Easy 1,5 - 2 Watt audio amplifier with a 741 Opamp & BD139/BD140 (schematic and demo)

Easy to make audio amplifier, made with a 741 Opamp and 2 complementary “end” transistors, the BD 139 (NPN) and the BD 140 (PNP). Important: supply voltage max. 12-13 V, otherwise the quiescent current gets out range. Earlier video (741 Opamp pre amp) is here

   • Biasing (small signal) Opamps and a good a...  

Read the comments section for good insight info.

Only works properly between 12 V and 13 V.

The advantage of using the Opamp proved to be: less components. BUT, important: there is “no” application to limit and protect for thermal runaway in this circuit.

So glue the 2 diodes to the heatsink, one 1N4005 to the heatsink of the BD 139 and the other 1 N 4005 to the heatsink of the BD 140. Make no shortcuts when you glue them, only the plastic part of the diodes may have contact with the (aluminum) heatsink. They can limit the maximum current that the circuit takes under a heavy load between 12 V and 15 V.

I tested the amplifier with a 3,3 Ohm/5 Watt power resistor instead of the loudspeaker for 1 hour at 200 Hertz at 14 V. The current went up to 600 mA at 14 Volt, the transistor heatsinks got hot, but but no audio distortion was visible on the oscilloscope.

So for normal use in an audio circuit (living room, 20 degrees celsius) this is enough stability. For maximum stability: use heatsinks and glue the diodes to the heatsink. In that situation it can work up to 50 degrees Celsius. Or glue the diodes to the plastic housing of respectively the BD 139 and the BD 140, when you don't use a aluminum heatsink and want to use it for small audio applications (say max 1 Watt).

Correction to the video: I say in the beginning that the input resistor is 100 K, but of course it is 100 Ohm…(!)

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Be aware of that, I saw on the internet my circuits reproduced in a poor or even not proper way. I can not help that, sorry. Upload 10 march 2019.