Circulation: 

Since 3/23/99

Editor,
Congratulations for your web-journal. It is unique and a joy to "receive", every month (if the May issue is on time, we'll forgive the April joke). ...I have not read anything about SRPP and mu-follower (with the possible exceptions of hybrid versions) operations. Are you planning to write something about them? or is there some kind of aversion for these topologies, which would be a real contradiction with the extraordinary creativity and openness displayed throughout the Journal....
Olivier
   
    We have been meaning to expose the inner workings of the SRPP circuit for some time now, but we just have not got around to it. Why? Because this circuit topology is so popular, but so little understood, we knew that the SRPP would require a lengthy explanation, which damped our ardor somewhat. Second, we had hoped to get a copy of the 1943 patent for the article, but we never did. In spite of which, here is what will surely prove to be the first of many articles on the SRPP circuit.
    Remember, if you have a request or suggestion of your own for either an article topic or circuit, please e-mail:         
                                 Editor

   This circuit has many names: SRPP, SEPP, Totem Pole, Mu Follower, Mu amplifier, Cascoded Cathode Follower, and its original name, the Series-Balanced amplifier (Feb. 1943, US patent 2,310,342. Just what "SRPP" means is uncertain; maybe it stands for Series  Regulated Push-Pull amplifier or Single-Ended Reflexive  Push-Pull amplifier.
   Wildly popular, this circuit was the defining vacuum tube circuit of the 90's. It appeared in high-end SE amplifiers, line stages, phono stages. Why all the fuss? This circuit promises the advantages of both the Grounded Cathode amplifier and the Cathode Follower: high gain and low output impedance.
  Yet, in spite of its popularity, few realize that it is not a single-ended circuit. In fact, most tube fanciers are shocked when told that this circuit is fundamentally a push-pull amplifier and not that radically different than the output stage of the a push-pull power amplifier like the Dynaco ST-70. "It must be single-ended…look there's no output transformer!" (A quick re-read of the June 1999 article on push-pull amplifiers is encouraged.)

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