Hi-Fi Stereo: Modular Amplifier Platform
For my high school graduation I bought a 5.1 channel surround sound speaker system. It came with an amplifier built in to a subwoofer enclosure and five two-way bookshelf style speakers. Over the years it turned out to be unreliable and was replaced a few times before I decided to build my own. When I set out to design a stereo amplifier my primary goal was modularity so I could make more than one for different areas of the house using the same designed circuit boards. I settled on an overall layout using single PCBs for each speaker driving channel and power supply boards which could each power up to three channels. This works out conveniently. A 2.1 channel setup would use one power supply board and three amplifiers to drive three speakers. A 5.1 channel setup would use two power supply boards and six amplifiers to drive six speakers. I decided to make 2.1 channel backplane and drive the subwoofer with an active crossover. In the future I'd like to expand the backplane to be adaptable for up to 7.2 channel surround sound. The way I chose to select amplifiers makes this convenient.
The first iteration of design consisted of a transformer, linear power supply, and two identical class D amplifiers.
First tests took place spread out on a desk. In the top-left corner of this photo you can see a series/parallel switch. I like to use transformers with two output coils and design my power supplies to be easily switched between 115V and 230V mains power so no re-design is required for use in Europe. The power supply in later photos is an improved version with surge and short protection added for safety and longevity.
After I was confident the system was working, I tucked it in the corner of my computer desk for a while and did a real-world use test for a few months. This was a replacement for my old broken amplifier for a while until this project was completed.
Building the Enclosure
Every project needs a functional and visually appealing enclosure, especially one I now look at every day next to my computer. I designed an enclosure to be made with aluminum front and rear panels with wood walls. To make the enclosure I used aluminum sheet stock and a piece of birch plywood.
The aluminum panels were CNC milled to approximate dimensions then buffed and sanded to create the brushed finish shown here. I used a table saw to cut the wood down to the correct sizes, then milled slots for ventilation and to hold the front and rear panels.
All the electronics mount to a piece of plastic with tapped holes to make it easy to insert and remove single boards as needed. Here you can see the rear panel has a power entry module and backplane for connecting audio input and speaker outputs. There are three speaker driving amplifiers, a power supply, and a subwoofer crossover mounted towards the front.
Wiring was completed by hand for a therapeutic experience and quality end product. Board-to-board connectors are used for power distribution. Everything else is either soldered or uses terminal blocks in the spirit of modularity.
Everything but the power entry module and power switch is wired here. Power, signal input, and speaker output wiring is separated in different areas of the box.
With a few more connections, wiring is completed.
The completed 2.1 channel stereo amplifier highlights some of my favorite design elements. It fits in well with the decor on my desk. I chose to leave the box a little unfinished to maintain the homemade appearance. As always I have a lot of ideas to expand upon this platform when it comes time that I want to build another one. I plan to add a tube preamplifier, preamplifier bypass, and expand to 5.1 channels. For now this box does the job well. It's nice to be able to fiddle around with subwoofer gain and high pass frequency to fine tune the sound to suit the type of music being played and the room it's in.