capacitor explanation?

1. Coupling capacitor(s) is connected between the output of the input stage and input of the intermediate or output stage of amplifier or preamp.

2. Output capacitors are to protect speakers from DC at the binding posts

3. Bypass capacitors are to separate DC feedback from signal feedback.

Conceptually, the coupling capacitors aid is separating each stage's design by blocking DC (since the caps are in series) and only allowing the AC portion of the signal. This aids in accurate transistor biasing and stage-impedance matching.

The output caps block DC before it gets to the speakers but also lowers the impedance at the output so that the load resistance is isolated and not mucked up by bias/transistor resistances so that proper bias design can be acheived. You want these to be fairly large so that the lowest audible frequencies will be unaffected.

The bypass caps are often used in common-emitter BJT designs so that the gain-reducing effects of the emitter resistance are elimiated during AC signal periods. It also aids in temperature stability of the DC bias by now including the emitter resistance (i.e., the cap is in parallel with the emitter resistor).

If you have anymore questions, don't hesistate to email me. Arthur

Aball's explanation of coupling capacitors is good. Coupling is actually a misnomer since they are actually in the circuit to block DC as he points out.

Output caps are also used to block DC but the part about output caps lowering impedance is wrong. Every cap has some impedance so it will actually increase the ouput impedance. However, as he points out, the trick is to make it large enough so the impedance is low enough compared to the load impedance that it doesn't have much of an effect.

I'm also not so sure about the DC stabilizing effects of bypass caps since they are open to DC and are not part of the DC bias. The emmitter resistor is part of the DC circuit whether you have a bypass cap or not.

Actually, "coupling" is a proper description. The capacitor couples two circuits together by the common capacitive reactance of both circuits. The capacitor provides current at the rate to keep the voltage across it constant, so a change in voltage results in a change of current. The voltage change of the input signal causes a current change in the coupled circuit - except for DC, which is blocked.

Output capacitors also can act as DC blocks but they can also be used for filtering. Output caps in some tube circuits bypass harmonic frequencies to ground, before they reach the xfmr.

Bypass caps are used on the cathode of a tube to help with self-biasing the circuit. The voltage across a cathode resistor will vary with the signal voltage across the grid. As the voltage across the resistor varies, so does that of the grid and the bias. If the cathode resistor is bypassed with a capacitor, the voltage across the resistor remains constant which keeps the bias voltage constant.