The Split-Load Circuit

Author: N.H. Crowhurst

Development of split-load phase splitter

One simple way to make a phase splitter is to connect half of the plate load resistance between B-|- and plate and the other half between cathode and ground. Since these resistances are equal and the same current flows through both, each will produce the same d-c voltage drop and the same audio fluctuations. When the fluctuation across the plate resistor goes positive, due to decrease in plate current, this same fluctuation will be negative across the cathode connected resistor because of the same decrease in current. This provides voltages of opposite phase, but we still must provide input to the tubes. The normal place to apply input voltage to a tube is between the grid and cathode, however, in this arrangement, half the total output voltage is between cathode and ground. This circuit uses the tube just to get phase inversion (to reverse the voltage between grid and plate) and does not achieve any useful amplification.

Using one-half of the 12AU7 as an example, an input fluctuation of 10 volts, measured between grid and cathode, will produce an output fluctuation of 100 volts - 50 volts at the plate and 50 volts at the cathode. When the grid-to-cathode voltage goes 5 volts positive from its bias point, the current through the tube will increase. The cathode will go 25 volts positive from its d-c operating point, while the plate will go 25 volts negative from the d-c operating point. As the cathode has now gone 25 volts more positive from ground, and the grid-to-cathode voltage needs to go 5 volts positive to cause

Currents and Voltages in Split-Load Circuit

this, the .grid-to-ground voltage itself must go 25 + 5» or 30 volts more positive to produce this swing. We therefore require a 60-volt peak-to-peak audio fluctuation from grid to ground to produce a 50-volt peak-to-peak fluctuation at plate and cathodet respectively.