Simple three way audio amplifier with LM380/LM384 and  power supply from 19V  adapter for note book  or laptop
Petre  Petrov,   Sofia, Bulgaria
 
Introduction
We are in need of simple  three way audio power amplifier for experiments.
Preferably, the audio amplifier have to work with 19V AC/DC adapter because these adapter are widely used for laptops and notebooks.
These adapters  are  widely available at low price, they can be acquired and replaced rapidly, they have long life, low weight and other advantages as certifications according many safety standards.
Here we  see very simple three way audio amplifier with 19V power supply.
 
Description of the circuit
Figure 1 presents the circuit diagram of the simple  audio amplifier with LM380 with three independent channels  and  power supply from 19V  adapter for note  book  or laptop,
The device is build around three popular and low cost  integrated circuit (IC) LM380/LM384.
LM380 and LM384 are produced in the same DIP package with 14 pin and can be used with the  same heat sinks.
The heat sinks are obligatory if we use the load  resistance of 4 Ohm or  power supply near to the allowed maximum.
If we use LM380 and LM384 near to ta maximum power supply, e,g,20V and if we have quiescent current of  25mA the package will be hot even at low levels of the output signals.
 
The maximum output current of LM380 is  around 1,3A and the power supply range is from 10 to 22V.
The maximum output current of LM384 is  around 1,3A and the power supply range is from 12 to 26W.
The minimal speaker impedance is defined to be 4 Ohm but 8 Ohm is preferred.
The output power of  LM380 (with Vs = 18 V, Rload = 8 Ω and THD = 10%) is minimum 2.5W .
The output power of  LM384 (with Vs = 22 V, Rload = 8 Ω and THD = 10%) is typically 5,5W .
Here we will use 19V power supply and we will have around 3W over 8 Ohm and that is enough for many home applications.
 
The  voltage gain of LM380 and LM384  is internally set to 50 and that is enough for our applications.
Both ICs have input impedance of 150 kOhm and that is good enough to use them with passive RC filters.
The volume of the three channels are controlled with separated  pots named P1, P2 and P3.
 
Each of the channels can be used with or without passive RC filter.
The switches S1 to S8  give several  configurations of the usage of the circuit.
S1 and S2 permit to use the devices with one common input of with three different inputs.
 
The couples S3+S4, S5+S6 and S7+S8 permits to switch On and Off  the input RC filters.
The switches S3 and S4 connect or disconnect the low pass filter (F3dB = 300Hz) to the input of IC1.
The switches S5 and S6 connect or disconnect the high pass filter (F3dB = 3.4kHz) to the input of IC2.
The switches S7 and S8 connect or disconnect the  band pass  filter (from 300 Hz to 3,4 Hz) to the input of IC3.
The movement of the cursor of the potentiometers  does not change significantly the characteristics of the RC filters.
The preferred value of the potentiometers P1, P2 and P3 is 1kOhm, but the circuit works good with 2,2kOhm.
 
The issue is that the 19V adapters for  note  books  or lap tops can be too noisy for the audio implications.
Consequently, we must use additional filters in order to reduce the noise and the ripples from the adapter.
Here we use LC filter which suppress effectively the ripples and the nose from the AC/DC adapter.
 
The output capacitors C7, C14 and C22 have to be selected according to the load resistances and the required pass band of each of the channels.
 
Conclusions
The circuit does not need any adjustment to work properly.
In case of need we can change the parameters of the RC filters according to the needs.
We have to use enough copper area around the ICs in order to provide proper cooling of the ICs.
 
List with figures
Figure 1. Circuit diagram of the  simple three channel audio amplifier with LM380/LM384 with  power supply from 19V adapter for notebook or laptop