Class A

[wheelema]

This term (class A) has got to be one of the most devalued marketing terms in the world. I have seen this used to describe gear costing from several hundred to several thousand dollars. Never seen anyone boast that their gear is 'Class B' (Behringer?) or 'Class C' (Nady?) however.

Does anyone know where it originated, how it was intended to be used, and whether or not just anyone can use it or are there standards that must be met?

 

[Massive Master]

It has nothing to do with marketing - It's a circuitry designation.

There's a lot to it - This was the easy one...

http://sound.westhost.com/class-a.htm


* Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1
* Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)
* Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.
* Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! ** This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF)
* Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - because of frequency limitations (and the fact that they nearly all seem to sound disgusting), many are only suitable for industrial control of motors and loud but crappy sub-woofers (this may change if transistors with an infinite bandwidth become available soon - yeah, right!) All Class-D amps have a major limitation in the output filter, whose response is highly dependent on the load impedance.

 

[boingoman]

In a class A device, the whole signal is reproduced by output device(s) that never turn off as the signal swings from positive to negative, and reproduce the whole signal.

In other classes, there are separate output devices (tubes, transisitors) for each half of the cycle. So in a non-class A guitar amp, one tube handles the positive side, one handles the negative. Each tube switches off when it is not used. The unit can run cooler, as the output devices draw no current when no signal is present.

One side-effect of non-class A operation is that at the point where the signal crosses from one device to the other (somewhere around 0 volts), there is a bit of distortion. This is called crossover distortion. Not necessarily a bad thing, but it is there.

This problem does not occur in class A, as the output device(s) handle the whole signal.

Class A runs hotter, as the output devices never switch off.

Class A is not a marketing term, but it sure has been used to sell a lot of gear.

 

[Massive Master]

Class A runs hotter, as the output devices never switch off.

My Bryston puts out over 1300 BTU's per hour. In idle. It's a friggin' pizza warmer.

 

[krimson]

My Bryston puts out over 1300 BTU's per hour. In idle. It's a friggin' pizza warmer.

I love my Bryston! It's a great room heater on those cold winter nights.