SRPP Once Again

   Last issue we covered the theory behind the inner workings of the SRPP circuit. This time we will look into using the SRPP topology in in power amplifiers. In addition, we will look into the possibility of using a variation on this topology to drive a purely reactive load. Finally, we will examine a new variation that should make the Mu Follower crowd happy.

Power Amplifiers
   As the SRPP topology shares the advantage of the of other push-pull topologies of delivering at least twice the idle current into the load, why not use this topology in a power amplifier? One reason against such a design move is that the SRPP is not a perfect push-pull topology in that the grids of the output tubes do not receive a balanced input signal, as the top grid sees not only the drive signal generated by the bottom tube's operation, but also the distortion harmonics it generates, which will not cancel in the output signal. Still, maybe a balanced symmetrical topology that displays well on an oscilloscope might sound skewed at our ears because of its selectively dropping out even harmonics. Maybe partially unbalancing a push-pull amplifier will please the ear more by offering a more natural harmonic structure.
   At its simplest, the SRPP only requires two tubes: something of a push-pull version of the
Simplest Amplifier Possible, no driver stage or phase splitter, just two output tubes. Because only triodes should be used to ensure a low output impedance, and because the output tubes must realize a high transconductance to allow them to forgo the need for a driver stage, the selection of output tubes is limited. The only few that come readily to mind are the triode connected EL84, EL86, and 8417. If a transformer is used at the input, then just about any power triode will work: 2A3, 300B, 6AS7.

An SRPP power amplifier

   Of course, if we are willing to abandon minimalist ideals and use at least one driver stage, then even a pair of 211s could be used. Whichever output tube is chosen, the output transformer should be capacitor coupled to the SRPP to prevent the transformer's core from becoming magnetized. As no net DC current flow through the output transformer, a nickel core transformer can be used. A small tweak can be added in the form of terminating the output transformer to the bottom triode's cathode rather than to ground, assuming there is a cathode resistor. This arrangement eliminates some of the bypass capacitor effect from the output.

Doubly Push-Pull SRPP
   Two SRPP stages can be arranged into a bridge amplifier. The load connects the top triode's cathodes together and the bottom triodes must be driven with a balanced signal. The advantages this arrangement holds over the single SRPP stage is that the power supply sees a constant current draw and the power supply noise drops out of the equation. The advantage of a constant current draw is that it prevents the signal being amplified from impressing itself on the power supply and thus re-circulating through the rest of the amplifier. The advantage of less power supply noise is obvious, but how it drops out of the output signal is less so.   

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