Big decisions

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There are plenty of decisions when you’re designing an amplifier. Some are pretty basic: wattage, amplifying devices, target costs. Others can get more detailed: class of operation, heat evacuation, cost.

In our mini series, we’ll imagine we’ve made those decisions and wound up tasked with building a class AB amplifier. We’ve further decided it’ll be a classic solid state design of 100 watts per channel into a 4Ω load, and our goal is to make the best sounding amplifier we know how. Time to roll up the design sleeves and put pen to paper. First, let’s make sure we understand what we’re doing.

The best way to think about a power amplifier is to divide it in half. The first half is the voltage amplifier, the second half is the driver and power stage. To understand better, here’s a simple image. Think of the first stage—labeled voltage gain—as the preamp where a smaller signal is made bigger. The gain of this first stage is typically 30, which means for 1 volt in, you get 30 volts out.

I like to imagine this first stage of a power amplifier like a bicycle wheel at a bike repair shop. The technician has your bike elevated on a work stand, and she can turn the pedals with just her hand. The rear wheel spins fast. But if she puts the bike on the ground, her hand doesn’t have enough power to spin the wheel. For this, we need a more powerful leg, or in amplifier terms, we need current—watts—that can take the louder (but weak) voltage amplifier stage and put some muscle behind it.

Tomorrow we start grinding uphill.

Ask Paul

The first of the new Ask Paul Video series is up. Go here: and signup to get daily updates when a new video is posted. This morning’s video is all about clipping.