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Class Act:
The Pros and Cons of Amplifier Design
by John Roberts
Many manufacturers would like you to believe that there is
only one "best" way to design a power amp. These companies
usually make only one type of amplifier. Perhaps they believe
their own hype, or more likely, they are limited to a single
in-house technology or by a specific target market. In the
course of designing power amplifiers for the different markets
we serve, Peavey has developed products using radically different
technologies. I will attempt to give an overview with specific
pros and cons of the different approaches.
For convenience, engineers usually characterize amplifiers
by circuit topology, the type of active components they use,
load type, and even operating voltage: CLASS A, B, C, D, etc.
Circuit topology describes how current is "steered," or controlled,
within a power amplifier before it is delivered to a speaker
load.
Class A is the simplest, most basic topology. Reproduction
of music requires speaker motion both in and out. To do this,
amplifiers must "source and sink" current. In a Class A amp
only one direction of current control is used. To generate
both directions of output flow, a constant current stage is
subtracted from a variable current stage.
Pros: Since neither output stage ever turns
off, device non-linearity and turn-on/turn-off time can be
minimized or ignored, resulting in very low distortion designs.
Cons: As the maximum output is limited to
the constant current stage, at idle this stage must put out
full power and the variable stage must absorb this full power.
Transformer, heat sink, and output stage must be sized for
continuous duty at maximum power. Because of cost and the amount
of waste heat generated by this approach, Class A only appeals
to esoteric hi-fi designers, where lack of efficiency or price
is no object.
Class B topology uses two variable output stages, one to source
current and the other to sink current.
Pros: This topology overcomes the poor efficiency
of pure Class A designs, only delivering power as needed. Transformer
and heatsinks can be sized to match typical demands of music
being reproduced.
Cons: Both output stages turn completely off,
then on again, during each cycle of a waveform. Time delay
and low-level non-linearities cause severe distortion, called "crossover
distortion," during transition from source to sink output stages.
This type of distortion is worst at low output levels. Pure
Class B is only used in the lowest-cost, lowest-fidelity designs.
Class A/B topology, as you may have guessed, is a combination
of Class A and Class B. Using two variable output stages like
Class B but keeping them from ever completely turning off,
you get near Class B efficiency with near Class A's low-distortion
performance.
Class C topology combines active devices with resonant magnetic
components for high efficiency at radio frequencies. This topology
is not used in audio-frequency designs.
Class D topology uses source and sink output stages that consist
of full-on or full-off switches. These output stages toggle
from full sink to full source at a rate significantly higher
than the highest audio frequency to be reproduced. The ratio
of time sinking to time sourcing controls the audio output,
with a 50% ratio delivering zero output.
Pros: Class D offers significantly higher
efficiency than even Class B, which at 1/3 power is wasting
more power inside the amplifier than it delivers to the load.
Losses in Class D designs are limited to turn-on time of the
switching devices and resistive losses in these devices and
output filtering.
Cons: Class D amps require more complex circuit
designs with extensive shielding and filtering.
Class G & Class H topologies are variations on Class B that
use multiple source and sink output stages. Low-level signals
are handled by one pair of output stages, while higher-level
signals are handled by other pairs. Each pair is optimized
for the power range it delivers.
Pros: More efficient amplifiers can deliver
the same output power with smaller transformers and less heat
sink.
Cons: Circuit complexity increases, which
adds cost. Switching distortion similar to Class B's crossover
distortion occurs at each output level transition.
Bridge Mode takes advantage of the fact that speaker loads
can be driven differentially. Using separate amplifiers to
drive both the positive and negative speaker terminals with
opposite-polarity waveforms yields an effective doubling of
the voltage swing for 4 times the power. It could be argued
that this isn't actually a topology, as each amplifier can
be any of the previously mentioned topologies; however, it
warrants discussion.
Pros: Provides high power levels using lower-voltage
components.
Cons: Increased circuit cost/complexity and
inability to ground reference either speaker lead.
Transformers are often used in the output circuit of audio
amplifiers to match the "real world load" to an impedance or
voltage swing that is more comfortable for the amplifier. Step-down
transformers are used with vacuum-tube amplifiers to match
the large voltage swing and high output impedance of tube circuits
down to speaker levels. Step up transformers are used to generate
70V and 100V constant voltage outputs used in the fixed sound/background
music industry.
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