Rotel and their class d amp technology

[SuBXeRo]

I was reading up on some rotel stuff and noticed that theur HT amps are all class d now as well as some of their stereo stuff. I thought it would ne interesting for you all to read so i copied the blip from their FAQ section about it. Also explains how class d works in comparison to an A/B amp.

Thoughts anyone?

How do class D amplifiers work?

A Class D amp works by taking the analog input signal and creating a PWM (pulse width modulation) replica of it—essentially a train of pulses, which correspond to the amplitude and frequency of the input signal. In its most basic form, a comparator circuit is used to match the input signal with the PWM signal. The PWM signal is then amplified by an output stage operating in switch mode, which is to say there are two states, on or off, at very high speed, corresponding to the PWM pulses. A linear amplifier’s output stages, by comparison, see a continuous waveform and, to avoid distortion, are on for more than half the waveform (Class A/B) or for the complete waveform (Class A), thus greatly reducing efficiency and generating heat. The amplified PWM waveform is low pass filtered to recover the audio waveform and eliminate spurious ultrasonic noise before outputting it to the speakers. This process seems digital but is in fact analog in nature. The signal is not “digitized”, i.e., assigned a numerical value; the PWM pulse train is an “analog” of the input audio signal. What distinguishes Rotel Class D amps from other designs on the market are innovations in the area of generating a highly accurate PWM signal (COM, which stands for Controlled Oscillation Modulation) and in the feedback circuits (MECC, Multivariable Enhanced Cascade Control) to provide a stable filter characteristic in spite of variable loudspeaker impedances. In simple terms, this means that our Class D amps offer full bandwidth performance at very low distortion in “real world” applications—just like our linear amps, but with the benefits of being smaller, cooler and much more energy efficient. Why aren’t there more Class D amps on the market? For starters, creating stable, full bandwidth Class D circuits while controlling RF/EMI byproducts isn’t easy. Few companies have the technological know-how to do it. It also requires extensive use of Surface Mount Devices (SMDs), again putting it out of technological grasp of most audio manufacturers. We have invested with a technology partner to realize these designs. Here’s another key detail that often causes confusion. The Switch Mode Power Supply (SMPS) is not what makes these “switching” amplifiers. As just described, the amplification stage is a high speed switching circuit and what defines this design as Class D. A Class D amp could, in fact, use a conventional power supply; and a linear amp could use a SMPS. A traditional power supply stores large amounts of energy, but wastes “excess” energy not demanded by the load in the process. An SMPS matches output to real-time requirements, supplying only the power required by the load, as a result operating very efficiently. The analogy is a water tank (linear supply), which is always being refilled and spills over if demand is insufficient; compared to an endless series of buckets (SMPS), which can be slowed down or sped up as required. The SMPS in our Class D amps reflects the fact that the Class D amplification circuit does not require the massive energy storage of a linear power amp, so the more efficient/compact SMPS is a better choice.

Why does Rotel use both Class A/B and Class D amplifiers?

Rotel has been designing, manufacturing, and refining class A/B amplifiers for over four decades now. Our analog designs have proven to be extremely reliable and have won many significant awards for their sonic excellence. Part of our reputation is due to the extraordinary amount of attention we devote to the power supply, especially the single most expensive component, the transformer. In order to ensure that this critical element meets or exceeds all of our specifications, we custom build and test all power transformers in-house. For some audio enthusiasts, analog power amplifiers remain the preferred choice because of their proven performance over several decades. Many of today’s home theater and multi-room audio applications however, require components that not only perform to the highest sonic expectations, but also fit into smaller enclosed spaces, such as custom cabinetry or equipment racks built into walls and closets. Electronic components in these close quarters generate a large amount of potentially harmful heat, most of which is generated by traditional analog amplifiers. In this situation, Class D amplifiers make the most sense. Whether or not you choose traditional analog or the new Class D amplifiers, you can be assured that all Rotel amplifiers will provide you with great sound and reliability for all your audio video needs.

[russ69]

Thoughts anyone?

Class D amps are getting better and some class D amps would be a good combo with most Klipsch products but so far, most of the high-end class D amps still fall just a hair short of the class A A/B designs. The area where they fall short is in top end extension and solid bass performance (mostly due to power supply).

[Quiet_Hollow]

....works by taking the analog input signal and creating a PWM

If you run an analog source okay, but this is a great way to make a digital source sound like junk..no matter how good one thinks it sounds. Avoid the A/D conversion of a digital source (ie. resampling) on the front side at all cost. It's an uneccessary step that adds audible distortion. The only digital amps that sound impossibly good are the one's that process PCM natively using IC's. Late model Panasonic, NAD M2's, TacT, Lyngdorf, Kenwood, Sherwood NewCastle, and most recently Pioneer.

In this particular context, Rotel is thumbing their nose at the consumer, essentially saying, "We could've made great class D amplifiers 15 years ago, but no one would've taken us seriously because our mavericks in sales brainwashed you so well on our legacy technology ...coincidentally, now that all of the free world is in a state of environmental fuss, our class D product can actually move off the shelves at our bloated price points."

...."or so the German's would have us believe."

[StephenM]

Here's what (little) I know about Rotel class D amplifiers:

http://www.hometheaterhifi.com/images/stories/amplifier_product_reviews/power_amplifier/2007-01-rotel-rb-1092-amplifier-thd-vs-frequency-20-volts-product-reviews.gif

http://www.hometheaterhifi.com/images/stories/amplifier_product_reviews/power_amplifier/2007-01-rotel-rb-1092-amplifier-thd-vs-frequency-30-50-63-volts-product-reviews.gif

Other pertinent reading material

http://www.audioholics.com/education/amplifier-technology/clone-amplifiers