The Circular/Balanced/Bridge amplifier is looked at for the last time...well, at least for the next few months. This time we debunk the myth that this circuit is something mysterious, radically different from all other amplifiers, defying both reason and math, and that only the chosen few can understand how it works. ("It must be Class A; all the ads say so.")
   This month's Design Idea was inspired by Italian audio circuit designer Andrea Ciuffoli's simple unity gain amplifiers. Just what would be the simplest amplifier possible? Take a look at page 11 and you will find one answer.
   Reader Ian has 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).
   In response to the pleas that we make the typeface larger in the Webzine, we have increased the point size to 12 pt.
   Remember, if you have a request or suggestion of your own for either an article topic or circuit, please e-mail:

Class A vs. Advertising
   Audio equipment has had a long history of fluffing in ads. Remember the cheap, small speakers that claimed to have a frequency response of 20-20kHz and $200 receivers that boasted 200 watts of peak music power when in fact they put out only 20 RMS watts? It became so ridiculous that in the U.S. the Federal Trade Commission (FTC) had to step in and restrict the fluffing of power wattage in advertising. While its rulings were off base in some regards (one hour of continuous output at the rated output wattage is excessive and places an unrealistically low output rating on tube OTL amplifiers, for example), something was needed to contain the marketing department's exuberance. But with high-end audio products, self policing, by competitor's rebuttal ads if nothing else, has been really the only constraint in place. Remember the coupling capacitor ad wars? Claims about an amplifier's output stage mode of operation and classification, however,  are much harder to verify and thus to police.

What is Class A?
   Amplifiers can consist of one just one active device (such as a sole vacuum tube, transistor, or a MOSFET) or many such devices. In the case of single device (single ended) amplifiers, almost all are run in Class A mode, which means that the device constantly conducts current throughout the entire waveform being amplified. If the device stops conducting during any part of the waveform, the output signal goes flat and it is no longer operating in pure Class A.

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