T-Class Explained

A paper by Jack Matheny, president of U.S. Amps, Inc.

 

 

 

Special thanks to caraudioforum.com for the opportunity!

 

 

U.S. Amps’ T-Class™ amplifiers incorporate a revolutionary new method of Digital Power Processing™, a proprietary technology of Tripath Technologies that provides superior performance compared to conventional methods of amplification.

 

U.S. Amps T-Class™ digital amplifiers provide the full range high-fidelity of Class “A” and “A-B” designs, yet maintain the high-power digital efficiency of less sophisticated and less capable “Class D” amplifiers. 

 

 

What is a Digital Amplifier?

 

All “amplifiers” digital or analog, really consist of two sections, the power supply and the amplifier section

 

The power supply, or the “engine” of the amplifier, is highly efficient, loosing little energy in the form of heat. 

 

The amplifier section acts as a “transmission” to convert the power supply’s energy into audible sound. 

 

It is during the analog conversion that the majority of amplifier heat and waste take place.

 

“Digital” amplifiers are high-speed, high-efficiency “transmissions”.  They work, or “switch” at much higher rates that analog amplifiers.  Digital amplifiers allow more of the power supply’s energy to the speakers with less heat and waste.

 

Both T-Class™ and “Class D” are over 80% efficient as a whole, (power supply waste included), compared to the 45 to 70% efficiency of a typical analog design.

 

 

U.S. Amps’ T-Class™ Verses “Class D”:

 

Both types function on digital principals, where they virtually “skip” the analog conversion process of “Class “A” and “A-B”. 

 

(Very important)  It is method and switching speed that primarily distinguishes T-Class™ from “Class D” in performance.

 

 

“CLASS D”:

 

“Class-D” amplifiers are unable to equal the performance of U.S. Amps T-Class™.  The fundamental technology of “Class D” is self-limiting.

 

Here’s Why: 

 

“Class D” switching uses pulse-width modulation (PWM) technology.  PWM switching is relatively slow, and fixed at a switching frequency anywhere from 75KHz to 200KHz. This produces audio output quality that is inferior to Class A or A-B, so efficiency is gained at the expense of sonic fidelity.

 

A basic rule of thumb of audio amplifier design dictates that the minimum circuit switching frequency is 10 times greater than the audible range.  This bare minimum is necessary to keep transistor “switching noise” out of the audible signal. 

 

To adequately reproduce the audible range, (20Hz to 20KHz) the “Class D” PWM would have to switch at a minimum of 200KHz, which represents the upper range (and cost) of PWM capability. 

 

As a result,

 

“Class D” is generally used only in low frequency or low fidelity applications where the performance level of PWM is acceptable.

 

Large speakers -such as woofers, or inaccurate speakers -like the full-range single cone drivers used in General Motors’ “Delco-Bose™” systems- help to hide the audible high frequency noise of “Class D” by virtue of their mechanical inability to reproduce high frequencies.

 

“Class D” is load-specific.  Both “Class D” and T-Class™ amplifiers require output filters to eliminate spurious digital “switching noise” from the output signal.  The slow switching speed of “Class D” necessitates a large, built-in, high-value output filter that acts as a passive crossover and is dependent upon the correct speaker impedance. 

 

Additionally, “Class D” operates at a relatively low voltage, much like a high current amplifier, and must be used into low impedances to make power with current.  This emphasis on current requires the output filter larger and more critical to impedance load matching.

 

This is why “Class D” amplifiers are specific “1 Ohm” or “1/2 Ohm”, etc.  Failure to observe the exact recommended speaker load is akin to connecting an improper speaker load to a passive crossover network, and will result in degradation of the already limited “Class D” frequency response.

 

 

U.S. Amps T-Class™:

 

To be sure, “Class D” is (was) a step in the right direction.  It does work, and works rather well in certain instances.  It is, however, a crude and incomplete technology when compared to “T-Class™ and DPP.

The underlying technology of U.S. Amps T-Class does not use PWM and is not pure analog.   T-Class incorporates cutting-edge Digital Power Processing™ (DPP*), a product of Tripath Technologies.  DPP combines the benefits of digital and analog with a completely new approach.

 

U.S. Amps’ T-Class™ amplifiers are among the first to apply this breakthrough technology.

 

DPP utilizes ultra-high speed digital switching that varies with the amplitude of the incoming signal.  At rest, with no signal input, DPP switches at an incredible 1.5 MEGAHERTZ!  (Wow!)

 

As the signal amplitude increases, the switching frequency of DPP decreases correspondingly, but never enough to degrade sound quality.  DPP maintains an average switching frequency of 600 to 800KHz while sampling and switching audible frequencies.

 

This incredible sampling and switching speed gives U.S. Amps T-Class amplifiers their full-range characteristics, and requires a smaller and less restrictive output filter.

 

T-Class amplifiers operate at a much higher voltage than most “Class D” designs, producing more power into 4 and 2-ohm loads.  Correspondingly, the T-Class™ output filter is much smaller than that used with “Class D”, allowing U.S. Amps T-Class™ to operate into various loads without affecting signal quality or frequency response in the audible range.

 

Typically, U.S. Amps’ T-Class™ amplifiers have THD+N of less than 0.08% over the full audio bandwidth.

 

Additionally, U.S. Amps’ T-Class™ exhibits ultra-low IHF-IM (high-frequency, inter-modulation) distortion -less than 0.04%.  Low IHF-IM reduces “listening fatigue”, even at excessive volume levels.  By any measure these superb specifications fall within the esoteric realm of  “audiophile performance”. 

 

U.S. Amps’ T-Class™ provides power conversion efficiencies of 80% to more than 90%, equal to or better than “Class D”.

 

Another amazing but predictable benefit of T-Class™ is its cool operating temperature.  When used within its designed power and load limitations, T-Class™ generates almost no heat.  What little heat that is produced is quickly absorbed by the standard-size U.S. Amps heat sink. 

 

U.S. Amps T-Class™ is the beginning of an audio amplifier revolution.   Soon the consumer will have access to new and powerful products that will fit in confined spaces, require very little power to operate, and produce incredible full-range digital fidelity.

 

Like any technology, DPP can be painted as great or terrible, depending on the facts examined, ignored, or otherwise twisted in some marketing campaign. T-Class™ has a mediocre signal-to-noise ratio (86Db) when compared to analog amplifiers; due to a certain amount of high frequency “noise” is present in the output content.  The high-frequency “noise” is well above the audible range, and is a by-product of the terrific DPP switching speeds, and the conscious use of the smallest possible output filter. 

 

The high frequency content of T-Class™ is extremely tiny, only about 200mv, and impacts harmlessly on the speaker voice coils.  By changing the output filter value it is possible to all but eliminate the switching noise, but doing so would have a slightly detrimental effect on two T-Class™ benefits, the full-range digital ability and 4 ohm to 2-ohm operation.

 

 

The Limitations of Digital Audio:

 

Like any other amplifier design, it is possible to distort, or “clip” the output of a Digital amplifier.

 

What few people understand is that any digital amplifier design, T-Class™ included, looses much of it’s efficiency benefits when it is used outside of it’s intended operating range.  When a digital amplifier “clips” or is driven into gross distortion, it starts to act more like a conventional class “A-B” design, both in power consumption and ugly waveforms.

 

A big reason that U.S. Amps’ conventional class “A-B” amplifiers are successful in SPL competition against “Class D” products is our firm commitment to power supply technology.  Once you get past the “clean power” capability of any amplifier, it more or less lies down and lets the power supply do the work.  U.S. Amps power supplies out-work the rest.  Our oversized power supplies only enhance the capabilities of T-Class™ operation.

 

 

Let’s Get Technical:

 

The following is a more in-depth look at “Class D” and T-Class technologies.  The section entitled “Class T Architecture” is a direct reprint from Tripath Technologies’ technical “white paper” and can be found in it’s entirety on the Tripath web site, www.tripath.com.  U.S. Amps is supplying this document for the sole purpose of education.  All material contained within is the sole property of Tripath Technologies and is not intended for redistribution. 

 

(Most sane people can stop reading now.  Don your pocket pen protectors, lab coats, and coke-bottle glasses if you wish to press on.)

 

What? Still here?  You have been warned.

 

“CLASS D” REVISITED:

 

Traditional “Class-D” technology is Pulse Width Modulated,  (PWM), and switches at a fixed frequency, generally between 100 and 200KHz.  A comparator circuit reconciles the audio input signal to a “triangle wave” that is switching at the fixed frequency of the “Class D” amplifier.

 

The triangle wave then drives output MOSFET transistors in a push-pull fashion, just like a normal analog amp, but at a much higher frequency.  (Analog amps rock on at 20Hz to 20KHz) 

 

The resultant high frequency signal must then be “choked down” -where a healthy part of the signal is absorbed by a low-pass filter (inductor and capacitor) positioned inside the amplifier, before the speaker, that serves to filter out the high frequency triangle wave, leaving amplified audio.

 

The audio quality of Class-D amplifiers is not only inferior because of fundamental problems with the slow-switching PWM approach. Typically, the output transistors (MOSFETS) do not switch perfectly and generally are not available as “matched components” Imagine an engine with pistons that do not weigh the same.  It would cause vibration.  It would severely limit RPMs.  You would have a  “Class D” engine.

 

The imperfect switching of the output transistors causes “ground bounce”, and “ringing” which adds noise. Further distortion is added by “dead time”.  “Dead time” is that nano-second something between when one output transistor turns off and the other one turns on (like a Class A-B amplifier).  This is commonly referred to as crossover distortion.

 

Finally, all the energy of the triangle waveform cannot be removed from the audio signal with a simple low-pass filter, and what remains is even more audible distortion.

 

As designed and manufactured today, Class-D amplifiers will never be able to approach the sound fidelity of a U.S. Amps T-Class™ amplifier.

 

If you wish to know more about the ever-changing and ever-improving nature of digital amplifier technology, visit http://www.usamps.com/.  Keep on the webmaster's rear end, he only gets better! 

 

Sincerely  -Jack