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Old 8th November 2002, 11:51 AM   #1
MGR is offline MGR
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Default Class G amps

Has anyone built the class G design of Douglas Self or a Class H amp in the QSC schematic site?I would like to know if they sound good.Almost all high power amps are either class G or classH.
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Old 8th November 2002, 06:15 PM   #2
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The NAD208 is a superbly designed class G amplifier.
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Old 8th November 2002, 06:26 PM   #3
halojoy is offline halojoy  Sweden
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Default Excuse me.

But what is a Class G amplifier.
I know only of A, B and some about H.

What ever happend to the C D E F - amplifiers?
And when will I and J amplifiers be introduced?

Does anybody know...
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Old 8th November 2002, 07:07 PM   #4
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class C: Each transistor conducts less than half a cycle. Since this
gives rather extreme distorsion it is not useful for audio, but is
very efficient and obviously useful for radio.

class D: Pulse width modulation, ie. very different from the other
classes. There were some class D amplifiers back in the 70s, but
I don't think they made much success then. There are some class
D kits around for audio, eg. from LC audio and Zetex, and I
understand that it is quite common in car stereo (for efficiency
reasons) and subwoofers (efficiency, plus distorsion figures are
much better for low frequencies). Class D is very efficient since
the transistors work in switching mode.

class E: Don't really know what it is, but is said to be a method
to reduce power dissipation and is only used for radio.

class G: Two class B amps in parallel connected two rails of
different voltages. The one with higher rail voltages is only used
for high power and is otherwise on standby.

class H: Varies the supply voltage according to power need.
Not sure how or why. Presumably the purpose is to increase
efficiency, so I suppose a switching supply with variable output
is used, but I'm only guessing here.

class S: Said to be a low-power class A amp driving a load which
is backed up with a class B stage, the latter making the load
appear to have a higher impedance. Don't think I understand
quite what this means.

(Source for those classes I didn't already know about is
Randy Sloanes power amp book)
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Old 8th November 2002, 07:11 PM   #5
halojoy is offline halojoy  Sweden
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Default Thanks, Crister

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Old 8th November 2002, 07:11 PM   #6
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Default amp classes

FYI - a cool chip to try class-h out with is the Philips TDA1562Q. The app note shows the entire circuit. You can make a 70 watt amp with only a 12v input, with no switching power supply.


I started this from memory, then decided to look it up. These definitions are from Audiolab's glossary

Class H amplifier - 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.

Class G amplifier - 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 D amplifier - 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 AB plus B amplifier - 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 AB amplifier - 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 B amplifier - 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 A amplifier - 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.

Matt
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Old 8th November 2002, 07:16 PM   #7
BrianGT is offline BrianGT  United States
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Douglas Self's book has a full chapter on Class G:
http://www.dself.demon.co.uk/book.htm

It if a pretty informative book.

He also has a small section of his site on Class G:
http://www.dself.demon.co.uk/ampins/g.htm

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Old 8th November 2002, 07:50 PM   #8
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There seems to be some problem with this description
of Class S. Anybody seen the IEEE definition?
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Old 8th November 2002, 08:10 PM   #9
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I found that and some similar ones, but also a non-IEEE
reference to a Class A and Class B.

The description bears a strong resemblance to a Stasis
amp, which was patent # 4,107,619 which describes
a Class A stage connected directly to the load, but
current bootrapped by an external amplifier, typically but
not limited to a Class B or AB amplifier.

woops! looks like the comment I was responding to
disappeared.....

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Old 8th November 2002, 08:15 PM   #10
BrianGT is offline BrianGT  United States
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I was having posting problems with uploading the file in ascii mode instead of binary. It should work now:

http://brian.prohosting.com/class_s.pdf

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