02/10/2023
What do the Direct Stream Digital (DSD) format and "class D" power amplifiers have in common? A lot more than you may think!
You've probably seen something like the image below before. (By Paweł Zdziarski, https://commons.wikimedia.org/w/index.php?curid=985994).
The top shows how a Pulse CODE Modulated (PCM) signal represents an analog waveform. Pretty much the world standard for digital audio so no need to dive into that further here. The bottom shows how a DSD Pulse DENSITY Modulated (PDM) signal represents an analog waveform.
Why is DSD/PDM interesting? Well, it is a simple 1-bit on/off switching system that creates a square wave that correlates to an analog waveform. It also happens to look exactly like a Pulse WIDTH Modulated (PWM) signal which is what all class-D amplifiers use! Or stated another way, a class-D amplifier is very similar to a DSD DAC, except the class-D amplifier operates at MUCH higher voltages.
So if class-D amplifiers are essentially simple switching devices, how do they work with analog and digital audio inputs? Great question!
With analog inputs, class-D amplifiers use something called a "comparator." The job of a comparator is to convert the analog waveform to a corresponding PWM signal. Although not an analog-to-digital (A/D) converter because the output is not technically digital, the function of a comparator is very similar to the function of an A/D converter. And like any signal conversion there is significant potential for signal change (distortions of various types).
OK, so how can we avoid these distortions? The short answer is to use a digital input. The process of taking a digital input and creating a PWM output is relatively simple math with little potential for errors and COMPLETELY bypasses the need for a comparator. A simpler and better solution.
OK, so now that we have a pristine PWM signal for our class-D amplifier we send it to the outputs and voila? Not so fast - literally! If the output transistors are made from Metal Oxide (MOS...the global standard for transistors for the last 50+ years) then we have another problem. MOS-FETs don't switch on and off fast enough and cleanly enough to do so without adding SIGNIFICANT distortions of their own to the output signal.
OK, so how do we fix that? Well, we design a feedback loop to reduce them. OK, so the problem is fixed right? Yes and no! We successfully minimize the measurable distortions but we also remove some of the original signal along with it.
Bummer! Any way around that? Yes! If we use Gallium Nitride (GaN) output transistors instead, they switch on and off ORDERS OF MAGNITUDE more quickly and cleanly than MOS-FETs! So much so, that we can feed GaN-FETs the pristine PWM signal we mentioned earlier and they will drive the outputs without using ANY feedback and achieve STUNNING sound quality.
This is why the Carina GaN does not use the DACs inside to create the signal for the power amplifier. The DACs inside are used to create the analog signal for the headphone amplifier and the PREAMP line-level output. The GaN-FET amplifier itself creates the analog signal that drives the speakers.
This entire concept of power amplifier design is NOT a new one! Wadia and other companies made products that operated this way (sometimes called power DACs) long before we did. They were ahead of the times! What is different about the concept today is the availability of GaN-FETs (in place of MOS-FETs) that allow the power DAC concept to achieve MUCH, MUCH higher levels of sound quality at significantly lower price points. That is EXACTLY what Peachtree Audio is all about!
