2. amps-preamps
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what is diff: Class "A", "A/B", and "B"

I am confused as to the meaning of the various categories of amps/pre-amps. Is a class A necessarily better than a class a/b or c etc. Would someone care to briefly list some of the brands that fall in these categories and why?
I know, it's a small question.
thanks,
jb
joeb
zaikesman
3,017 posts
 
No, if I were a betting man, I wouldn't put money on any of these answers. Why do I get the feeling class of operation is being confused with single-ended vs. push-pull? Where is any mention of bias level? What is this stuff about "adjusting power envelopes" and "gain levels" and "switching"? No, no, no. I'd bet none of this is really correct (if I were a betting man), just more confusing. I can feel it in my gut! Well intended, yes, and touching on some valid points, but nowhere a fundamentally sound, reasonably comprehensive answer to the technical question asked. Like where a sine wave ends up relative to a zero-crossing line on a graph depending on its amplitude, and what that implies. Or how output devices operate. And what is non-linearity. But I am not qualified to explain it all either, so I'll wait for some EE to come along and set things straight. Or surely there must exist some good links for this stuff?
vantageaudio
182 posts
 

Solid state devices (ie: output transistor) work by using the base current to control the collector current. Vacuum tube devices (ie: power tube) work by converting a voltage input into an output current.

With respect to SOLID STATE devices:

CLASS A bias means the device base current is set to allow collector current to flow through the device. This basically means there is current flow at all times, regardless of signal input.

CLASS B bias means the device base current is set to zero and therefore NO collector current flows through the device and the device is cut-off. When an input signal is present the base current will lift and allow collector current to flow and the device is now conducting.

With respect to VACUUM TUBE devices:

CLASS A bias means the grid voltage is set to allow anode current to flow at all times.

CLASS B bias means the the bias point has been set to a point where the tube has almost stopped conducting. This results in a larger input signal being required to drive the tube and an output current for only half of the waveform. To amplify each half of the waveform it is necessary to use 2 tube devices in a "push pull" arrangement, with one device taking care of one half of the waveform and the other device taking care of the the other half.

CLASS AB bias is somewhere in between the above 2 classes with usually a small amount of bias applied

The subject needs more than a quick simple explanation to understand. I hope the above reads OK and I have not further confused anyone..??

Regards, Richard.

supertrain_196038ed
7 posts
 
I have a reciever with a class "G" rating. What does a class "G" do differently from "A"-"A/B"-ect? I would appreciate any info.
twl
3,251 posts
 
Sorry you guys didn't like my "power envelope" description.
twl
3,251 posts
 
Amplifier Design Principles

This section will detail design philosophies of amplifiers and the various classes of amplifiers applied in audio. For example:

Class A - transistor amp conducts for the entire cycle of input signal, conduction angle 360 deg. Runs hot, as the transistors in the power amp are on all the time, but has high sound quality.

Class B - positive and negative halves of the signal dealt with by different parts of the circuit, the output devices switching continually. Runs cooler, but the sound is not as pure.

Class AB - biasing the transistor amp at a non-zero DC current much smaller than the peek current of the signal source. Second transistor conducts during negative half cycle of waveform and the currents from the 2 transistors are combined at the load. A compromise between sound quality of Class A and efficiency of Class B. Most amp designs employ this method.

This is a direct copy from the Audioholics web site.
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