This
Issue

Circular
Amplifiers -Part 4-

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:

Editor

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.

In This
Issue

1
6
11
13

Circular Amplifier: Class A?
Vacuum
Tube Plate Curves -Part 1-
Simplest
Tube Amplifier Possible
E-mail
Publishing
Information
Glossary
of Audio Terms

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