Need a simple power amplifier to drive a loudspeaker that doesn't require any messy set-ups to get going? Then this is the amplifier for you! This minimalists design uses just seven transistors to get the job done. A long-tail-pair, VAS, drivers and output transistors. The output is configured as a Darlington emitter-follower push-pull stage. I would not suggest that the +/-35VDC rails specified be exceeded. It will probably run at +/-42VDC, but I cannot guarantee the longevity of the amplifier.

Build two of these modules for a 100W stereo amplifier. It is also possible for two of these to be in BTL (Bridge-Tied-Load), although, this is untested. I have not tested the stability of this amplifier into a 4 ohm load.

Specifications:

Power Output: 112W peak into 8 ohms; 56W RMS into 8 ohms

Frequency Response: 20Hz - 20kHz

Input Sensitivity: 2.41Vp-p (864mV RMS) for full output

DC Offset (@+/-35VDC): ~+/-50mV

Total Harmonic Distortion (THD): Not measured (should be less than 0.1% into 8 ohms @ 50W RMS)

Stability: (@8 ohms) Unconditional

Note 1: To reduce DC offset, change R4 from 1.5k, to 820ohm 1/2W

Note 2: There is a possibility of thermal run-away, so it would be a good idea to mount D1 and D2 from the underside of the PCB bent to come in to contact with the heatsink with a drop of thermal transfer paste, to keep the output transistors Vbe "tracked". As the amp heats up, the diodes will conduct more, lowering the bias current to the output stage, reducing the thermal run-away effect.

Link to my YouTube video about this amplifier build: