From: DPierce@world.std.com (Richard D Pierce)
Subject: Mr. Pierce responds to Re: Class C amplifiers, says Mr. Pierce.
Organization: The World Public Access UNIX, Brookline, MA
Date: Thu, 17 Dec 1992 14:58:44 GMT

In article <1goubrINNlf5@uwm.edu> randyd@csd4.csd.uwm.edu (Randall Elton Ding) writes:
>In article <41484@unix.SRI.COM> henry@ginger.sri.com (Henry A. Pasternack) writes:
>>
>>   In a recent posting, Dick Pierce referred to Class C amplifiers.
>>
>>   I'm curious to know the brands and model numbers of some of the
>>more popular Class C audio amps.
>>
>
>What?  If I recall correctly, Mr Pierce said class C amps are NOT used
>for audio.
>
>I worked on a class C amp in the military, it was an old tube type sonar
>transmitter with big 4PR60 tubes for the output stage.  The tubes were biased
>so high that only the top 1/3 of the waveform was amplified.  You could
>get away with this type of biasing since the amp was only designed to
>amplify a single frequency, 8KHz I think.
>
>There is no way you can use a class C amp for audio, period.
>And why would you want to.

Did I say that? Really? REALLY?

No, I don't think so!

One way of viwing what amplifier "class" means is through the concept of
"conduction angle." This is the phase angle through which one side or the
other is turned on, active, conducting, whatever. It describes how much of
the full waveform a particular output device is handling. 

Class A has a conduction angle of 360 degress, that is, for the entire
waveform. Class AB has a conduction angle ranging from less than 360 to
greater than 180 degrees, that is for less than the full but more than
half the waveform. Class B is a conduction angle of 180 degrees, precisely
half the waveform. And Class C is less than 180 degrees.

The class of operation is determined by (among other things) the bias
level set for the output devices. The class also determines how much power
the amp must dissipate at idle. A simple gedanken will reveal an ideal
Class A amplifier using perfect output devices will be dissipating a
minimum of 50% of its rated power at idle (and often more in reality). A
Class B amplifier will be dissipating 0% at idle (since with 0 signal,
the output devices will be off, but will start to conduct as soon as there
is even an infinitesimal signal). Class AB will be somewhere in between
(typically, about 40% in reality). Class C will dissipate no power until
the signal reaches the poiint where the output devices start conducting,
thus will be dissipating 0% of it's power for "outputs" of 0 to some level
where the outputs start conducting.

Now, if you go out and design yourself a Class B amplifier with completely
linear devices in a perfect world, everything is OK. You have the
distinct advantage that you have an amplifier whose output is "linear,"
but you have essentially no idle dissipation. So you can get away with
smaller heat sinks, lower ratings on components, smaller supply, and so
forth. In otherwords, you can get more power for less money.

But if you use non-linear devices (like tubes or semiconductors), and the
devices have characteristics which are temperature sensitive and couple
them with less than perfect components which are also temperature
sensitive, whose to say that your perfect Class B amplifier doesn't
occasionally wander into Class C operation. No, no audio amplifier may
have been designed as Class C, but if it's SUPPOSED to be Class B, I'll
bet you it runs Class C much of the time. 

The early Phase Linear amps were designed as Class B. But simple
temperature effects, biasing errors, component drift and so forth
basically gauranteed that they seldom ran pure Class B. Most that I saw
ran sloppy enough that the suffered from horrible crossover discontinuity
that is the hallmark of marginal Class C operation. This lead to their
simultaneously poor sound and poor measurements at low power.




-- 
|                Dick Pierce                |
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