To recap what's going on here, this amp has active crossovers that drive my 4-way active speakers. So for each speaker driver in the 4-way, there's an amp board and a crossover board. I designed the crossovers based on the setting I arrived at with my miniDSP setup - the DSP setup is what I was using before I made this new amp. DSP is digital and adjustable, while this new amp and crossovers are analog and fixed. I can only change the output level on each channel, not the crossover slopes or frequencies.

Getting all of the amp boards mounted on the heatsinks and wired to the power supplies and output banana jacks took the better part of a full day. A big part of that time was me testing each board before installing it to make sure they are working correctly.

Then more metal fab, I used some scrap aluminum extrusion to make the two decks that the crossover boards will sit on. I had a genius idea to bend tabs down to support the ends of the amp boards, but that failed when the tabs broke off. Worth a try, but I wish I'd tried on a scrap of the same aluminum before cutting the shelf. Then I had to cut the other shelf in the same way so it wouldn't look odd!
And yes, I tried annealing the aluminum with a torch to make it softer - it didn't work.

With the first crossover board hooked up I can turn it on and see how much of a turn-on thump it creates. Not too bad, but I can eliminate it altogether by muting the inputs to the amp boards until the power supply for the crossovers stabilizes. Worked great and it only added one more full day to the build time!

Thanks again to Rod Elliott's ESP website for a suitable muting circuit to use:
http://sound-au.com/articles/muting.html

The output of each crossover board stays disconnected from the amp boards for about two second after you switch the amp on, then the relays close and connect them. I used relays because I have a bunch of them that I got for next to nothing several years ago.

When you switch the amp off, the muting circuit detects the loss of AC voltage from the transformer and immediately turns the relays off, disconnecting them from the amps. That eliminates the sound that the crossover boards make as the power supply discharges.

With the crossover installed and the metal front panel screwed in place, I moved the amp down to my listening room to setup and use. It's not finished, but it's finished enough to use (or so I thought...) and I need to take a break from the project to do other things.

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