Why don't Aragon, Bryston, Rotel, Parasound, Proceed, etc make Class G amps?

[Ricky]

Does anyone beside me wonder why just about every major amp manufacturer doesn't use Class G designs in their amps? Most amps are A/B with varying bias in pure Class A. These new Outlaw/NHT 200 watt monoblocks go from A/B to G at 80 watts. So above 80 watts, you are listening to G. If Class G sounded better than A/B, wouldn't companies like Bryston, Aragon, Classe, etc implement it? Some of their amps retail $800-1000+ PER channel, so sound quality is key for survival of the fittest...yet none of their amps have Class G designs.

[Lex]

I guess we need a class G explaination, I am personally not familiar with it.

Lex




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[Ricky]

Lex,

Class G seems like another way to implement A/B sound.

Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers are single-ended designs with only one type polarityoutput devices. Class A is the most inefficient of all power amplifier designs, averaging only around 20%. Because of this, class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power.The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.

Class AB operation allows both devices to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demands. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.

Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs.

[David Meek]

Ricky,

Thanks for the concise explanation. My question is: Are the Class G amps prone to higher NOTICABLE distortion?




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[Lex]

I'll go out on a limb here and say:

Odds are pretty good that if pro audio manufacturers are going to class G, then class G could be a dirty way to generate lots of power. Not necessarily dirty, but not as pure as Class A, Class AB, but less expensive.

High end manufacturers may have no desire to change, because they know there are sonic compromises to be made. Maybe...

Only speculating. Jon, what's your take on this?

Lex




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[Kevin P]

What Lex said.

When I read the description of "Class G" I was thinking, wow, lots of added noise and distortion generating switching circuits. Sounds like a good idea for pro amps where they need to handle nasty peaks but it doesn't sound as pure as class AB amps.




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[dsmith]

Ricky, you may find this webpage of interest. This is at Doug Self's audio site. Doug is a fairly well known (in audio circles) British eccentric with a solid background in audio electronics, as you will see if you explore his site.

Don

http://www.dself.demon.co.uk/ampins/g.htm

[Kevin_McC]

What class are Carver amps? Your description of Class G brought Carver to mind for some reason.

[JonMarsh]

The biggest advantage, IMO, to CLASS G, is the reduction of power dissipation at lower power levels in the output stage, and the ability to use smaller, ligher, less expensive heatsinks. It does require switching voltage levels.

The worst case power dissipation in the amplifier and for the heatinsks in an AB amplifier with low bias occurs at about 35-40% of maximum output power. If you can use a multi-level or switched level power supply, then the voltage accross the output devices is greatly reduced, and hence, the power dissipation.

Class G and Class H are pretty similar in their ultimate effect, and also in that they can introduce small switching distortions. Now, these may not be very audible, becuase they occur mostly at high signal levels, unlike crossover switching distortion, which occurs at low signal levels (zero crossing).

So, you know, as an older curmudgeonly kind of guy, (but also an ex musician and pro sound guy), I have to make a few silly statements about "high end" home amps

"Real men" don't use switched voltage level power supplies or other tricks to cut costs in heatinks.

"Real men" use heavy duty torioidal transformers and large, low ESR and ESL capacitors, because of the high short term (transient) output capaibilty in relation to their thermal limitations (unlike switching power supplies). :B

"Real men" like big heat sinks that get quite warm from high levels of Class A bias, because they can heat their cabin in Colorado or Winnipeg by listening to music! :LOL:

"Real men" don't use global feedback in power amplifiers... just ask Neil, Charlie, JC, or Victor Khomenko. :W

"Real men" don't care what other folks do, and listen to what pleases them.. b 8O


-Jon




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