Tag Archives: Power Amplifier

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Choosing inputs

We started a little mini-series on how amplifiers work. I know that some of you gloss over these details because you’ve heard them before or you don’t care. Others have large appetites for learning and it is to those hungry readers I continue.

If we look at the block diagram of a power amplifier in this post,  we note two main sections: input and output. The job of the input stage is to take a small signal from the preamp and make it 30 times bigger. Let’s talk about that stage today.

The input stage of a power amplifier is a big preamp. It is also the one stage that makes nearly all the sonic difference in an analog amplifier. This is the most critical stage to get right. Just like a preamp’s architecture has everything to do with its sound, the amplifier’s input stage is where all the magic happens.

If we look at an amplifier like the BHK, we’ve used a vacuum tube to provide the gain. In most solid state amplifiers—those that are not hybrids like the BHK—this task of amplifying the small input signal is handled by any number of clever schemes. I have engineered simple op-amp style architecture with a single diff pair feeding a gain stage, to more complex versions known as full complimentary where there are multiple diff pairs and gain stages. The means to build a high voltage preamplifier are as many as there are amplifiers. Every engineer has their take on what sounds best in this all critical stage.

One technique we pioneered many years ago, though I am sure we weren’t the first, was the use of a separate power supply for this input voltage gain stage. It’s what we’ve done in almost every amplifier we’ve ever built and the improvements are clear to hear. In this scenario, there are two power transformers (or at least two separate windings on the main transformer) inside the amp: a small and a large one. The small transformer and its associated power supply feed the input stage, while the behemoth transformer is kept separated for the output stage. Here’s where we can get tricky. We can easily regulate this input stage, we can make sure it is never impacted by demands for power on the output stage. Thus our internal preamp is pure and undisturbed by our subject of tomorrow’s post, the less important but certainly not unimportant, current amplifier.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

For those interested, here is how an audio power amplifier works.

Peeking under the covers

It seems I may be alone in my enthusiasm to read about high dynamic range loudspeakers and systems in these blog posts so we’ll move on. That’s fine, it’s just that I am currently immersed in the subject because of our work on the new line of Arnie Nudell speakers. We’ve had some excellent work finished by our driver manufacturer including a new midrange ribbon that has me swooning!

That said, I’ll keep on getting excited about high efficiency, high dynamic range solutions, but meanwhile, we’ll switch gears on these posts.

One question I get asked a lot is how a power amplifier works. Generally, the question comes up because power amplifiers seem somewhat of a mystery. Big, heavy boxes, with collections of strange components inside.

To start off the discussion let’s imagine the use case for a power amp—one we’re all familiar with: an input to connect the output of the preamp or DAC, and an output that connects to loudspeakers. What happens in between? We know a preamp is incapable of driving a speaker because it doesn’t have an essential element. Wattage. So, what happens? How does the power amp take the weak output signal from the preamp and give it wattage, muscle, power?

Let’s start with a simple diagram of a power amplifier.

Note there are 3 blocks. An input amplifier (U1) an output amplifier (U2) and a power supply. These are the three critical elements within any analog power amplifier. The 3 elements are:

  1. Voltage gain stage
  2. Current gain stage
  3. Power supply

Tomorrow we’ll start with the voltage gain stage.