Paul McGowan ...
Sorry about the headline, just couldn’t resist. We are going to start delving into the working of Class D amps, also incorrectly known as “digital amps” and correctly labeled PWM amps – short for Pulse Width Modulation.
As is our custom we’ll start with an overview and then get into details.
It’s perhaps easiest to break the amp down into four sections: input, converter, output, output filter.
The input to the PWM amplifier is what we’ll focus on today in an effort to keep some measure of brevity going.
The input stage is just like any input stage of every power amp out there. Single ended, balanced or both, the task of this stage is to accept the output of a preamp or DAC with an easy load and prepare the signal for conversion to PWM. This stage is particularly important if the incoming signal is balanced and the designer chooses to handle the balanced signal properly.
By properly I am referring to getting the signal in and taking advantage of a noise and distortion reduction advantage that a proper circuit affords us – in a process called common mode rejection – a subject I have covered at length in prior posts but would be happy to cover again should you all want to read it. Not every amp handles balanced signals properly but I would guess most do.
This input stage is pretty important and in large part, can determine much about how the amplifier sounds. In our case we spend a great deal of engineering effort to get this right. The type of semiconductors or tubes used to create the input circuit, how it’s biased, what kind of power supply is used, the voltage of the power supply – all important as this stage is the make it or break it stage of most Class D amps.
In our case, I would refer back to the original Hybrid Class A design we did of a few years ago. The HCA-2 was one of the first of the (then) new breed of Class D amps and it featured many of the innovations and design structures we use today in an amplifier.
That circuit used high voltage, was fully balanced at the input (so you had good common mode rejection), used a combination of JFET’s and bipolars and was class A.
Whatever the designer chooses to do to this stage really sets the bar for how the amp sounds – and that’s critical.
