Audio amp 1.0
My six-channel audio amplifier, version 1
This is an audio amplifier I built for my apartment. I've never had a proper surround sound system, even though my computers can output 5.1 surround. (And I use my computer to listen to music and watch movies, as well as gaming.) My original goal was a six-channel amplifier that can output least 50 W per channel.
I actually started this project back in 2006. After reading a bit about high-performance audio amplifiers, I decided to not try to design and build my own design from individual components. The LM3886 chip amplifier has been around for several years and is perfectly adequate for my purposes. I want a good amplifier, but I'm not an audiophile and I'm not worried about shaving off hundredths of percents of distortion.
In the summer of 2006 I gathered the parts for the amplifier and constructed a case for it. I had a nice large power transformer and some finned heat sinks that were salvaged from an old linear DC power supply, and I built the case around them. After doing so, however, I decided that the output of the transformer wasn't quite what I needed, so I disassembled the transformer and started to wind a new secondary for it. Then I started grad school and didn't have free time any more.
Three years later, the transformer was still in pieces but I decided that it was time to finally put the amplifier together. Since I was in a smallish apartment at the time I didn't really need hundreds of Watts of output power. Instead, I used a 24-volt power supply (a leftover from my spun mirror project), as a single 24 V supply is enough to power an LM3886.
The LM3886 is essentially a high-power op-amp, though it does have some features specific to audio applications (such as mute). So I needed an interface board for each chip to hold some external components, such as power bypass caps, resistors to set the gain, and filters. When using the 24-volt power supply, the amplifiers must operate with a single-sided supply, so I also needed to provide the chip with a DC bias voltage. I designed the interface boards so that they would also work with both positive and negative supplies in case I ever finished rebuilding the transformer.
The boards I had made are shown at right. Although I wanted to use parts that I had on-hand, I decided to have the boards made professionally. I've etched my own circuit boards before, but the result was never very clean and I don't really have a way of drilling the holes with any precision. I used BatchPCB to get my boards made. They're the cheapest PCB prototyping service I've seen, they work from standard Gerber files, and the resulting boards were quite nice. I designed the boards using FreePCB, but the BatchPCB automated DRC software doesn't like the way FreePCB generates its Gerber files (using negative fills). Next time I have to design a board, I think I'll try gEDA.
Oh, and here's another cool thing about BatchPCB. Do you like these interface boards? I made the design public; you can go and order some for yourself.
Diagrams for the single and double supply circuits are shown at right. They are pretty similar to the circuit diagrams in the LM3886 datasheet. I provided a connection for the mute pin, even though I didn't make any use of it in the single-sided supply version of the amplifier. The power supply bypass caps on the board are fairly small, but with the rather low-power switching power supply they were enough. For a power supply based on the big transformer, some hefty power supply bypass caps are required off the interface boards. The large capacitors on the populated boards shown at right are there to provide AC coupling betcween the amplifier output and the speakers.
I mounted the boards by fastening each LM3886 to the heat sink. The boards are small enough that no other support is needed. The twisted pairs of orange and blue wires are the audio input, the large gauge wires are power supply and ground, and the individual red wire going to each board is the mute. Incidentally, most of the wires I used in this project are extras that I pulled out of the Rodgers Cambridge 220-II when working on its expression indicators.
I mounted the switching power supply inside the enclosure where the power transformer will eventually reside. Because it may get rather warm inside there and I don't want the power supply to overheat, I removed it from its plastic case. I'd like to be able to reuse the power supply, so I left its power input plug in place, and just soldered wires to its connections on the power supply's circuit board. I removed the power output cable (in favor of much shorter wires), but that will be fairly easy to solder back in place. I made little balsa-wood frames to hold the supply up off of the amplifier case and to support its heat sinks when the zip-ties that hold it in place are tightened. I don't want to put any more mechanical stress on the board than necessary.
To get power into the enclosure I mounted a power input jack in the right side of the enclosure. It is a three-prong jack, same as is used in computer power supplies. This one is filtered as well, which seemed like a good thing. I also mounted a circuit breaker and a power switch in the right side. These were all components that I had sitting around; I probably salvaged them from laser printers years ago.
The audio input is a line of six RCA jacks in the right side of the enclosure. I put an in-line plug and jack between them and the amplifier boards to make the wiring easier to handle inside (most of the rest of the wiring is run straight through and soldered, so it would be hard to take this back apart again). The audio output is on the left side of the enclosure, as can be seen in a photo at right.
The finished product is shown at right. For a pre-amp, I scored a 24-channel Tascam stage mixer that a local TV station was auctioning off. (Because the whole project wasn't ridiculous enough already!) Aside from some pops when the amplifier is powered on and the output isolation caps charge up, I was pretty happy with the results. Since then I've gotten a bit further with the big power transformer ... but that's another story.