ABLinear output
Class AB
The output devices remain active through most of the waveform, combining aspects of Class A and B operation.
- Strength
- Mature, widely used designs
- Plan for
- More heat, weight and rack ventilation
Amplifier knowledge
Output stages, clearly explained
Class AB, D, G & H
The letter on the specification sheet describes how an amplifier delivers power—not how good it will sound. Here is what each class changes in a real cinema system.
At a glance
Four approaches to the same job
All four can be engineered to a very high standard. Their most obvious differences are efficiency, heat and packaging.
DSwitching output
Class D
The output stage switches rapidly and an output filter reconstructs the audio waveform with very low power loss.
- Strength
- High efficiency and channel density
- Plan for
- Quality of the module, filter and implementation
GStepped supply rails
Class G
A linear output stage works from lower supply rails, switching to higher rails when the signal needs more voltage.
- Strength
- Less wasted power than conventional AB
- Plan for
- Additional supply and control complexity
HTracking supply rails
Class H
The supply voltage changes with signal demand so the linear output stage sees only the headroom it needs.
- Strength
- High output with reduced dissipation
- Plan for
- Design complexity; naming varies by maker
A note on G and H: manufacturers do not always use these labels in exactly the same way. Treat the published topology as a starting point and assess the finished amplifier on its measured and specified performance.
Practical comparison
What changes in the room and rack?
The topology becomes relevant when it affects ventilation, circuit capacity, physical space or the number of channels you need to install.
| Class | Efficiency | Heat | Typical size | Strongest use case |
|---|---|---|---|---|
| AB | Moderate | Higher | Larger / heavier | Dedicated channels where space and cooling are available |
| D | High | Lower | Compact | High channel counts and space-conscious cinema racks |
| G | Moderate to high | Lower than conventional AB | Varies | Powerful linear designs with improved efficiency |
| H | Moderate to high | Lower than conventional AB | Varies | High-output designs that manage rail voltage dynamically |
These are useful tendencies, not performance rankings. Actual heat, weight and output depend on the individual design, power supply, channel count and operating conditions.
Choose for the system
Start with the installation—not a favourite class
- 01
Define the loudspeaker load
Use sensitivity, impedance behaviour, listening distance and target level to establish the real power requirement.
- 02
Count the channels properly
Separate the demanding LCR channels from surrounds and overheads. A mixed amplifier strategy can be the most rational solution.
- 03
Design the rack and power
Check ventilation, available rack units, weight, circuit capacity and how several channels behave when driven together.
- 04
Compare the complete specification
Look beyond headline watts to noise, gain, load stability, protection, connectivity, warranty and service support.
Our view
Topology is a design choice, not a quality badge.
Modern Class D can be exceptionally quiet, powerful and controlled. A well-executed Class AB, G or H amplifier can be equally convincing. The right answer is the amplifier that meets the loudspeaker, channel, thermal and service requirements of your system with sensible reserve.