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Class A amplifiers are primarily the audio circuits that I work with. I have not tested, worked with, or listened to any Class D audio amplifiers. I have avoided Class D amplifiers because of a previous experience in radio broadcasting with PDM modulation that was introduced in 1975. As the transmitters aged, the sound quality degraded, probably due to component value changes with age. The system in 1975 used 70 KHz switching. Current PDM systems use much higher switching frequencies. With the higher frequencies, PDM circuit boards are subject to effects common with RF circuits. PDM circuits are prone to anomalies that can degrade performance.
Class D Mechanisms:
The incoming audio signal is converted into a series of very fast high-frequency on-off pulses. The pulses are all the same amplitude,
but their width determines how long they stay on and changes to match the original audio signal. The amplifier's power transistors
act like switches, rapidly turning fully on or fully off in sync with the pulses. On the output a special filter, usually made of
coils and capacitors, smooths out the rapid pulses. The high-frequency parts are removed, leaving behind the original audio waveform.
Class D amplifiers are much more efficient than Class A, capable of delivering a power output of 200 watts a channel or more while
drawing less utility power.
Reading about the process of the input signal of a Class D amplifier being converted to high-frequency pulses and an output stage operating as on-off switches, I have to wonder if the high-frequency switching and simulated switched analog output are generating a potpourri of harmonics, and are any of those harmonics leaking through the output filter?
Apparently there are problems related to Class D amplifiers. In particular, the high switching frequency and circuit board layout. Switching frequencies above 100 KHz will be more susceptible to circuit design and circuit board layout flaws. Such high frequencies require engineering similar to RF transmitter design with careful attention to circuit layout. You can read Rod Elliott's page on the subject. Rod has done extensive testing of PDM topology.
Class A Mechanisms:
Class A amplifiers provide linearity and low signal distortion, are simple and stable, and, for the DIY builder, are easy to work with.
They provide very good signal reproduction as the operating point is halfway between cut-off and saturation. They offer a broad and flat
frequency response.
About four years ago I became interested in low-power audio based on the notion that one should not have ear-damaging power in an audio system. That is, of course, assuming one values keeping their hearing in prime condition for years to come. For low-power systems, Class A amplifiers are a good match. At lower power levels, relatively speaking, the low efficiency of Class A is not a big issue.
Russell O Hamm paper, tubes versus transistors Loudspeaker distortion Selecting capacitors for the best performance Amplifier damping factor Push-pull balance - W T Cocking Cathode follower output Half-watt 6SN7 headphone amplifier
last update 07/06/2025
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