Introduction
I use one of my Linux servers as a MP3 player, which is outputted to a car amplifier powered by an ATX computer power supply. This setup is quite messy in terms of wires, so I've decided to remove both from their respective housings and place them in a nice, custom-built box. Right, so while I'm about to do that, aren't there any other goodies to add, like a VU meter and volume control? Yes, which is what this project is about — how to built a stereo amplifier from various components and parts. Everything is modular, so the system components can easily be changed, upgraded, replaced or have new goodies added.
The Power Supply Unit
I am using an ATX computer power supply.
Standby/Power-On Indicator
The PSU provides us with a standby rail (+5VSB, purple wire), which is used to power circuits that require power input during the powered down state of the unit. A red LED, powered through this rail, will indicate that the unit is plugged into the main power. This LED should then turn to green when the unit is actually switched on. For this function we must use the PWR_OK (+5V, gray wire) signal.
Two interesting aspects of the required circuit are that we can use an H-bridge to switch the polarity applied to a bi-colour LED and that the switch for the H-bridge needs to be of a special configuration.
Schematic for the standby/power-on indicator as built by me. (See text for values, explanations and warnings.)
© Robert I. Sadler, 2009
Amplifier
The amplifier is an old car audio amplifier.
VU Meter
Being a product of the Eighties, I consider a VU meter as a definite must in any stereo system. Details on how to build this module can be found here.
Volume, Tone & Balance Control
I would like this amplifier to have a volume control. This can easily be achieved with a logarithmic potentiometer on the input, but by using a circuit based on the Philips TDA1524 chip I also get tone (bass & treble) and balance control. Details on how to build this module can be found here.