Stereo Amplifier

The amplifier is set up for a voltage gain of about 20, with a high pass filter to reduce the gain near DC. Because of this, DC offsets are not amplified so no audio coupling capacitors are used, which helps to keep the sound quality optimum and save large, costly non-polar caps on the speaker outputs.

All 1% metal film resistors are used for lower noise. The input attenuator is a rotary dual 10K log pot. The input switcher is a rotary 2P4T switch. The power supply uses a 50VA transformer at around 16-18V per side, for around +/-22-25V for the rails. A big 8A bridge rectifier and some 4700uF caps are used. Oh, and we chose a heatsink too. (although I think it should have been a bit larger) The LM1876 is available with an isolated tab, so you don't have to use a mylar washer and nylon bushings to connect it to the heatsink.

The headphones derive power from the main amp outputs, reduced in volume by some resistors. Although a dedicated headphone amplifier would probably be better, the sound quality seems ok. The resistors were chosen to play a 32 ohm headphone at about the same listening volume as the speakers at the same volume setting. Instead of the ever-so-annoying "plug in the headphones and the speaker will cut out" which means that you have to find somewhere to hang the headphone cable when you're not using them, we implemented a speaker cut switch. If you leave headphones plugged in they play all the time. If you don't want the speakers on, just flip the switch to use headphone only.