Ask S&V

Steve Skaff

I suspect we will indeed see more Class D amps exactly because they are far more efficient than other technologies. For those who aren't familiar with this type of amplifier, it's quite different than traditional power amps, which simply increase the amplitude of the analog input signal without changing its waveshape. A Class D amp converts the input signal into very high-frequency pulses of constant amplitude but varying width, a process called pulse-width modulation or PWM. The varying pulse width represents the input signal's amplitude and frequency. (PWM isn't the only technique used in Class D amps, but to explain the others here would be impractical.) The pulses are then replicated at a higher amplitude, filtered to recover the original waveshape, and output at a higher power than the input signal.

This process is very efficient—typically in the 90-percent range—whereas conventional power amps are normally less than 50-percent efficient, dissipating most of their power as heat. Thus, Class D amps can be built much smaller and lighter, and they run much cooler than conventional amps.

There are those who say that a Class D amp does sacrifice sound quality, which is true if it's not designed or implemented well. But recent advancements have overcome this problem. For example, the Pioneer Elite SC AVRs use Bang & Olufsen's ICEpower Class D technology, and they sound great to me and many other reviewers. Of course, it all depends on how the amp is designed and built, and manufacturers have less experience with Class D than other technologies, so it's certainly possible to find Class D amps that sound like crap.

Sony vs. LG
I need a 32-inch LCD TV, and I'm considering Sony's KDL-32XBR9 and LG's 32LH40. Do you have any pros or cons on either model? They are both 1080p.

Tom Mines

These are very comparable models in terms of features; for example, both offer 120Hz operation and are Energy Star 3.0 certified. The LG boasts 80,000:1 dynamic contrast ratio, while the Sony claims 40,000:1—not that I pay any attention to this particular spec, which is universally bogus. The LG is a bit less expensive at $950 compared with the Sony at $1100.

I suspect the Sony is a better performer, though I haven't seen either of the models you mention yet. If the LH40 uses the same type of LCD panel as LG's other models, the black level will be relatively high, though the viewing angle will likely be wider than the Sony's.

In the Long Run
I have a 25-foot HDMI cable running from my Onkyo SR-576 receiver to an LCD TV. However, it only delivers audio and no video to the TV. I was told by the manufacturer that the receiver's HDMI output is limited to 15 feet. I know there's no problem with the HDMI cable because using it to connect the output from my AT&T U-verse box directly to the TV works fine.

Is it possible to boost the HDMI output from the Onkyo receiver? If not, how do I find information about the output on other AV receivers in order to find a model that delivers a signal that can reach 25 feet?

Jim Fanucchi

Unfortunately, I know of no way to determine if a particular AVR will drive an HDMI cable of a given length. Twenty-five feet is generally too long for an HDMI connection to work reliably from many devices, though I have a 10-meter Ultralink HDMI cable that seems to work fine with most of my equipment. And your cable works with the AT&T box, so it's capable of conveying HDMI over that distance.

One solution is an HDMI booster or extender from companies like Gefen, Key Digital, and DVIGear, but they generally cost more than $100—sometimes a lot more. Most of these products use CAT5 or fiber-optic cable for very long runs. Another option is the PureLink HDC Fiber Optic HDMI Cable System, which uses fiber-optic cable to send HDMI up to 100 feet; the 33-foot length is $550.

If you have a home-theater question, please send it to scott.wilkinson@sorc.com.