Several months ago, I gave in to my son’s fervent wish for a Sony PlayStation 3. I don’t have the space for multiple entertainment systems, so this acquisition forced me to abandon my lovingly chosen audio system, which included a Sony NS999ES disc player, a separate digital-to-analog converter based on the wonderful Analog Devices AD1853EB evaluation board, and a class-D integrated stereo amplifier built from a kit offered by Hawk Audio of Ledegem, Belgium.
I decided to assemble a new audio-video system based around the Sony PS3. Some aspects of this switch were obvious: The PS3 plays almost any kind of disc, so it simply replaced the NS999ES as my disc player (sadly, new versions of the PS3 cannot play superaudio CDs, as older versions could). A potential showstopper loomed when I discovered that the PS3 outputs high-quality audio only as an optical digital signal in the TOSLINK format. But then I realized that my AD1853EB converter board happens to have a TOSLINK input jack (thank you, Analog Devices).
The amplifier was the rub. I’d gotten used to the precise, detailed sound of my Hawk Audio class-D amp, which is based on amplifier modules built by Hypex Electronics of Groningen, Netherlands. In comparison, the sound of conventional class-AB amps seemed muddy and loose. But my Hawk Audio amp was too big to fit in the small cabinet I had available for my audio stuff. And I wanted a class-D amp for more than its terrific sound—the high efficiency of class-D amps was also appealing. In that small cabinet I also had the PS3, which throws a surprising amount of heat. I didn’t want an amp that would throw much more. For that reason, too, I didn’t want an excessively powerful amp—20 or 25 watts per channel would be perfect, I figured, because my speakers, from Omega Speaker Systems of Norwalk, Conn., are quite efficient.
So, class-D, compact, about 25 watts per channel. Cheap would be nice, too. Could I get a nice little stereo amp with true high-end sound for less than US $500? I became kind of obsessed with the idea. Audiophiles can be like that.
Some exhaustive Web searching convinced me that nothing like my ideal amplifier was available on the market ready-made. All the class-D amps I could find had power levels below 15 or above 50 W per channel, the latter with price tags generally above $1200. They also weren’t compact enough to fit in my little cabinet. If I was going to get that perfect amp for my setup, I’d have to build it myself.
My interest increased when I discovered an assortment of class-D audio amplifier evaluation boards produced by the major manufacturers of class-D amplifier chips. Such boards are available from Texas Instruments, Zetex Semiconductors, International Rectifier, National Semiconductor, Maxim, Toshiba, Analog Devices, STMicroelectronics, ON Semiconductor, and others. These are boards that the chipmakers produce to show off the capabilities of their amplifier ICs, so they are rigorously and cleverly engineered. Typically, to make a complete amplifier out of them, all you need is an appropriate direct-current power supply and some miscellaneous hardware: a metal chassis, some switches and audio-connector jacks, a potentiometer to control the volume, and so on.
I picked the Zetex ZXCD50STEVAL class-D board for three reasons: Its output power level of 25 W per channel was perfect for me, Zetex has a reputation for excellent-sounding amplifiers, and the board was readily available from electronics-supply retailer Digi-Key.
The Zetex board’s documentation specified a DC power supply capable of providing 2.3 amperes at a voltage close to but not exceeding 25 volts. As with many of these amplifiers, the power supply is single rail, which means a positive voltage and ground (for contrast, a dual-rail power supply is a positive voltage and a negative voltage and ground). For an audio amplifier, you don’t want just any power supply. You want one capable of feeding big and fast transients. In other words, when you’re playing a disc of Beethoven’s Ninth Symphony, you don’t want your power supply to choke when the disc player feeds it those tremendous crescendos.
I did some more Web searching and eventually came across the Ï¿11 power supply designed by AMB Laboratories in Sunnyvale, Calif. AMB, which is run by the superbly talented software engineer Ti Kan, specializes in circuits for headphone amplifiers. The Ï¿11 uses a pair of metal-oxide semiconductor field-effect transistors, or MOSFETs, as ”pass transistors,” which regulate the output voltage, keeping it very close to the desired value.
For people interested in building the Ï¿11, AMB sells a printed circuit board and about half a dozen of the needed components. Kan also provides excellent and incredibly detailed documentation for the power supply. You buy the rest of the components, including a toroidal power transformer, from your favorite electronics supplier.
Headphone amps typically use less current than audio amps designed to feed larger speakers. But as I was scanning Kan’s documentation for the Ï¿11, this sentence caught my eye: ”The high-current MOSFETs are not normally the limit of how much current the Ï¿11 PSU could supply, as long as they are adequately heatsinked.”
If you’re using a nice metal box as your chassis, you can’t do much better than to heat-sink your power transistors to that. So I did [see photo above], and I picked a power transformer from Amveco capable of handling 3.9 A.
To heat-sink the power MOSFETs to the chassis, install them on the inside bottom surface beneath the area where you intend to mount the Ï¿11 circuit board. Position the transistors precisely so that when you bend their three leads upward, at a 90-degree angle to the body of the transistor, the leads will poke up through the appropriate holes in the Ï¿11 PC board. The transistors are in a TO-220 package, so you need just a single screw and nut to mount them. The metal mounting tab connects internally to the transistor’s drain terminal. So use a good quality, electrically insulating but thermally conductive pad between the back of the transistor and the metal chassis bottom. And also use either a Teflon mounting screw and nut or some Teflon washers so that the screw itself does not make an electrical connection between the tab and the chassis.