Reader Rowan has asked for some direction in building a tube microphone preamplifier. Two topologies are shown.
   In the last issue, reader Ian made a great suggestion:  a tube amplifier that would mimic the mixed class of operation of the Pass Labs Aleph amplifier (SE / push-pull Class A). This month we have a brief follow up by presenting a current source alternative to using a choke.
   Remember, if you have a request or suggestion of your own for either an article topic or circuit, please e-mail:

  We were hoping to let the Circular/Balanced/Bridge amplifier rest and not make an appearance this issue, but it is here in an article on tube buffers. Expect more on this topology next month as well, for we are not nearly done excavating this vein. The circuit from the July article, "Unbalancing Acts," is back in the same buffer article. In this same article you find the secret to optimizing the White Cathode Follower as a Class A output stage. Headphone fans take note.
   I have found that many tube circuit neophytes and, unfortunately, far too many tube gurus have only the Grounded Cathode amplifier under their belt, so to speak. Good as this circuit is, a bigger repertoire is welcome. (I am saddened by sights such as the earnest tube fancier wasting his time trying different brands of the same valued plate resistor in an attempt to make the 12AX7 based line stage he owns better drive the 15 feet of high capacitance interconnect to the amplifiers.) The aim of this journal is to increase the vacuum tube circuit vocabulary of the readers.

A Quick Overview
    Right after the Grounded Cathode amplifier, the second most common tube circuit in use is the Cathode Follower. The simplest buffer that can be made with tubes, it is used to match a signal from a high impedance source to a low impedance load.   

    The circuit is single-ended and Class A by design, as there is only one active device. The plate is, in AC terms, but not in DC terms, grounded and thus the Cathode Follower other name: the Grounded Plate amplifier.

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