Design Idea: Current Balancers 

   In previous issues, we have stressed that lowering the noise floor restores more of the music's subtle nuances and, thus, allows more of the tube's virtues show. We have also stressed that not all noise comes from the power supply, that the signal itself within the amplifier can re-circulate through the power supply and create smearing and reduced dynamics at the output.  So here is a design trick that helps SE designs compete with balanced designs. Balanced circuits benefit from the canceling of current swing within each stage.  As one phase leg swings negatively, the other swings positively; the net current change for the stage: zero.
  In an SE circuit, as the plate swings negatively, the current flow increases; thus, the net current change for the stage is more than at idle. This increase in current flow will tug at the power supply connection and pull down its voltage, if that connection has any impedance greater than zero.

An SE stage that has been rebalanced by the addition of a Cathode Follower 

   Now, if a seemingly redundant Cathode Follower is added to the circuit, the current can be rebalanced to zero net change. The Cathode Follower's output is not used, although it certainly could be. The Cathode Follower's purpose is to draw an anti-phase current signal from the power supply to cancel that of the Grounded Cathode amplifier. (Remember that while a Cathode Follower works in voltage phase with the Grounded Cathode amplifier, its current draw is in anti-phase with it.) When a Grounded Cathode's the current flow increases, the plate swings negatively. When a Cathode Follower's cathode swings negatively, the current flow decreases. And as most small signal triodes come two per envelope, finding a triode for the Cathode Follower is easy enough.
   It would seem that as long as the plate resistor equals the Cathode Follower's cathode resistor, the current use would balance, but it will not in practice. The gain loss through the Cathode Follower must be accounted for first. If we take the Cathode Follower's gain and multiplying it against the plate resistor value and set this new value as the Cathode Follower cathode resistor. Then it will balance.                       

// JRB   Copyright © 1999 GlassWare   All Rights Reserved