Hands-On Project: Low-Cost, High-End Class-D Amplifier

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.”

Photos: Glenn Zorpette

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.

The Ï¿11 can give you any voltage from 5 to 36 V, depending on the value of its resistor R10 and zener diode D5. To get around 24 V, I used 10K ohms for R10 and 12 V for D5. Those values gave me a rock-steady 24.23 V.

I had to deal with only one unexpected problem. I found that, for reasons that still elude me, the only way I could get the power supply and the amplifier board to work together in the same metal box was by completely isolating the signal ground from the power ground. Before separating them, I blew fuses twice and got hideous static sounds. To separate the grounds, I grounded the Ï¿11 board and also my power-entry module to the metal chassis. Then I grounded the Zetex amplifier board and associated components, such as the 100K potentiometer I used as a volume control, to the ground tabs of the RCA jacks I used to input the music signal. Just make sure that the Zetex board and none of its associated components touch the chassis ground and you’ll be fine. For this reason, do not buy input RCA jacks that ground to the chassis—which is what most cheap ones do.

I splurged and bought an NKK illuminated switch, whose clear plastic toggle has a built-in LED that lights up pleasingly in blue (green and white are also available) if you connect it to an appropriate source of switched current. The Ï¿11 has such a source right on the board, along with a space for a current-limiting resistor to accommodate whatever indicator LED you might choose. For the NKK switch, I found by trial and error that 332 ohms were exactly the right amount of resistance to give a level of illumination from the switch that was suitable for both light and dark rooms.

For a volume-control pot, I used an ALPS RK27112A00AK. It’s conductive plastic, dual-gang, 100K logarithmic taper. At about $12 at Mouser Electronics, it’s a fantastic value. But if you’re kind of obsessive, do go ahead and get the 100K TKD 2CP-2511 or 2CP-2508 at Michael Percy Audio or Parts Connexion. It’ll run you between $62 and $100. Only you can say if they’re worth it, my friend.

I added a toggle switch on the amp’s front panel that lets me bypass the pot and use the amp with a preamp. I’m not doing that now, but it’s nice to have the option. Plus, this switch gave me symmetry around the volume control knob with the other switch, which lets me select between two signal sources (for example, disc player and radio).

I connected the amp to my PS3 and to my Omega speakers, which are about 91 decibels efficient. I fed the PS3 a CD (Lucinda Williams’s Car Wheels on a Gravel Road ). With the volume knob turned up only about a third of the way, the sound was about as loud as I could tolerate. With efficient speakers, you could shred your eardrums with this thing.

You’re reading this article because I liked the sound of the amplifier so much that I actually sat down and bothered to write these paragraphs and curse while squinting perplexedly through all my Digi-Key and Mouser invoices to cobble together a parts list. I also nagged IEEE Spectrum’s photo editor, Randi Silberman, to take pictures of the amp. The sound is much better than the very good Rotel integrated amplifier I had in the same setup briefly before I finished the Zetex amp.

Total cost of the amplifier: $523.43—not even $25 over my goal. If I were building this under government contract, I’d get an award. If you’re discriminating about audio, curious about class-D amplifiers, handy with a soldering iron, and frugal by nature, here’s a good way to fall in love with your CD collection all over again.

Photos: Randi Silberman