Posted on Feb 5, 2014

An old nonfunctional DVD player donated by the Engineering Department was gutted to provide an appropriate enclosure for the audio amplifier. To mount the nearly 16 lb toroidal transformer and 2in diameter filter capacitors, various sheet metal braces were machined in the shop. The 25A rectifier bridge was bolted to a heatsink attached to the bott

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om of the enclosure, and a temporary brace was attached to hold the mains power switch and fuse holder. The current state of the amplifier construction process is shown in Figure 1. Because the +12V, -12V, and +5V regulators can only take a maximum of 30V at the input, a smaller 24VCT 700mA frame-type transformer was also mounted to power the lower voltage circuitry (preamplifiers, delay circuitry, output relay controllers). The delay circuit originally presented was modified to allow for an adjustable delay setting via a potentiometer, and a LM311 comparator was included to make a definite logic level transition. The modified power supply circuit is shown below in Figure 2, and the assembled delay/voltage regulation circuit is shown in Figure 3. Upon turn on, the delay circuit and main high current power supply worked correctly. Mains unloaded voltage was measured at +/-34VDC, and the smaller power supply produced as expected +12V, -12V, and +5V, as well as the 0-5V logic level delay signal. Since the reservoir capacitance of the small power supply is only 1000uF per rail, local capacitative decoupling will be important for clean voltage levels in the preamplifier circuitry. To prevent ground loops, the center taps from the transformers are not connected directly to the chassis, but will be linked to the chassis through the circuitry at the point where the audio input connector grounds are attached to the case. This...

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