Operational Amplifier – How to Make a Bad Audio Amp


After discovering how great op amps are, and that some extremely good ones–especially at low power levels–are available at reasonable prices, I wonder why all audio amplifiers, big or small, don't achieve excellent performance by simply combining a good small-signal op amp with a simple output stage.

I mean, with opamp there is no need to worry about all these bias voltages & temperature stability, just stick opamp and any unmatched darlington transistors, and you are good to go.

Any pitfalls?

Best Answer

The use of opamps in amplifiers can drastically simplify their design, but opamps aren't perfect. If they would have infinite amplification over their full bandwidth they would tend to oscillate, so they are internally compensated, which limits their bandwidth. A limited bandwidth makes the amplifier prone to Transient Intermodulation Distortion (TIM), a type of distortion much more annoying than harmonic distortion (HD).

The reason only HD is published, and TIM never is, is that it's much easier to get good-looking HD figures. Who wouldn't be impressed by a figure like 0.01% harmonic distortion? Most customers don't realize that this figure is totally irrelevant because the total system's distortion is for the most part determined by the speakers, which easily add a few percent distortion.

The power stage isn't without its problems either. Class A amplifiers are hardly used because of their low efficiency. Class B or AB amplifiers have a crossover distortion where one transistor takes over from the other. This is a non-linear distortion which can't be compensated by feedback. May not be true. If someone can enlighten here I'd love to hear it..

A final quote on opamps:

"There is no such thing as an unconditionally stable op amp unless it lies on the table with power disconnected" [1]

Further reading
[1] Intersil appnote AN9415: Feedback, Op Amps and Compensation