Classy Audio Amplifiers, Part 3

Looking at the rear of the Crown DC300 amplifier with its ‘raw power’ transistors

Before discussing the Crown DC300, I want to introduce the Class D amp; final amplifier architecture examples, that discuss the topic regarding Class G , H and other amplifiers, will appear in my final Part 4.

Class D amplifiers came about in 1958 but have improved exponentially in performance since then. If we want the best architecture for best efficiency, smallest footprint, good for battery operation as in portables or wearables, the Class D amp stands out among the crowd.

Figure 1: Top image shows a conventional audio power amplifier; Bottom image is a Class D amplifier

Class D amplifiers, especially with high efficiency GaN power transistors, can eliminate the need for a heat sink in many cases.

Audiophiles have always commented that the best sound comes from vacuum tubes. There are schools of thought that say FETs, not BJTs, are more closely related to the type of ‘pleasing’ distortion and sound that vacuum tubes produce in audio. The controversy continues even today as it has since the vacuum tube began to lose importance in most electronic designs.

Figure 2: Block diagram of a Class D open loop amplifier (Image courtesy of Reference 1)

GaN power transistors in Class D

A great audio amplifier will have excellent Total Harmonic Distortion (THD) and Intermodulation Distortion (IMD). GaN FETs have very close to ideal switching characteristics with low prop delay and very fast slew rates. They also have short dead times for lower open loop distortion. See this article on EPC’s website.

The Crown DC300

The DC300 output stage used paralleled single-diffused power transistors in a circuit (Class-AB+B) that had enough speed and reliability not seen before in a power amp. The amplifier employed a great amount of feedback that enabled the DC300 to set new standards for fidelity and helped make solid-state more than just hype. A U.S. patent number 3,493,879 was issued for the design.

The name DC300 was chosen because the amp was direct-coupled for DC operation with a powerful 300W of 8 ohm stereo power. Another mention was that in 1958, the DC3 was a well-known commercial aircraft: The Crown was ‘flying higher’ than other amps at that time.

The raw 500W of power in a 45 pound chassis was a great improvement over 500W vacuum tube amps that weighed in at 200 pounds (no disrespect to the sound of tube amps!)

The output power section architecture is remarkably similar to the Phase Linear 700 (See Classy Audio Amplifiers, Part 2 for those details) Q110–113 and Q115–118 output power transistors are 2N3773. All of these transistors are selected for optimum matching.

A Crowning achievement

Crown now belongs to the renowned Harman International company as of 2000. The famous JBL speaker line is also a part of Harmon.

The modern CROWN by Harman Macro-Tech 12000i has a whopping 4.5kW power output at 4 ohms and weighs in at only 29 pounds.

Reference

1 Class D Audio Amplifiers: What, Why, and How By Eric Gaalaas, Analog Dialogue 40–06, June 2006, Analog Devices

2 Guidelines for Measuring Audio Power Amplifier Performance, Application Report, by Richard Palmer, SLOA068A–October 2001–Revised September 2019, Texas Instruments