75 A. This will result in a direct voltage of±43 V.