Loudspeaker Box Amplifier schematic

Posted on Sep 24, 2012

This original design is a variation on a well-known design, examples of which can be found in a great many texts. My variation was to add a second differential stage to replace the usual common emitter-plus-constant current source. Doing so opens up a second inverse feedback path. The signal path being a series connection of the differential amp inverting inputs, and the feedback being the non-inverting inputs. To this end, the constant current source does not include bypass capacitors to render it a DC-only stage.

Loudspeaker Box Amplifier schematic
Click here to download the full size of the above Circuit.

The other feedback path is the usual one: output to the non-inverting input of the first differential stage. The idea is to add lots of inverse feedback in order to linearize the output transistors, as the non-linearity of these devices makes them rather poor analog amplifiers. The other design criterion was the use of components readily available at a hobbist's store: "Radio Shack". No "exotic" components that would require a special order are used here. To this end, the output stage is a semi-complementary design, although this is not optimum so far as fidelity is concerned. (Radio Shack doesn't offer complimentary pairs of power BJTs.) Since the amp is intended to work into the "wide band" (60Hz -- 18KHz) speakers that this store sells, the gain is deliberately kept down. With the design given here, the max output is 6.0W. Even here, since these speakers are rated at 3.0W, you could blow speakers if the volume is turned up too high. Limiting output is accomplished with the initial voltage amp (Q1). This stage has a gain of 7.5, so that with a 1.0VRMS input, the output voltage becomes slightly less than 7.5VRMS. The gain of the circuit from the output end of the volume control to the speaker is unity, due to the large negative feedback. You could drive the power output as high as 10W, as there is plenty of "head room" in the power supply design. (Again, this was done to improve the fidelity as much as possible with a...

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