Simple Amplifier With LM1875 And Up To Six Transistors

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For testing and experimenting purposes we in need of power AC or DC amplifier with low cost and easily reparable.

In these cases the THD and the IMD of the amplifier are not very important.

It is important to have simple solution of power amplifier which can be adapted to a particular application with external passive filters at the input and the output.

Also it is interesting to use the same amplifier with different configurations of elements (kind of modularity or configurability) as output transistors, filtering capacitors, type of the heat sinks, etc. to adapt the implementation to different field of application.

About LM1875

LM1875 is low cost and very popular and useful integrated circuit (IC).
It is designed to be stable with gain higher than ten.

Its main application is stand alone audio power amplifier but with two or more additional transistors we can obtain configurations adaptable to more applications.

If the load capacitance is large, e.g. above 1uF it is recommend connecting with series with output of the IC resistor above 1Ohm with appropriate power dissipation.

Otherwise the IC may begin to osculate.

Another method of driving high capacitive loads is to connect resistor around 10Ohm in parallel with high current inductor of 3-10uH between the output of the IC and the highly capacitive load.

LM1875 is limiting the output current to around 4A but from practical considerations as power dissipation it is good to use maximal output current up to 2A.

LM1875 has maximal power supply of +-30V

Usually there is no need to add protection diodes to the output of the IC.

Nevertheless two additional diodes (D1 and D2) as shown of Figure 1 can be useful for addional safety precautions.

When the temperature of the IC exceeds +170C the IC shuts down.

It is operational again when the die temperature drop to around 145C.
The thermal resistance junction – case is 3C/W.

The gain product bandwidth of the IC is typically 5.5MHz.

The typical slew rate of LM1875 is 8V/us.

The maximal output power of the IC is typically 25W over load of 8Ohm at power supply of +-25V.

Description of the circuit

Figure 1 is showing the circuit of the simple modular power amplifier.

It can produce more than 120W of output power.

The amplifier it built around LM1875 and has several configurations over the same printed circuit board (pcb).

The modularity of the solution can provide at least four possible configurations.
C3 and C4 are obligatory in all configurations.

1. In the simplest case the IC will work alone and only C5 and C6 from the large filtering capacitors.

In that case the output power is determined only from the IC, the size of the heat sink and the power supply and can go up to around 25-30W.

The load resistance is 4Ohm or above.

2. In the second case the IC will work with couple of output transistors, e.g. Q1 and Q4.
The filtering capacitors should be at least C5 C6, C7 and C8, each of them for 2200-4700uF/35V.

In that case the output power can go to more than 60W and load resistance can be 3.2Ohm and some times below.

3. In the third case the IC will work with four output transistors, e.g. Q1, Q2, Q4 and Q5.
In that case it is preferably to use at least six from the the filtering capacitors, e.g. C5 to C10.

The output power can go to more than 100W and load resistance can be as low as 1.6Ohm (preferably no less than 2Ohm).

4. In the forth case the IC will work with all output transistors Q1 to Q6.

In that case it is preferably to use all large filtering capacitors C8 – C14.

The output power can go to more than 120W and load resistance can be as low as 1.6Ohm.

In all possible configurations the power supply is limited by the power supply rating of the IC.
As mentioned above the limits are +-30V and preferably up to around +-28V.

Also in all cases the IC and the transistors should be mounted on the same heat sink and the thermal protection of the IC will protect to some degree the transistors.

All transistors do not work with initial (quiescent) current and are blocked at very low level of the output signal and that is reducing the dissipating heat.

The short circuit protection of the amplifier is not very important when the amplifier is in the box with the loudspeakers.

That is why the usage of two fusible F1 and F2 can be considered enough for that amplifier.
The size of the fusible F1 and F2 in the power supply depends on the maximal current required from the amplifier and from the total size of all filtering capacitors on the board.

F1 and F2 can be for 4 to15A and sometimes more.

The gain of the amplifier is equal to
G = 1 + R3/R4
and should have the minimal possible value.

That will keep the THD and IND to low level.

The loads are connected to the connectors CON2 to CON5.

The resistor R21 should be below the load resistance and with appropriate power dissipation.

R5 to R10 are obligatory and should have the minimal possible values (usually 10Ohm/2W)
R11 to R16 are obligatory and also should have the minimal possible values (usually 0.15Ohm/3W)


  1. Polarity of the capacitor C6,C8,C10,C12,C14 are wrong. In this circuit positive terminal of the capacitors are connected to the ground (-ve) pin.


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