Here is propose a project ofan AB class power amplifier, at its simplest, assembled with common compoments(not very expensive), based on traditional diagrams : a symmetricaldifferential input stage, a cascode stage driver and a MOSFET output stage.
The printed circuit board isvery compact , and is composed of two subsets : the command stage and theoutput stage. All resistors are 1/4 watt 1 % metal film (except if otherwisestated).
Constant current sources of+/- 1 mA are carried out around T5 and T6. The diodes D1 to D6 allow the use oflow noise transistors type BC550C and BC560C, the transistors T7 to T10 formthe stage driver. The potentiometer P1 allows the adjustment of the quiescientcurrent to 100 mA per output transistor.
The symmetrical power supplyis entrusted to a large transformer of 625 VA, 2 * 51 volts + bridge rectifierand 6 reservoir capacitors of 4700 µF, which gives an output voltage of + and -70 volts per rail.
The output power is 200 Wrmsunder 8 ohms or 350 Wrms under 4 ohms. Distortion is lower than 0,02 %, dampingfactor is better than 300, signal-noise ratio is 112 db (balanced A at fullpower), the input sensitivity is 1,2 volts (200 W under 8 ohms).
Diagram :
The capacitor C3 is notreproduced on the diagram and on the PCB (it is correct).
Proposal of the command stage PCB :
Transistors T9 and T10 mustbe assembled on a common heatsink with 5° c/w thermal resistance or on theoutput stage heatsink.
proposal of the output stage PCB :
The points marked A, B and Chave to be connected between the two PCB.
The ground (earth) points ofPCB's and loudspeaker ground (earth) must compulsorily be connected in starwith the 0 Volts of the PSU.
The power transistors mustbe assembled on a heatsink with a thermal resistance less than 1 ° C/W. Theymust also be electrically insulated from the heatsink by using a mica insulator+ heat-conducting compound or a silicone insulator.
It is possible to use theMOSFET output stage PCB which is also reproduced on the site, but take carewith the thermal dissipation of the output transistors (this circuit is perfectfor power of 100 Wrms under 8 ohms or 155 Wrms under 4 ohms). It is possible touse two PCBs in parallel to double the number of output transistors, do notplace the limitation diodes, the fuses and the RC output circuit on the secondPCB.
Bandwidth :
Bandwidth is limited from 5Hz to 53 Khz (at - 3 db) by the input module (C1, R1, C2, R2). It can bemodified according to the application desired. It is useless to increase thebandwidth to the botton (bass) because little useful signal is present at thebottom of the spectrum. In public address sound systems the bandwidth is oftenlimited between 20 and 35 Hz. The loudspeakers are thus protected against toohigh extreme-bass signals and some more power is available for the remainder ofthe spectrum.
