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.

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.