Six Channel Amplifier Electronics & Audio

I started work on this in late 2007 and didn’t finish it until 3 years later, in 2010. That might seem like I took some time off in there somewhere, and I did, but not much. This was a massive project, mainly because I wanted to do as much of it as I possibly could from scratch.
This amplifier was build specifically for the three way speakers I made around the same time. The idea was to have each of the six amplifiers drive one of the speaker elements (tweeter, midrange, woofer) individually and connect it through a digital crossover. Normal stereo speakers have a passive crossover built in, and that separates the bands of frequency in the audio signal and sends it to the right driver (speaker). That wasn’t complex enough for me!

It really started with the design, layout and manufacture of the individual amplifiers:

These are the circuit boards that I made, etched from double sided copper clad board. The “silkscreen” on this was actually etched from the copper on the top side of the boards, and I thought that was pretty neat. Each board represents a single 100 watt amplifier, and six were used for this.

Here’s the schematic for this amp (click image to open full size pdf version):

The amp boards populated with components:

I called these “Patchwork”, because I took inspiration from a few different amplifier designs for this one. I really didn’t have a clue what I was doing when I started to “design” this, but I had enthusiasm and the desire to learn. I’ve always been the type the does his learning “on the job”, while working through different iterations to try new things. It took most of a year to beat the basics into my head, and another before I finally got to this stage. If you look closely, you’ll see the “2009” on these.

Of course, amplifier boards are great, but they need a place to live, and that brings me back to familiar ground – building stuff. As usual, I wanted to make as much of the case as possible, spending as little as possible and using materials I already had as much as possible. The biggest stumbling block was the large heat sinks these amplifiers would need to keep cool. These amps are class AB, so they don’t run a high idle current, but they can produce significant heat when the volume goes up.
I had a pile of aluminum door threshold pieces that I was saving to bring to the scrap metal place, but thought that it might be possible to use these to make my own heat sinks:

So I cut them into pieces 6″ long:

And bolted them together with long pieces of threaded rod:

Of course, I had a good amount of sanding to do on the back to make them smooth enough:

Given how much big heat sinks cost and how I couldn’t even find any this big, making these was worth the time and effort.

Originally I thought that I would mount the amplifier boards directly on the heat sink, like this:

But decided it would be better to bolt a wide aluminum angle on instead. This does two things: situates the boards neatly inside the case, and straightens the curved heat sinks:

The amp board shown here was an earlier test build. While prototyping the amp circuit and board layout, I made several changes and made new circuit boards for nearly each one.

Next I made the chassis for the case and used a piece of 20 gauge stainless steel that I bent into a “U” shape for that:

This was also scrap that I got for free from a job where they took it out of a floor grate that was just inside the main entry doors.

I went back and forth on how I wanted the front panel to look, but finally settled on this lamination of solid maple and black walnut:

This amp has a power switch, but it’s at the back, so no holes through the front.

The power to run those six amps comes from a large (barely fits!) toroidal transformer rated for 700 VA:

This transformer puts out 40-0-40 volts AC for a split supply when rectified of +56VDC and -56VDC.

Along with amplifier design, I had to learn the basics of power supply design as well. There are two of these, one for each “side” (left and right) of the amplifier:

Each power supply board supplies three of the amp boards. The spacing for the fuses was tight, but I added pieces of thin clear plastic between them to insulate. This was the biggest board I had at the time and had to really squeeze things in to make it all fit. The big capacitors are for smoothing the AC ripple – more is better.

The back panel has the inputs for each amp, the speaker terminals and the power switch. This amp is meant to be left on continuously, so no reason to put the switch in a more convenient place:

Making good progress here with all of the amp boards installed. The blue wires go to the individual speaker terminals and are routed beneath the aluminum angles:

As you can imagine, wiring for something like this can be tricky, especially if you want to avoid hum from ground loops:

To provide even more shielding for the amps, I added another sheet of aluminum to box them in and ran the supply wiring along that:

And went back to my stash of door thresholds for the tray that you can see here that holds the input wiring. Neatly mitered in the corner, no less:

Another key component for an amp this big is a soft start that limits the inrush current:

It’s thermistor based and has an RC timer to turn on the relay after a few tenths of a second. This is yet another example of a circuit that I had to learn about and design.

Everything installed and ready for the first tests:

Of course, I had been testing all the way through, so I knew it would work. But there’s always some trepidation when first powering something on that you’ve poured literally years into. Last thing I needed was for it to go up in smoke because I hooked up a wire wrong, or a loose screw rattling around and shorting something that’s live.

After I was satisfied that everything was working as it should, I went back to finishing the case. First I painted the heat sinks flat black to make them look better. I then made the lid from more maple and used cedar strips to form a grill:

Originally, I was going to leave the cedar natural, but figured black would look better:

It’s 2018 as I type this and the amplifier has been in continuous operation since finishing it more than 8 years ago. It is connected to a digital crossover and use it to drive my three way speakers. Earlier today I brought the amp out to my shop to blow the dust out and do a kind of show and tell for a video I recorded: