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Tripath's Digital Revolution

How the Class-T® digital amp chip will transform car audio (and the rest of consumer electronics)

Heads up!

Welcome to this article from the Crutchfield archives. Have fun reading it, but be aware that the information may be outdated and links may be broken.

Car Amplifiers — Your audio horsepower just got supercharged
Car audio is in the midst of a major technological transformation. I'm not talking about satellite radio, mobile video, surround sound, or HD Radio technologies. I'm talking about . . . amplifiers, of all things, and specifically about Class-T® digital audio amplifiers developed by Tripath Technology. Whether you are a car audio hobbyist, a professional mobile system designer, or just someone who listens to the radio on your factory deck, these compact and highly efficient amplifiers promise to alter dramatically the way we listen to music. In fact, the implications of Tripath's Digital Power Processing® technology extend well beyond car audio. Digital amplifiers like the Class-T® are poised to impact everything from flat-panel televisions and broadband internet access, to cell phone technology in the very near future.

The juice behind your tunes
Amplifiers. They're just not very sexy. In the world of car audio, flashy head units, touch-screen monitors, and ground-shaking subwoofers usually hog the spotlight. However, their more bashful brothers — car speakers and amplifiers — are arguably the two most important components in any system. As Scott Memmer of notes, "Since amplification is the last stop for the audio signal before it goes to the speakers, the amp merely takes whatever signal is delivered to it (after tone adjustments, balance/fade, etc., have altered it) and increases it. But that 'merely' makes all the difference in the world, and it's the reason people will pay thousands of dollars for an aftermarket system that delivers awesome bass." It's simple really: an inferior power supply = inferior sound quality.

How amplifiers work — a short explanation
There are three basic stages, or sections, in the process of audio amplification. The preamplifier section is where the signal from your source (radio, CD, cassette, DVD, MP3 player, etc.) is adjusted via bass and treble controls, crossovers, equalizers, and other sound shaping controls. Throughout the preamp section, the audio signal is a low-voltage electrical signal.

The power supply section provides the energy used to boost the audio signal. In order to prevent corrupting the audio signal with extraneous noise, the power supply section should be physically separated from the preamp and power amplifier sections.

The amplifier section contains the signal boosting output transistors that take the increased current and voltage from the power supply and kick the audio signal up to a level that will drive a speaker.

This very basic amp diagram shows the three main sections of an audio amp.

Depending on the application, these three sections — the preamplifier, the power supply, and the amplifier — can be linked together in a variety of ways.
  • An integrated amplifier consists of the preamp, power supply, and amplifier sections all contained within a single component; car stereos typically include integrated amplifiers.
  • Many aftermarket car stereos, however, offer preamp outputs, which allow you to bypass the receiver's internal (and usually weaker) amplifier and route the audio signal through an external power amplifier.
  • High-end car and home stereo systems will sometimes consist of separate source, preamp, and power amplifier components.
For a more detailed explanation of how amplifiers work, see our article Comparing car amps — a look "under the hood".
Amplifier Classes
Amplifiers fall into one of several classes based on the type and layout of their components. Each design has its own strengths and weaknesses. We measure the performance of an amplifier by measuring two criteria. Two of the most important characteristics that define each class of amplifier are:
  1. Distortion — how accurately the amplifier reproduces the audio signal put into the amp. Measured as a factor of Total Harmonic Distortion. A THD figure of 1% or below is generally considered undetectable to the human ear.
  2. Efficiency — how much of the power going into the amp is converted to pure sonic output (vs. being lost in the form of heat)?
Here's a quick look at the most important classes of car audio amplifier, focusing on how each class differs in terms of sound quality.

  • Class A amplifiers are noted for the accuracy of their output and their signature warm, pleasing sound. However, a pure class A amplifier is very inefficient (only around 20%) and runs very hot. The output transistors of a Class A amp always have current running through them, so the amp is basically always "on." The constant flow of current through the output transistors (even with no audio signal) creates lots of heat, and "wastes" input energy. Because of this, Class A amplifiers are a relatively rare design for a car audio amplifier — most car amplifiers that boast "Class A" circuitry are really Class A/Class AB hybrids.

  • Class AB amplifiers run cooler, and consequently more efficiently, than Class A amplifiers. Although they also allow current to run through the output transistors when there is no audio signal, the current is at a much lower level than in the Class A design — they're also always "on", but they use a lot less energy while waiting for an audio signal, which allows Class AB amps to achieve 50-60% efficiency. Yet, since they require built-in heat sinks to dissipate heat build-up, class AB amps are housed in relatively large (and weighty) chassis. Class AB is by far the most common design for a car amplifier.

  • Class D amplifiers boast higher efficiency (some approach 90% efficiency), produce less heat, and draw less current than traditional Class AB designs. They use output transistors as switches to control power distribution — the transistors "turn off" when there is too much voltage across them. Because they produce much less heat, Class D amplifiers can be housed in a much smaller chassis than a Class AB with the same power output. The downside is that the on-off switching of the transistors produces high levels of distortion, particularly in the middle and upper frequencies. Class D amps are usually used to power subwoofers, since low-frequency distortion is hard to detect.

  • Tripath's Class-T® amplifier design combines the low distortion and excellent sound quality of the Class AB design with the efficiency advantages of the Class D design. Tripath uses switching transistors to achieve very high power efficiency (around 90%), low heat production, and compact chassis-size. Yet, they've managed to solve the high-frequency distortion that plagues Class D amps.

Class-T chip
An inside look at a Class-T® amplifier.

I talked with Shawn Scarlett, Senior Product Marketing Manager at Tripath, to gain a better understanding of how Class-T® amplifiers manage to avoid the high-frequency distortion associated with the Class D design.
Tripath HQ
Tripath's headquarters in Santa Clara, California.

CA: Can you explain in broad strokes (for the layman) how the Class-T® amplifiers work in comparison to traditional AB amp designs?

Tripath: Traditional Class AB amplifiers work by keeping the output within the linear operating range of the output transistor. While in the linear range, the output is equal to the input multiplied by some amount of gain. So if the input doubles so does the output. The two signals are the same shape, just different scale. The disadvantage is that the linear range of transistors is fairly small; in a way, the transistor is never able to fully turn-on. While the transistor is "half-on" it's only delivering half its power to the speaker, the other half is going into heat. Add to that the "bias", or amount of energy the amplifier uses to keep itself running, and you end up delivering less than half the power you use to the speaker.

"Switching amplifiers" [Class D and Class-T®] work on the concept of turning the amplifier fully on or off, so very little energy is wasted. You do need to use a bit of power to make the switch from on to off, but in the end, the amount of heat you generate falls to about 10% of the total energy used.

Let's put some numbers with that. Let's say you have a 4 x 100-watt amplifier. You can deliver a total of 400 watts to the speakers, but if it's an AB amplifier, you're generating about 400W of heat. If you add them together, you're pulling 800W from your power supply (or battery).

If you change to a switching amp, that same 800W from the battery can deliver 720 watts to the speakers. You can have a 4 x 180-watt amplifier without pulling any more power, and you will only generate 80W of heat.

Or, if your design is limited by the amount of heat you can handle (like in an automotive head unit), and you have the same 4 x 100-watt Class AB amp, its heat-sink can dissipate a total of 400 watts of heat. Now with that same heat-sink on a switching amp you could generate 3600 watts: Four x 800 watts of power, with 400 watts of heat! Obviously, you'd really need to beef up the battery in that case.

This efficiency gain is common to most switching amplifiers. Historically, the gain in efficiency has come at the cost of fidelity [Class D amps]. So switching amps were limited to subwoofer applications [because low frequency distortion is virtually inaudible to the human ear]. This is where Class-T® comes in.

Class-T® is the name for our proprietary architecture that improves on general switching amps. We use a combination of "predictive" and "adaptive" processing. On top of that, we use a very high switching frequency. The basic idea is that we look at the incoming signal to determine the best way to encode it, making sure to minimize interference or mistakes. We then use feedback, or "adaptive" processing to analyze the output and keep the system stable. The high switching frequency allows us to correct any issues quickly before they become audible. Because of the robustness of the system, we can maintain our fidelity even with mismatches in the output FETs [Field Effect Transistors], power supply "ripple", and other issues that normally require significant engineering time and manufacturing cost to prevent. The Class-T® technology allows us to reach very high fidelity levels and keep THD+N [Total Harmonic Distortion plus Noise] figures better than many A/B amplifiers, while still delivering the efficiency of a switching amplifier.

Tripath Lab
The lab.

CA: Do the Class-T® amplifiers adhere to the new CEA-2006 certification (20-20 kHz @14.4V into a 4-ohm load at 1% THD)?

Tripath: The CEA-2006 specification really shows off the advantages of the Class-T® topology. Standard AB amplifiers show a gradual increase in THD over their operating power range. When Class AB amplifiers are used near their specified [maximum RMS] power, they produce significant distortion. The feedback loop inside the Class-T® amplifiers maintains very low THD curves until the part nears clipping, so that when the amplifier is operating at loud volume it still has remarkably low THD+N measurements and a great sound.

The CEA-2006 certification is related to the final product itself, not components, so it's not appropriate to say that Class-T® amps are certified. What is critical is that normal practice in the industry is to spec output power at saturated square wave, or other sketchy limits. The problem is that the amplifier ends up sounding terrible long before it reaches that power level, so it's not a useful spec. Hence the CEA-2006. The new spec rates the power at a useful distortion level (1%). Because of the advantages of Class-T®, measuring at this point gives a big power advantage to the manufacturer and the end user. The user gets more power where they need it, and the manufacturer can showcase that fact and compare against other amps.

CA: What do you see as potential ramifications for the future of car audio? With ever more powerful internal amplifiers being incorporated into in-dash receivers, will users have a greater chance of blowing their factory speakers, and consequently need to upgrade to higher quality car speakers?

Tripath: There are three major ramifications for aftermarket car audio.
  1. The first is that high power head units will take a big hunk out of the need for stand-alone amplifiers. The Panasonic head unit offers more real power then many outboard amplifiers. Why pay for and install an amp, when you can get the same power from the head unit? This will mark a new level of performance for people who want to just replace their factory components. Refitting a high-power head unit and new speakers will offer a major performance upgrade without rewiring.

    Panasonic's MXE CQ-C9800U uses a 60W x 4-channel Class-T® amplifier that gives you power comparable to that from an external 4-channel amp.
  2. The second big impact is the space and heat savings. With the extra space created by getting rid of the heat sinks, OEM's [Original Equipment Manufacturers, the companies that make factory stereos] can add better DSPs [Digital Signal Processors] and bigger LCD monitors.
  3. The third big impact is that with Class-T® we can begin to cram more channels into the head unit. Why just 4 channels? Some OEMs are already looking at 6 channels for in-car-theater, but what about 8 or 12 channels? With the horsepower in the DSP, you can use active crossovers and then separately amplify each speaker driver. Best of all, you could still fit it a normal single-DIN package.
Overall these will definitely continue the trend to greater demand for speaker and wiring upgrades.

CA: What type of amplifiers do you expect to introduce next year?

Tripath: We're working on two parts specifically targeted at automotive head units. The first is a digital controller for Class-T®. This will make it easier to for OEMs to build-in Class-T® amplifiers to their head units, hopefully making it a bit more common and accessible.

The other part is a 4 x 100-Watt version of our current part. Imagine a head unit putting out that much power! Look for some units built around that part in a few months.

For the home theater market, we have just introduced a new family of amplifiers that will make Class-T® very competitive with traditional designs. We expect these parts to be a big hit for A/V units and "all-in-one" DVD players. The new parts are optimized for power output of between 50 and 150 watts. One of the neat aspects of the new family of amplifiers is that it allows the manufacturer to scale the power to fit the needs of the system.
Alpine MRD-F752
The Alpine MRD-F752 5-channel amp uses Class-T® technology.

Digital Amplifier Revolution: The future is bright
The market for Class D and Class-T® switching amplifiers is still relatively young and untapped, but the technology clearly holds major implications for consumer electronics. The Tempe, Arizona-based consumer electronics market research firm Forward Concepts completed a comprehensive market study in June 2004. Dr. Daniel Sweeney, the author of the report, concluded: "Class D amplification is emerging as a truly disruptive technology and one that may eventually replace linear amplifiers in most audio frequency applications." Tripath, with its high-fidelity Class-T® amps, is consistently being touted in the CE industry press as a "a company to watch with its unique product solution."

In fact, Tripath has already had considerable success marketing Class-T® to the consumer electronics industry. Back in 2002, Blaupunkt became the first car audio manufacturer to harness Class-T® technology, winning a prestigious CES 2002 Innovations Design and Engineering Showcase Award for their 7-channel OD7500 PA Series car amplifier. Panasonic introduced the first in-dash receiver utilizing Class-T® amplifiers this year, the MXE CQ-C9800U. Home audio manufacturers are also well aware of the size and efficiency advantages of Class-T® amplifiers. Tripath's customers include Denon, Hitachi, Motorola, Onkyo, Sanyo, Sampo, Sharp, Sony, and Toshiba. And in April, Tripath announced new design wins for flat-panel TVs from Samsung, TCL, and Eizo.

The key to Tripath's continued success is the advanced sound quality of the Class-T® design in comparison to Class D amplifiers. Judging by the reaction of CrutchfieldAdvisor's resident rock star/car audio reviewer extraordinaire, Charlie Pastorfield, I'd say the future is looking very bright. Pastorfield summed up his appreciation of the Class-T®'s performance in the Panasonic MXE CQ-C9800U like this:

    "After a couple of hours of listening, I became a Performance Digital believer. The stereo spread was noticeably more pronounced, and I kept hearing little details popping out (reverb on a snare drum, vocalist's lips smacking, amplifier buzz) that I wasn't accustomed to hearing, making the recordings seem more lifelike. And it wasn't that the mix was being reproduced inaccurately by the Tripath amplifier. On the contrary, I was hearing all the information on the CD reproduced pretty damn close to perfection, just like you'd hear on a high-dollar home stereo system.

My thanks to Shawn Scarlett of Tripath Technologies for his help with this article.