Class A Amplifier operation is where the entire input signal waveform is faithfully reproduced at the amplifiers output as the transistor is perfectly biased within its active region, thereby never reaching either of its Cut-off or Saturation regions. This then results in the AC input signal being perfectly “centred” between the amplifiers upper and lower signal limits as shown below.
So at this point we have the signal and power grounds connected. Next is the chassis. There are a number of ways that we can accomplish this connection. The key thought is to maintain the shock protection while using it as an EMI shield. Sometimes it can connect directly to the ground at the input jacks. This was a fairly common practice in the earlier days of electronic equipment. These were also in many cases what I would consider lower fidelity equipment and things that did not benefit from three wire AC mains. I do not recommend this as there is the possibility of introducing the EMI noise from the chassis and AC mains earth ground into the signal path. Most methods of making the connection involve resistors, capacitors or rectifiers. I suspect all will work. My preference is for a parallel combination of a metal film resistor of about 120 ohms (1/2 Watt is fine) and a type X2 capacitor in the range of 0.1 to 0.22uF. Type X2 capacitors are rated for use with AC mains circuits. I have seen ceramics and polys used there but since they are not usually AC mains rated I strongly recommend against them. The capacitor and resistor provide sufficient isolation between the chassis and circuitry to allow the chassis to be an effective EMI shield but not induce the EMI into the active circuitry.
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