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Since I'm using two different transformers, I need two separate power supplies. These supplies have a soft start circuit built in and in this video I go over how that works.
In a nut shell, the thermistor is all that's needed for the soft start to work. But there are a couple of good reasons why it's better if you bypass it after it has done its job.
First is that if you bypass it, it will cool down and last longer.
Second is that when the amplifiers are not drawing enough current, the thermistor will cool down anyway, and as it does its resistance goes up, which lowers the supply voltage.
When you short it with a relay, the relay carries all of the current without heating up. But there has to be a short delay before the relay switches on so that the thermistor has time to work. I used the switching circuit from Rod Elliott's website to do that, and he gives a more detailed description of how it works: sound-au.com/pr...
To power the soft start bypass I dropped the 40VAC from the power transformer down to less than 10VAC with a resistor and a pair of 1uf capacitors. That lower AC voltage is then rectified to power the circuit and switch the 12VDC relay.
This has the advantage of not needing a small transformer to run the soft start. Also it doesn't heat up like resistors would to drop the rail DC down to a usable voltage.
On the board as well is what's known as a ground lift. It consists of a 10 ohm power resistor and two diodes wired back to back. The lift "separates" the supply ground from the chassis (earth) ground, but still provides a physical connection between the two. This can help greatly with ground loop problems that cause hum.
In the video I also show the power distribution boards that feed the individual amp boards. These reduce the wire runs inside the case and also put the supply capacitors very close to the amp boards, which should improve (very, very slightly) transient response. They are also a convenient place for the rail fuses for each amp board.
Since I have two supplies and 10 amplifiers, one supply will power 6 amp boards (these will drive the tweeters, midranges and midwoofers in my 4-way speakers) and the second will power the other 4. That other 4 will drive the woofers in my 4-ways, plus the bass shaker in my seat and have one channel left over for a possible centre channel speaker.
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