DIY Amplifier
After many failed attempts at findind a suitable, cost effective, amplifier solution to drive all the ceiling speakers throughout our house I decided to finally give up and make my own.
So I got a hold of two 25W amp kits based on an LM1875 IC, and a power supply suitable to drive both of them.
Below are pictures of firstly the power supply driven by a 160VA 18-0-18V toroidal transformer, and secondly of the two amps mounted together and attached to the heatsink on the rear of the case. Each transformer can comfortably drive 4 x 25W amps.
The case is just a cheap 19″ rack mount enclosure from Jaycar which is quite easily customisable with adjustable mounting rails, and is pretty solid once assembled. As you can see below, plenty of space for another 3 power supply circuits and 6 amps, to make the unit up to its capacity of 8 channels. I could potentially squeeze in another 2 channels, but then heat starts to become more of a problem, as does finding rear panel space for connectors.
Yes, I know the heatsinks aren’t straight.
Add four dual 50kΩ log pots for stereo volume controls, a power switch and LED on the front, and the amp is ready to go. Well, except for the fact that there’s only 2 out of 6 channels built so far.
The biggest challenge of this project was eliminating the hum that I first heard when I turned it on. I knew the electronics were fine, as the amp was very quiet when I first tested it outside the casing. The hum was very much indicative of interference, likely caused by a ground loop.
The first step was to change the wire from the input terminal, to the pot and then to the amp. I made the mistake of using cat6, as it conveniently has the right number of wires. But no good for unbalanced audio. The noise reduced significantly after replacing it with shielded coax. But it was still there.
I soldered in extra ground wires, I tied the ground to the mains earth, I moved wires away from the transformers, all to no avail. Then eventually when probing around with the multimeter, I accidentally shorted out the negative input signal wire to the casing of the volume pot, and the instantly disappeared. Now I have zero hum all through the volume range. And the 25W/ch is plenty for the ceiling speakers. Not sure about driving outdoor speakers for the deck (when it gets built), so I may need to make a 2×50W amp for those.
Next step is to introduce a microcontroller to drive volume and turn channels off by relay (power saving) via RS232.