Balanced Line Stage

The Circuit
   A balanced line stage could be made out just one 6021 or 6111 configured as a differential amplifier, as long as a very low output impedance was not needed. But if needed, then the differential amplifier will need augmenting with dual Cathode Followers. If the load to be driven is a brutal 600 ohms, then a pair of very large valued coupling capacitors will in addition to the Cathode Followers  be needed to ensure low frequency bandwidth.

   Always appreciate your circuits with comprehensive explanations! I'd like to request a balanced line stage preamplifier that uses:
   1. Both (+) AND (-) B supply voltage.
   2. Subminiature tubes like 6111, 6112, or 5719 which
       are the last tube generation.
   3. Would be even more interesting with subminiature
       phono equalizer.

Thank you again for a great site.


Subminiature tubes

   Yoon is right about these tubes being the last tubes designed. Often called "pencil tubes" because of their diminutive size, they were some of the most carefully built and most vigorously tested of all tubes, as their main intended use was in demanding military applications, such as missile guidance. If the transistor's development had been stalled for one decade, these tubes would probably be the standard tubes used in audio today, as their small size and high performance would have compelled their use in home stereos.

  The circuit begins with a differential amplifier whose outputs are connected to the negative supply rail through voltage dividers. The outputs of the dividers feed the grids of the Cathode Followers, which then capacitively couple to the output jacks. Notice that the voltage dividers eliminate the need for internal coupling capacitors by stepping down the plate voltage. But there is a cost: they also represent an insertion loss of -4 dB and the added noise from the high value of resistors used in making the dividers. However, the loss of gain is not crippling in most situations and the noise contributed by the resistors is less than what the dividers will magically subtract from the power supply noise. Magically? Here we have an example of a circuit that works with the causes of noise and ultimately lowers the output noise.

The Noise Cancellation Trick
  Let us review the noise present in a split rail power supply. This assumes that the two rails were derived from a single center-tapped transformer winding or at least two identical transformer windings wired in series. As long as both rails share an equal, but opposite voltage and both are equally resistively loaded, the noise present on each rail will be both equal in amplitude and frequency, but opposite in phase.   

   The only real liability to the subminiature, other than potential non-availability, is the limited cathode-to-plate voltage (usually 165v) and the low dissipation capability (usually 1.1w). Tube sockets might hard to find as well, but circular 8 pin IC sockets are close in size (the tube's pins are just wires that can be readily bent).

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