SRPP Demystified

SRPP

There is considerable misunderstanding and mystification regarding this particular circuit, including even what it's called. "SRPP" seems to mean: Shunt Regulated Push Pull, although you see different words assigned to the acronym.

As for what, exactly, this thingy is, it should be immediately recognizable to anyone with a solid state design background: it's an active pull-up/active pull-down circuit. (It greatly resembles the output stage of the TTL family.) The main difference is that it is quasi-complimentary by necessity since there is no such thing as a "P-Channel" VT. This means it is, indeed, push-pull by definition. The upper triode acts to source current to the load, and the lower triode sinks current from the load. As with any push-pull topology, it reduces distortion by nulling even order harmonics.

The big point of departure from solid state is that the SRPP is balanced for three load conditions only: a dead short (makes both triodes into grounded cathode stages -- not very useful) an open circuit (or at least a very high load impedance -- if there is just one path for the current, equal currents must flow through both triodes) and the one impedance for which it was designed. For any other load conditions, the SRPP goes out of balance, and distortion rises, more or less, rapidly. This is OK, considering the purpose for which the SRPP was originated: a line driver. As a line driver, it operates into the characteristic impedance of the T-line.

As an audio circuit, this leaves a lot to be desired. If it works into Class A*₁ grids, then it's OK since that's nearly an open circuit. The trouble starts when the driven control grids are driven positive, and draw current. Under grid current conditions, the resulting impedance is neither constant nor linear. This is not the type of load an SRPP wants to see. Including such a driver will lead to poor clipping behaviour. Unfortunately, it is grid current conditions where you'd like to include active pull-up to not only supply that grid current, but supply it from a Lo-Z source to minimize distortion.

You also see SRPPs used as audio finals. This, too, is not the place for this since any speaker represents anything but a constant load. Speaker impedance varies not only in magnitude, but also phase angle. This will play hell with an SRPP, as it will be operating off its optimum load impedance almost all of the time. That will generate lots of avoidable distortion which will require that much more NFB to correct. This is not what you want in good open loop design.

As for how the SRPP develops voltage gain, consider the R_P. The far end of that plate load resistor has a cathode follower sitting on top of it. This means that the voltage across that resistor is much less than it would be if connected directly to the DC rail. The effective AC resistance is much higher than its DC resistance. This gives the lower triode more voltage gain that it would otherwise have. The next question becomes: can we make that resistor even larger to increase voltage gain? If you break the DC coupling, the answer is "yes". This gives us the variation called a "Mu stage", so called since its voltage gain can approach the amplification factor (μ) of the lower triode.

Mu Stage

The AC coupling allows for a larger R_P since it is no longer doing double duty as a cathode bias resistor. The mu stage no longer has any pretension for being a balanced topology. It is designed for large voltage gains. Is this a useful topology? It was back in "the day", however, it is obsolete and should not be used. Today, we have solid state devices which can operate as excellent CCSs. You will do much better loading a triode with a constant current, as this gives a horizontal loadline that both maximizes output swing, voltage gain, and minimizes harmonic distortion. Back in "the day" we didn't have the ICs, BJTs or MOSFETs that could have made for decent CCSs. Before then, your only other recourse was to use a pentode as a CCS. That would give you something quite mu stage-ish anyway.

If you need lots of gain, then use a solid state CCS. These old fashioned circuits serve no purpose these days, other than nostalgia appeal, or audiophool trendiness on the part of those who like "exotic" circuits for the exoticness.