Push-Pull Amplifiers -Part 2-

This Issue

  This month we are going to take a tangential look at the Circular/Balanced/Bridge amplifier that was briefly covered last month. This circuit bewilders so many tube fans that I am tempted to apply for a doctorate in psychology using an analyses of the mental block that occurs when some look on it for my doctoral dissertation.
  We are lucky to have John Atwood tackle the subject of pass devices in a series regulator. He covers triodes, pentodes, transistors, and MOSFETs. We also have an article on simple tube shunt regulators and a hybrid design. (Regulators will be beaten to death, by time we get done with them over the next months.)
  The second part of reader Yoon's request is fulfilled this month with a balanced phono preamplifier. This is some-thing I would love to build. I have always wanted to try a balanced feed from the phono cartridge, as the floating nature of the cartridge's windings would lend themselves to this use. And having a balanced output from the preamp would allow for easy phase reversal, whether the rest of the system were unbalanced or balanced in design.
   A new request for a circuit comes from reader David, who wants a Common Cathode based headphone amplifier for driving Grado headphones. The circuit does not receive a detailed explanation, as it is based on the June Circuit of the Month coverage of the Common Cathode amplifier from the GlassWare Web site.
   Remember, if you have a request or suggestion of your own for either an article topic or circuit, please e-mail:


   Last month we tackled some push-pull amplifiers that did not look like push-pull amplifiers: the SRPP, the White Cathode Follower, and finally, the strange looking Circular/Bridge/Balanced amplifier. We also promised that this month we would cover in greater detail this altogether-too-confusing-for-most-tube-fans, power amplifier topology.
   Just as before beginning weight training, a good stretch is needed, so too before understanding a new, strange looking topology, a certain mental limberness is required. Let us begin with the word "circuit" and its meaning. A circuit is something that completes itself in the same way a circle does. In electronics, it is not enough to have all the pieces of wire and components soldered together, there must also a flow of current. A battery with a resistor connecting across it qualifies as a circuit, as a current flows through the battery into the resistor and then back into the battery, etc.   

Illustration A

Illustration B

   In illustration A above, a circuit is realized by the flow of current from the battery's negative terminal to its positive through a diode, a lamp, a resistor, a fuse. In illustration B above, a circuit is also realized by the flow of current from the battery's negative terminal to its positive through  same components, although differently arranged. It is as if a necklace had been created out of electronic parts. (Understand, the string was not the wire in between the parts for the wire itself is a device; the string is the current.) Notice that if the diode's orientation had been not preserved, no current would flow, but that the other devices could be flipped without effecting the current flow. Some devices have a polarity that must be observed, if current is to flow through them, such as diodes, batteries, triodes, MOSFETs, some relays, and transistors; while others conduct regardless of orientation, such as resistors, fuses, tube heaters, chokes, wire, lamps, and even some FETs.

In This Issue


Push-Pull Amplifiers, Part 2
Tube Voltage Regulators, Part 2
Balanced Phono Stage
Tube Shunt Regulator   
Publishing Information
Glossary of Audio Terms 

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