Direct Coupling

Author: N.H. Crowhurst

Direct coupling presents a problem

An alternative to resistance-capacitance coupling is direct coupling. This system requires several separate supply voltages, however, and there is a problem in getting them all set - and maintained - at their right values. For example, if the working plate voltage of the first tube is 175 volts and the next stage needs 7.5 volts bias, the second-stage cathode must be exactly 175 + 7.5 = 182.5 volts positive from the first-stage cathode. This voltage must be maintained in addition to the voltage supplies needed to provide plate current.

If the plate voltage should happen to be 170 volts and the second cathode is still positive 182.5 volts, its bias will become 12.5 volts, which is too much. When more stages are added to this system, a very small error in voltage at the input end can result in later stages being biased completely out of operation.

The problem then is that a number of supplies are needed, and they have to be very accurately controlled to the right value - which may vary with room temperature or a number of other things.

Such designs have been developed for computer applications, which are not audio and therefore will not be discussed here.

A two-stage amplifier using resistance coupling

Another system that uses resistance coupling has just two supplies, one positive and one negative. To bring the voltage at one plate down to a suitable level for the following grid, two resistors are used as a voltage divider. Of course these resistors will divide the available fluctuation at the plate as well as dropping the steady voltage.

Suppose the gain of the previous stage is 10 (as calculated at first), and the plate potential is 175 volts. If the negative supply is 250 volts, like the positive supply, the resistors that give the correct steady voltage for the following grid must give - 7.5 volts at their junction, or be in the ratio of 182.5 : 242.5. The fluctuations will be divided by (182.5 + 242.5)/242.5= 1.75. The gain from the grid of the first stage is now 10/1.75 or 5.7, being first multiplied by 10 in the tube, then divided by 1.75 in the resistance coupling. Resistance-capacitance coupling gives better results than direct coupling. Its main disadvantage is that it ceases to be effective at some low frequencies, fixed by the time constant of the resistor-capacitor combination.