There is a revolution happening in audio. The days of heavy, hot, power-sucking audio amplification are fast passing, replaced by an era of a new type of digital audio amplifier, the “T” amp.
These amplifiers are over 80 percent efficient, are cheap and best of all, sound great. They are available in board form now at places such as eBay, Amazon, Parts Express and MCM Electronics. Some audiophile audio companies are even building them.
What is a “T” amp? The T comes from a company called Tripath Technology Inc. named after its CEO, Adya S. Tripathi. This was the first company to make the integrated circuits that comprise these amplifiers. Their first IC was the Tripath TA-2020, a 20-watt-per-channel amplifier chip. These were used in very small battery-powered audio amplifiers (one in particular by a company called Sonic Impact) and sold in places like The Sharper Image for around $30. The word quickly went around — these cheap little plastic amplifiers with the big, round volume control outperformed some audiophile amplifiers that were selling for over 10 times their cost. A 2005 review on the site BoingBoing.net raved about the Sonic Impact amp quality (http://tinyurl.com/rwee-sonic).
Fig. 1: At left, the Lepai 2020A 40-watt amp. At right, a two-watt-per-channel stereo amp. Both were bought through eBay.HOW THIS IS POSSIBLE
How could this be? Fact is, it was good — and still is today. Rebuilding the Sonic Impact audio amplifiers by replacing the (very) cheap chip resistors and capacitors used in them with audiophile-grade parts quickly became a cottage industry. Often those parts cost many times more than the entire amplifier itself, but since the amp was such a bargain to begin with, no one minded. People were even hauling 12-volt car batteries into their living rooms to power these little amplifiers. I had one of the early Sonic Impact amplifiers and found that with the right high-efficiency speakers, it truly sounded magical. I also found it to be incredibly cheaply made.
Tripath began making bigger amp chips, but the cat was out of the bag. Soon lots of IC companies were manufacturing digital amp chips, and since Tripath could not compete with the likes of Texas Instruments and the Asian IC manufacturers, it merged with a company called Etelos in 2008, which itself went out of business in 2013. However, lots of Tripath Class-T amps are still out there for sale. One of my favorites is the Lepai 2020A. It is one of the better ones, and a genuine bargain for around $20. The amplifier is available through Amazon (http://tinyurl.com/rwee-LEPAI) and at many other places.
The big problem with the 2020 IC is its power output — it’s only around 8 RMS watts per channel. It didn’t take long for the other manufacturers to come out with much more powerful ICs, and the race was on! I currently run a digital amp board at home on my TV set that is rated at 50 watts per channel — it’s actually it’s more like 40 watts RMS; but still plenty to play my old Advent speakers loudly. The amp runs on an old laptop power supply and the entire amplifier is a board about 3 inches on a side with a small heat sink on it. Since the digital power IC is so efficient (over 85 percent from power supply input to sound output) very little heat is produced. Fig. 1 shows the board I bought on eBay (http://tinyurl.com/rwee-TDA7492) — a solid 40 watts per channel of true HiFi audio for under $10, shipped from China — using an old laptop power adapter for its power supply. To the right of the penny is a small 2-watt-per-channel T-amp, also readily available from eBay.
HOW THE DIGITAL AMPS WORK
Think of AM transmitters like the Harris MW5, MW50, Gates 5 or Continental Power Rock. The digital amplifiers work on the same pulse duration modulation (PDM) principle these transmitters use. How do they sound so good? It has to do with using a very high switching/sample frequency — usually above 1 MHz. Using a high frequency allows for a very high-resolution sampling of the audio and also means that the filter that removes the sampling stuff can be a very simple gentle filter improving frequency response, distortion and efficiency.
These amps also work on what is known as the Bridge Tied Load (BTL) principle. Their outputs are balanced “bridge” type amplifiers, not unlike the “balanced” op-amp outputs commonly used by most of today’s broadcast equipment. This makes it possible to swing twice the voltage across the load as a single-ended amplifier can, resulting in four times the output power (Power = V2/R, so twice the voltage results in 22, or 4 times the output power). Your car stereo probably also operates this way, as does your HDTV set.
In fact, T-amp ICs were originally designed to be used inside HDTV receivers because of their small size, low heat, high power and high efficiency.
A good description of how a BTL amp works, courtesy of the Texas Instruments E2E Community website (http://tinyurl.com/rwee-BTL), has the following:
BTL is an acronym for “bridge-tied load.” A bridge-tied load configuration consists of one amplifier driving one side of a load and another amplifier (with an inverted signal from the first amplifier) driving the other side of the load. This results in 2x more voltage swing across the load than you would have in a single-ended configuration where one side of the load is tied to an amplifier and the other side to ground. Twice the voltage swing across the load equates to 4x the power to the load (P = V2/R). So, a BTL load configuration offers 4x more power to the load than a single-ended configuration from the same supply voltage. Be aware, that BTL amplifiers dissipate 4x the heat of a Single-Ended amplifier under the same supply voltage and load conditions. Refer to the TPA7x1 series parts in the applications section for more on BTL. Also note that the output DC blocking capacitors are NOT required in the BTL configuration. Since the load is tied across two amplifiers with the same DC bias and the signal across the load is the difference between the amplifier outputs, the DC bias is removed.
APPLICATIONS
As mentioned above, with BTL, a downside is that each output is normally biased at half the power supply voltage. In practice, this isn’t a problem when running in the BTL mode because both output terminals are biased at this voltage so the difference between them is zero volts. If you have to reference one output to ground (as in using a BTL type amplifier for a headphone amplifier or other circuit requiring a single-ended common ground output), you can simply put a 1000 microfarad capacitor in series with each output to block the DC component (putting the capacitor’s + lead towards the amp).
Being BTL, these amps have some other interesting characteristics, including the ability to output a mono audio output from a stereo input by simply connecting the load from the + terminal of one channel and the – terminal of the other. I do this at WGLS-FM to drive the ceiling speakers in the lobby and restroom. The high-power amps can also drive a 25- or 70-volt speaker distribution line directly.
One more trick: As these are “balanced bridge” outputs, you can use an amp like the $25 Lepai, connected with two 1000 uF capacitors per channel as mentioned above, as a fine-sounding “matchbox” unbalanced-to-balanced converter — or even as a distribution amp by placing a 100-ohm resistor in series with the leg of each balanced output. I’m sure that you’ll come up with even more possibilities.
How do these amplifiers sound?
In a single word: Outstanding. They add little to nothing to the audio appearing at their inputs. Their output is smooth, clean, quiet and punchy, free of much of the strain of similar (or up to 10x the) cost of Class AB single-ended audio amplifiers. They put out practically no heat; the heart sinks remain barely warm to the touch even at close to full output. Output powers can run into the hundreds of watts, and still cost well under $40 for the boards. Assembled digital amps are also available all over — and offer similar great values.
If you want to dip your toe into the world of high-quality digital amplification, I recommend that you purchase the Lepai amp mentioned — unless you want a piece of history; then you can buy a Sonic Impact clone from Parts Express (http://tinyurl.com/rwee-dayton). Either way, I guarantee you’ll be blown away by the incredible sound of the mighty T-amp!
Dana Puopolo is chief engineer at WGLS(FM), Rowan University in Glassboro, N.J.
