Design Idea:
(L+R)  & (L-R) in One Stage

   Those readers old enough to remember quadraphonics might remember some of the matrixing circuits used to decode the quadraphonic signal. Or maybe some of readers are working on a tube FM stereo decoder or modern home theater 5-channel surround sound system that uses tubes. In all these cases the following design idea may prove interesting, even helpful.
  Combining two signals into one is easy with a simple tube mixer. Two triodes and one plate resistor will work. Adding a negative power supply rail allows increase the cathode resistor values so as to bring the gain down to unity and lower the distortion by virtue of the cathode degeneration.

Mixing and subtracting in one circuit

  This design idea uses two fewer triodes and one more coupling capacitor than the last two circuits. It offers both functions, (L + R) and (L - R), in one stage, unity gain, and low distortion. Superficially, the circuit looks like a cascode, but it works quite differently. Each grid receives signal and no gain results.
  The secret to unraveling how this circuit works is to realize that there is only one current path at idle, but two paths when processing signals. Let's start with the bottommost cathode resistor. Pretty much independent of what goes on at the triode's plate, the cathode follows the grid's movement; if the grid swing positive, so does the cathode. The signal  superimposed on the cathode resistor defines a signal current that flows through the entire stage up to the B+ connection. And as the topmost plate resistor is in this current path, it will experience an anti-phase signal development in response to this signal current.
  Now if the topmost triode's grid sees a positive pulse signal, the top triode will conduct more current in response. But where will this increase in current come from? One source is the bottommost triode and the other is the 10k resistor that is terminated into the capacitor that connects to ground. (Remember we are dealing with AC signals only.)

Mixer with no gain

  Subtracting one signal from another is not as easy without a feedback loop. The Differential amplifier performs the function of L-R, but adds unwanted gain and distortion.

Differential amplifier used to subtract one signal from another, unfortunately with gain


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