Stereo Amplifier

Introduction

My friend Chris finished school and thought that a nice way to celebrate would be to get a decent stereo. He had a limited budget, and a very good idea of what he wanted: good bookshelf speakers and a high-end no-nonsense amplifier. The speakers ended up being PSB Image 2Bs. I have two pairs of these and they’re great. He got the speakers on a closeout deal for much less than the regular price. But the amplifier he wanted was $800, and his stereo budget was already dwindling, so I suggested that we build something.

If your idea of an amplifier is a lot of lights and buttons and surround sound effects, you might as well go and buy the latest home theatre amplifier. But if your goal is to just make little wiggles from your iPod or computer into bigger wiggles to drive your speakers, then a simple homebuilt amplifier might be the answer. With modern power amp ICs doing most of the difficult stuff, it’s quite easy to make a very high quality amplifier for low cost that rivals even some of the “high-end” audio brands. Audiophiles (and I’m not one, thank god) might argue up and down about why an IC amplifier is poor, but I’ll take precision-manufactured chips over esoteric overpriced discrete transistor or tube designs any day. I like that clean, crisp digital sound, and most people my age do also.

When I was a kid my neighbours threw out an old stereo system from the 70s. I scavenged the amp and speakers from the curbside and got them working. The speakers kind of sucked, but I ended up using the amplifier for many years. The thing I noticed right away was that the amplifier was pretty noisy and had a lot of junk like tone controls and rumble filters and things that I didn’t really need. So one of the first things I did was to rewire it and take out all the preamp parts. I saved the volume pots and just set them up as input attenuators. And low and behold the thing actually sounded pretty good! That’s when I figured out that maybe all this preamp stuff, EQs and other “sound enhancing” features aren’t all that great after all. Simplicity really is better in most cases.

The Design

Chris and I discussed the requirements. He wanted to drive two speakers and some headphones. He wasn’t sure how many inputs he needed, but definitely more than two. We looked around on National’s website and found some good looking power amp ICs. I ordered some samples and they came in the next week. We opted to use the LM1876, which can do 15W per channel x 2 channels. Although this might seem like a low-power amplifier, it proves to be more than enough for very loud listening levels in an average room. I drew up a schematic and laid out a PCB which we etched in my workshop. And we took a shopping trip to the parts store to get all the bits and pieces. In total the amplifier cost around $75.

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 aren’t 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.


Andrew fiddling with the headphone circuit.

Not shown in the schematic is the headphone jack parts. 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.

Conclusion

Building good one-off projects always takes a long time. This is usually because of all the mechanical parts, such as drilling and cutting holes in the enclosure, making cable assemblies, etc. This project was definitely no exception, with most of the time spent drilling and cutting the enclosure to mount all the parts. However, the results were very gratifying. Chris has been using the amp every since we finished it, and it’s still serving him well. Even with my ear right up to the speaker with no sound playing I couldn’t hear any noise from the amp itself. And music sounds clear and crisp without any noticable distortion. All in all, it just plays nicely and I could listen all day. And with such simple controls it can’t really get much easier to use! So before drooling over weird overpriced amps at your local hi-fi store, consider building something just as good by yourself.

Schematic