Audio Amplifier MOSFET 200 W
Here is propose a project of
an AB class power amplifier, at its simplest, assembled with common compoments
(not very expensive), based on traditional diagrams : a symmetrical
differential input stage, a cascode stage driver and a MOSFET output stage.
The printed circuit board is
very compact , and is composed of two subsets : the command stage and the
output stage. All resistors are 1/4 watt 1 % metal film (except if otherwise
stated).
Constant current sources of
+/- 1 mA are carried out around T5 and T6. The diodes D1 to D6 allow the use of
low noise transistors type BC550C and BC560C, the transistors T7 to T10 form
the stage driver. The potentiometer P1 allows the adjustment of the quiescient
current to 100 mA per output transistor.
The symmetrical power supply
is entrusted to a large transformer of 625 VA, 2 * 51 volts + bridge rectifier
and 6 reservoir capacitors of 4700 µF, which gives an output voltage of + and -
70 volts per rail.
The output power is 200 Wrms
under 8 ohms or 350 Wrms under 4 ohms. Distortion is lower than 0,02 %, damping
factor is better than 300, signal-noise ratio is 112 db (balanced A at full
power), the input sensitivity is 1,2 volts (200 W under 8 ohms).
Diagram :
The capacitor C3 is not
reproduced on the diagram and on the PCB (it is correct).
Proposal of the command stage PCB :
Transistors T9 and T10 must
be assembled on a common heatsink with 5° c/w thermal resistance or on the
output stage heatsink.
proposal of the output stage PCB :
The points marked A, B and C
have to be connected between the two PCB.
The ground (earth) points of
PCB's and loudspeaker ground (earth) must compulsorily be connected in star
with the 0 Volts of the PSU.
The power transistors must
be assembled on a heatsink with a thermal resistance less than 1 ° C/W. They
must also be electrically insulated from the heatsink by using a mica insulator
+ heat-conducting compound or a silicone insulator.
It is possible to use the
MOSFET output stage PCB which is also reproduced on the site, but take care
with the thermal dissipation of the output transistors (this circuit is perfect
for power of 100 Wrms under 8 ohms or 155 Wrms under 4 ohms). It is possible to
use two PCBs in parallel to double the number of output transistors, do not
place the limitation diodes, the fuses and the RC output circuit on the second
PCB.
Bandwidth :
Bandwidth is limited from 5
Hz to 53 Khz (at - 3 db) by the input module (C1, R1, C2, R2). It can be
modified according to the application desired. It is useless to increase the
bandwidth to the botton (bass) because little useful signal is present at the
bottom of the spectrum. In public address sound systems the bandwidth is often
limited between 20 and 35 Hz. The loudspeakers are thus protected against too
high extreme-bass signals and some more power is available for the remainder of
the spectrum.
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Dear Sir, In order to have a wide bandwidth (20kHz - 500kHz) what should be the design change that I've to make ?
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