Research Phase
Books from Douglas Self and Bob Cordell had significant technical and practical insights. Though not as technically detailed, Randy Sloan's book was also helpful. Extensive web research was also done, with diyaudio.com being a good source. Bob Cordell was retained for a couple hours of consulting time. The eventual design is heavily based on the work of Self and Cordell. High level decisions reached:
- Design completely analog Class A/B Amplifier with carefully designed output stage biasing to minimize crossover distortion.
- Use traditional 3-stage design (Input, VAS, and Output stages) along with high impedance input buffer.
- Use Bipolar Junction Transistors throughout, op amp for input buffer stage.
- Use Two Pole compensation to reduce distortion at 20kHz.
- DC power supply of +/- 63VDC to deliver the 56VDC needed (after accounting for power supply droop and rectification losses) to deliver 40 Volt RMS output for 200 Watts power into 8 ohms
- 1000 VA Toroidal transformer to minimize stray magnetic fields, deliver sufficient power to drive 4 ohm loads, both channels driven at full power, with sine wave test signals.
- AC couple the 120 Volt mains to guarantee no DC component from the mains is applied to the transformer, eliminating potential source for noise/hum.
- Carefully design grounding scheme to eliminate ground loops and hum. Use well configured star ground. Keep signal grounds isolated from power grounds via 10 ohm resistor from signal ground to star ground.
- Use one amplifier board per channel, each mounted on its own heat sink.
- Use the heat sinks as an enclosure design element - make them the sides of the amplifier.
- Use black anodized aluminum for enclosure, providing attractive look, and allowing laser etching of logo and wording on panels.