Digital switching amplifiers have been around for a number of years, and they are especially popular for use with subwoofers. The reason for this is that it is expensive to design and build one that works well throughout the audible spectrum (20Hz – 20 kHz), but inexpensive switching amps work fine in the low frequencies, where subwoofers operate. They run cool and are small in size. So, subwoofers everywhere are using them.
Only a handful of designers have been building top notch full range switching amplifiers. Even though they don’t necessarily have a chassis full of parts, they have been high in price. This is partly because they don’t have much competition, but also, it is very tough to have them switch at really high frequencies, and it is the high switching frequency that allows a good sound reproduction at the top end of the audible spectrum.
Over the past few years, Paul McGowan, of PS Audio, has gained international renown for his development of products that deliver clean AC. The voltage coming out of our wall sockets is full of noise, not only harmonics of the 60 Hz, but motor noise from our refrigerators and other household appliances, along with that of our neighbors who are on the same supply circuits. There have been lots of products developed that filter out some of the noise, but none that completely eliminate it. That is, none, until PS Audio introduced the Power Plant P300. Later came the P600 and the P1200, the latter of which I use here in our lab. It converts the incoming AC to DC, then starts over with a completely clean 60 Hz regeneration from that DC. It is big, heavy, and expensive, but boy does it work.
With that fundamental experience in designing power supplies, Paul decided to go into power amplifiers, where the power supply continues to be critical.
The HCA-2 is PS Audio’s first digital switching power amplifier. It delivers 150 watts x 2 into 8 Ohms and 225 into 4 Ohms. It is referred to as Class D (for the digital switching), with two gain stages (input and output, with a buffer in between), and uses very little negative feedback.
As you may remember, most amplifiers are Class A/B. This means that there is a small amount of current running through the output stage (Class A) even when no music is playing, and at low levels of sound, this current goes to the speaker instead of being dissipated as heat. At higher sound levels, the current has to be turned on (Class B). Depending on how much of the power is in Class A vs. Class B, the amp can run hotter at idle than when the music is playing loud.
Class D amplifiers, like the HCA-2, have no current running in the output stage at idle, like a Class B stage. However, they differ from Class B in that, when the music comes through, the output stage is switched on and off (Class D), rather than current running continuously during the music (Class B). The HCA-2 input stage runs in pure Class A. The output stage is where the switching is applied.
The trick is to switch the current on and off so fast that you hear it as being continuous, like the 60 fields per second on our TVs being fast enough that we see it as continuous motion. In order to have this work properly, and sound good, you need a switching frequency of 10 times the highest frequency that you are listening to (not an industry definition, just the opinion of some, including me). So, assuming an average human ear hearing up to 20 kHz, you would need a switching frequency of 200 kHz minimum. The big bucks switching amplifiers of the recent past use frequencies sufficiently high to achieve this. Some amps use less than the minimum, and they sound good, but not great. The HCA-2 uses a switching frequency of 500 kHz, which more than meets the standard, and yet the price is modest. How did he do it? Well, the technology has matured, it is available, and somebody had to be first. That is Paul McGowan.