The custom built stereo amplifier has 3 inputs, 2-auxiliary and 1-projector input. The projector input has a noise gate. The input connectors are stereo (3-circuit) ¼" plugs with tip being left channel, ring being right channel and sleeve used as common ground.
The amplifier has 3 outputs, left, sub-woofer and right channel. The amplifier can be used to drive external power amplifiers or can be connected directly to 8 ohm speakers for 10 watts of power each channel. The output connectors are ¼" mono (2-circuit) plugs with tip being hot and sleeve is ground. You can use a stereo connector for the output but leave ring disconnected. All components are conservatively rated. The output transistor heat is dissipated by an over-sized heat sink arrangement. The amplifier can be operated for extended hours.

A switched noise gate is provided on the projector input. With the noise gate switched on, when the audio drops to a pre-set threshold, the gain of the input is pushed down an additional 10db.
This is used to mask minor noise present on the sound track. A 10db reduction in gain is enough to reduce minor noise to where it is not noticed. Higher reductions of say 20db or 30db are noticeable and distracting drops in the audio. Sound tracks with high levels of noise are beyond the ability of a noise gate to mask without chopping the audio. The Threshold is adjustable from -35db to - 50db.
The audio path through the noise gate is passive, there are no active amplified circuits. Audio from the projector input goes through light driven resistors (LDR) then to the projector volume control. Left and right projector audio is sampled through simple amplifier stages. The amplified audio sample drives a 1N4148 diode to create a DC voltage proportional to the projector audio. This DC voltage then turns on the MPSA14 transistor controlling the LDR's. A network of resistors and capacitors create a time constant to control the DC voltage.

There are 2 power supplies, one in the lower pre-amplifier chassis and one in the upper output amplifier chassis.
The power supply for the pre-amplifier chassis is a 38 volt low current DC supply. The power supply for the output chassis uses a single transformer with dual 24VAC windings and powers 2-35 volt 2-amp DC supplies. One supply provides power for the left channel output amplifier and the other supply for the right channel output amplifier. Both supplies provide power through isolation diodes to the sub-woofer output amplifier.

A TEST/HEADPHONE jack on the front panel allows stereo headphones to be plugged in or can be used to provide signal to an oscilloscope for testing.

The inputs and outputs are all located on the rear of the amplifier. There is a 5db gain trim control for each channel to allow balancing the channels.
All labels, front and rear, were made by printing onto clear labels and then laminating the labels to project the printing. This type of label is very durable and will hold up well under normal use.
Constructed using aluminum chassis and aluminum chassis covers with a front panel made of steel.

The amplifier is made up of two chassis bolted one on top of the other. The 3 output amplifiers are in one chassis and the pre-amplifiers and associated circuits are in the other chassis
Pictured below is the output amplifiers chassis under construction. You see the output transistors with wires that will connect to the power amplifier circuit sub-chassis. Three black heat sinks are mounted on the outside of the chassis just opposite the output transistors. Besides the heat sinks, the output transistors are mounted on 1/8" thick aluminum.