A discrete but current solution is the one that makes use of power field effect transistors (Power MOSFETs). According to proponents of this technology, power MOSFETs have a feature that closely matches the old valves used in ultra-linear audio amplifiers.
Such amplifiers, according to these adepts, have a purity of sound that is not achieved by equipment that makes use of bipolar transistors, integrated circuits or even using digital technologies. According to these adepts, this type of amplifier has a much purer sound quality, with an extremely low distortion rate.
For readers who wish to have sound quality, no matter the technology, we give an amplifier circuit that has an interesting solution: a power MOSFET and a Power Darlington, given the difficulty of obtaining the P-channel MOSFETs.
We caution, however, that the pair of transistors used may not be very easy to obtain, which must be taken into account by the assemblers before beginning any type of practical work. This circuit consists of a common drain voltage follower and a current source consisting of the Darlington NPN.
With a current set at 2.2 A the circuit can provide a power of 17 W on an 8-ohm speaker. However, the circuit performance is low, as it needs something around 80 W from the source to provide this power.
In figure 1 we have the diagram of the circuit that must be fed by 40 V source with at least 3 A of capacity.
As the transistors dissipate good amount of heat due to the low efficiency of the circuit, they must be mounted on excellent heat radiators. A suggested circuit board is shown in figure 2.
Note that the capacitor C5 must be of the depolarized type and that the zener diodes are 1 W. The voltages at the main points of the circuit are shown in the diagram.
MATERIAL LIST
Semiconductors:
Q1 - IRF350 or equivalent - POWER MOSFET
Q2 - TIP141 - Darlington Power NPN Transistor
Z1 - 18 V x 1W - zener diode
Z2 - 3V0 or 3V3 - 1 W zener diode
Resistors:
R1 - 270 ohm x ½ W
R2 - 8.2 k ohm x 1/8 W
R3 - 12 k ohm x 1/8 W
R4 - 100 k ohm x 1/8 W
R5 - 2.2 k ohm x 2 W
R6 - 1 ohm x 5 W - wire
R7 - 10 k ohm x 1/8 W
Capacitors:
C1 - 5.6 uF x 100 V - polyester
C2 - 47 uF x 35 V - electrolytic
C3 - 4 700 uF x 50 V - electrolytic
C4 - 47 uF x 50 V - electrolytic
C5 - 10 uF x 100 V - polyester
Several:
FT - 8 ohm x 40 W - loudspeaker
Printed circuit board, 40 V x 3 A power supply, wires, solder, etc.
