A linear audio amplifier module that can deliver up to 20W RMS at 0.1% THD into four and eight ohm loads depending on supply voltage. The design of the module is based on an LM1875 integrated power op-amp instead of discreet parts. This module requires a suitable split-rail DC power supply to operate.
The LM1875 is a single power operational amplifier. It is used in the non-inverting configuration in this circuit together with additional supporting components that may be necessary depending on application. These additional components can be omitted during construction if they are found to be superfluous in a particular application. The table below describes the function of each component. Those marked with an asterisk are not strictly required for the circuit to operate as an amplifier.
Table 1: Component function
||Static drain. This is to protect against large voltages from developing across C1 and C2 due to electrostatic discharge. This can happen when the input connections are exposed or touchable.
||This resistor sets the input impedance. This forms a highpass filter with C1 and C2 with a cutoff frequency of 1/(2*pi*R2*(C1+C2)).
||These resistors set the voltage gain of the amplifier. The gain is given by 1 + R4/R3.
||This network reduces any tendency for the amplifier to oscillate due to positive feedback loops such as those from input-output coupling from long cable runs, etc.
||These protect the amplifier from becoming unstable when driving capacitive loads such as long loudspeaker cables and piezoelectric tweeters. R6 is located inside L1. L1 consists of twenty turns of 0.7mm diameter enamelled copper wire wound on a 6mm former.
||Input coupling capacitors. These provide AC coupling.
||These ensure that the amplifier has unity gain at DC. Plastic film capacitor C3 is to provide a high frequency bypass to non-polarised electrolytic C4 as some electrolytic capacitors can have a diminished high frequency performance.
||This provides high frequency noise suppression.
||C7,C8 and C9,C10 provide low and high frequency local supply decoupling respectively. Note that C7 and C8 are only to provide local decoupling and will not suffice as power supply smoothing capacitors.
The PCB is 59mm long and 33mm wide and uses a single-sided layout. The LM1875 located at the rear edge with the tab overhanging the edge of the board to allow fitting to a separate heatsink. The grounding is implemented so that the power and signal ground paths are kept separate. This is important as voltages developed across the power ground trace resistance will be superimposed onto the input signal without separation. The power and signal ground lines return to pin 2 of K2 from the right and left sides of K2 respectively. An M3 mounting point is provided in each corner of the board. Sufficient clearance provided around the locations of the mounting points to isolate the ground from the fixing screws. This is required to avoid ground loops when fitting the module onto a conductive chassis.
Power and cooling:
The LM1875 will dissipate up to 35W of heat in normal operation (and substantially more if exessive suppply voltages are used) and the cooling system must be sufficient to dissipate this heat. The LM1875 datasheet should be consulted for detailed information on power dissipation and cooling requirements. It should be noted that the metal tab of the LM1875 is internally connected to the negative supply rail (pin three). A power supply that can deliver at least 40W continuous per module should be used for driving 8 ohm loads, and 50W per module for four ohm loads. This assumes a supply voltage of ±25V.