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LM1875 Stereo Power Amplifier


I wanted to build a power amplifier to go between my PC and a pair of stand mounted loudspeakers. I like the minimalist approach to things so with this in mind I set about how I would construct it


                   


I decided to use a picture frame as the chassis because I thought this would make it look a bit different! The circuit design is based on the National Semiconductor LM1875 power amplifier IC. Two of these can deliver about 20 watts per channel which suits me fine. I chose not to use a split power supply and went for a single rail instead, with old fashioned output capacitors. HiFi purists might throw their hands up in disgust at this, but I didn't want all the extra protection circuitry that would be required to stop DC from reaching the speakers in the event of a chip failure, to get in the way of simplicity. Fuses it was gonna be!

The circuit is pretty much based on the chip manufacturers data sheet, but I've beefed up the power supply with 20,000uF of smoothing. Resistors are 1% tolerance metal oxide and internal wiring uses reasonable quality speaker cable (Chord Campana) rather than bell wire. The feedback resistor Rf in conjunction with resistor Ra sets the overall gain to 23 (Rf/Ra+1) making the input sensitive enough to be driven directly from the source (or via a passive volume control)

The sound quality achieved is really quite impressive for such a simple design, with virtually no hiss or hum from the speakers. The circuit diagram is shown below




Amplifier and PSU schematic (only one amplifier channel shown)




The amps were built on veroboard. I was concerned that they'd be unstable due to component positioning, but the boards are small and the amps behave very well


                   

How it was made...

                           

The whole thing was constructed on an 8”x10” picture frame (from B&Q) and the chassis is actually the MDF back sheet from the picture frame. This needs to be braced both for strength and for mounting the mains transformer. The top can be covered or painted. I chose light wood effect Fablon

                             

Top and bottom sides of the chassis showing the main components mounted ready for wiring. IC cooling is courtesy of a couple of discarded AMD processor heat sinks

                             

The power supply components are housed inside a cake tin with a removable base (John Lewis) which is held to the chassis by four blocks of wood glued to the inner side with bolts protruding downwards. The removable base was replaced by a steel mesh grille (also from B&Q) glued in with araldite

                             

The power supply board is home to the bridge rectifier and fuse holders and was made by cutting away separate areas of copper clad board with a knife. This board forms the central junction point for pretty much everything, including the 'star' earthing. To stop it from tarnishing (as copper does) it was given a coat of clear varnish

                     

Front and rear views with the cover removed
  

                    

The completed wiring. Underneath was finished off with domed rubber feet and some mesh grille left over from the cake tin!




The finished power amplifier