Min or max?

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Now that we’re all amplifier designers, building our mythical solid state class AB amplifier, let’s get down to brass tacks. (For what it’s worth “getting down to brass tacks” at one time referred to the 19th-century American practice of using brass tacks to spell out the initials of the deceased on the top of their coffin.)

We know that the basis of the class AB design is to keep both output transistors always on for small signals. The question we will want to answer is how much? In other words, do we want the first 1/4 watt, 1/2 watt, or full watt to be class A? More? Less? And, how do we make that determination?

The minimum class A-ness is easy. We need enough current to eliminate the crossover notch distortion I showed you in this post. Typically that’s not a lot, and the small signals that operate in class A are small indeed. So, what’s going to determine how much higher we go?

In the BHK amplifier, for example, the first watt is class A. That might not sound like a lot, but think again. The fact the BHK is a relatively big amplifier—500 watts per channel into 4Ω—means that 1 watt of class A generates a lot of heat. If the same amp were 1/10th the output wattage—50 watts into 4Ω—we would have considerably less heat generated from that single class A watt.

But, here’s the thing. On an average efficiency loudspeaker, most music is not going to need much more than 1 watt. Thus a lot of the quiet to medium-loudness passages will be through a class A amplifier.

This is all fine and desirable if you’re building a big and powerful amplifier with massive heat sinks enjoyed by the BHK. If your design budget doesn’t allow for that robust of a chassis, then you have to travel down the path of compromise.

Don’t despair.

Engineering at any price point, from the ultra-expensive to the beer-budget class, is all about compromise.

The trick is in knowing what parameters to employ in making those trade-offs.

We’ll wrap up tomorrow.


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