Class G Amplification

[DecibleLvr]

What are the merits of Class G Amplification? How does it compare to, lets say, Class A Amplifiers?

[lynnm]

I have an old Hitachi SR-2004 receiver which is set up for class G operation.

Basically the power amplifier section consists of a main power amp rated at 200 w/channel. There is some sort of circuit that kicks in a second set of power amps should the power demand exceed 200 watts. The second stage increases power o/p capability to 400 watts/channel.

I cannot state categorically what class the main power amp operates in but my best guess is that it is works in AB mode.

I used it with my Pioneer HPM 200's which while excellent speakers are very inefficient and that combo has one hello of a lot of punch. Once I bought the KLF30's I went to a decent 30 w/ch. tube amp and find the sound more natural.

If anyone is interested in some vintage POWER!! I still have the receiver available for sale or exchange for toys

[Ray Garrison]

First, note that while class A, AB, B, C, and (I think) D are recognized as defined, standard classifications and mean the same thing no matter which manufacturer is using the term, other classes (H, G, K, EIEIO) are not so universally defined, and someone's class "G" amp might not be operating in the same "class" as someone else's class "G" amp.

Having said that...

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 B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.

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 AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.

Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off they do not theoretically dissipate any power. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.

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.

Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. The efficiency of class H is comparable to class G designs.

[kenratboy]

Nice definitions, I have never found a complation like that. I be mored smarted now than before.

[djk]

"Because of this, class A amplifiers are single-ended designs with only one type polarityoutput devices"

Actually, most class A amplifiers use both polarity types. See DIY references below.

"class A designs are inherently the most linear, with the least amount of distortion"

An urban legend, cannot be proven with test equipment. See the Douglas Self book below.

"Class AB operation...the inherent non-linearity of class B designs is eliminated"

"Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio."

More urban legend. The 'Blameless Class B' amplifier from Self clearly demonstrates this. For those of you with the 'show me, I'm from Missouri' mindset: buy the circuit boards for the Trimodal amplifier. You can change from class A to AB to B at the flip of a switch.

http://www.signaltransfer.freeuk.com/trimodal.htm

http://www.rbookshop.com/engineering/a/Acoustics/Audio_Power_Amplifier_Design_Handbook_075064527X.htm

Those actually interested in DIY amplifiers should check of the PASS site:

http://www.passdiy.com/legacy.htm

Of interest might be:

Construct A Class-A Amplifier:(Audio, February 1977 by Nelson Pass)

A 20W class A design with all NPN outputs.

The Pass A-40 Power Amplifier:(The Audio Amateur, 4/1978 by Nelson Pass)

A 40W class A design with both PNP and NPN outputs. The circuit board for this is still available from AudioXpress for $6 !

Zen: (by Nelson Pass, and several follow ups in TAA over the years)

A 10W class A MOSFET design using both P and N channel IRF HexFets. Variations with all N channel devices. Variations with a lightbulb replacing the current source device.

[Curmudgeon]

I've got a couple of units laying around here that use that type of amplification. Both are early 80's vintage Yamaha pieces: a R-1000 receiver and a B-6 amplifier (pyramid shaped). These amps both use a high-speed comparator circuit to monitor the audio signal and switch to high power output when musical transients call for it. The comparator circuit switches to high power just slightly before transient needs it and leaves the power up for about a half a second afterwards so that the sound is seamless. The upside is great dynamic headroom (3 Db) so that the amp won't clip unless really pushed hard and the physical size of the amp is small. The downside is the amp doesn't like low impedance loads and will get hot if listened to at loud levels for a long period of time. Overall they are nice sounding units when matched with appropriate speakers. The amp design is similar to the Carver magnetic field amplifiers of that time period.

[TheEAR]

What are the merits of Class G,it hits the G spot fast! LOL

And who knows the famous class H by Poonasonic(Technics),I know it and the receivers sounded horrible at best.Grainy as sand itself and as pure as as the sewers.

The highest quality amplifiers are class A or AB,very few non class A/AB amps sound good.Many fancy hocus bogus amps came from Japan and most sounded real BAD.

Who will come up with Class Z?

[kenratboy]

Well, car audio is THE field for efficent amps. Min input -> max output, oh, and screw sound quality!

I mean, I put a $200 Sony head unit in my car, and it puts out over 200 watts (really, not fake specs) and, well, if that was class A or AB, I would need a 1000 amp alternator and a nuclear reactor cooling system

[chickey]

The later Carver designs, like the Carver Research Lightstar (which I run with my Chorus II's) used a tracking power supply, but run class A at the outputs. The tracking power supply overcame the problems with low impedance loads that had plagued earlier generation amps based on this principle. This design is also what's incorporated into the Sunfire amps and subwoofers. In the case of the latter, it's what provides such prodigious output from such a small footprint.

C

[kenratboy]

Imagine a subwoofer bursting into flames