Audio power amplifiers have a lot of class. Well, they have at least six classes. Let’s look at some of these so we can have the best audio amp design architecture for our needs.
Class A: The “audiophile” amp
When I was an engineer at Empire Scientific in the early 70s, we had a couple of people who were “Golden Ears” — -they had an exceptional talent for discerning good sound from great sound and could hear very low amounts of distortion present in the music. Class A power amps have the lowest distortion, best linearity, and low noise, with the caveat that it is the most inefficient audio power amp architecture. Don’t even think about using it above 25 W, as it only has a 10% to 30% efficiency — -you can make toast on it. This amplifier is conducting 100% of the time.
I had to go back to a 1966 article (Reference 1) to find a use for the Class A amp. Back then, solid-state TVs using line cords, typically used a 65/70V power supply, which was considered ‘low’, to design a Class A power amp to provide a 0.75W or higher power to the speaker, using low voltage transistors at low cost (I guess everything is relative to the 60s here) See Figure 1 for such a Class A amplifier circuit.
Now that’s a heater for your home!
Class A amps nowadays may be useful in something like a headphone pre-amp.
Class B: Better efficiency
In this design, from 1965, we have transistors conducting on the positive side of the audio input waveform and another set of transistors conducting on the negative side of the waveform: a push-pull architecture. Class B amps can provide up to 75% efficiency. One little drawback here is the crossover distortion that the “Golden Ears”, looking for high-end audio amp performance, will be able to hear.
In the next couple of parts to this series I will be discussing Class AB, Class G and H, and Class D. I will also discuss my experience with the Crown DC 300 pre-amplifier with a Phase Linear 700 power amp that we used at Empire Scientific in the 70s to power Empire speakers at audio shows, using an Empire phono cartridge and Empire turntable with records.
1 Low-cost power amplifiers with silicon transistors, Dwight V. Jones, GE Company, IEEE Transactions on Audio and Electroacoustics, Vol. AU-14, №1
2 Thermal and Power considerations in Class B transistorized amplifiers, Gideon F. Inbar, MB Electronics, IEEE Transactions on Audio, Vol. AU-13, №4, 1965