A few months later I was sent an amplifier to review for the magazine (Performance Auto and Sound). It was a USamps USA-700T. Another Class T amp. When I made some calls to the tech folks at USamps, I realised this was no marketing scam.
Further research revealed the amp was a variation on the Class D design. In a Class D design, the output devices are cycled fully on then off at a high enough rate to produce an 'average' output depending on how long the devices are on, compared to being off. This is called Pulse-Width-Modulation (PWM). It works very well, but not for high frequencies, as the music quickly approaches the switching frequency, and all sorts of harmonics and distortion is added to the music. So, these monstrous amps are relegated to a life of running massive subwoofers at outrageous power levels, with great efficiency.
Class T was invented by the engineers at Tripath Technologies. Their amplifier design uses a proprietary circuit to control not only the modulation rate, but also the modulating frequency of a Class D topology amplifier. This is indeed an interesting theory, but will need to be put to the test.
The 700T was compared to my friend Scott Ion's reference home amp, a large unit made by Aragon. We also gave my old gold-coloured first-generation Tube Driver 1500 a test. With the Aragon amp, we experienced a soundstage that was from speaker to speaker (8 feet apart) and extended about 3 feet behind the speakers. Instruments were evenly distributed across the soundstage, and front to back. Everything sounded very nice, and easy to listen to. When we moved to the Tube Driver, the sound wasn't as tight, and the stage became a bit less 'deep', shrinking to about 2 feet. Everything sounded very good though, and we were both pleased, if not slightly surprised by the performance of this 'car audio' amp.
Up last was the 700T. The stage depth dropped again to only a few inches. Also, there seemed to be gaps left of center, and right of center in the soundstage. Almost like listening to a poorly set up home theatre. Good strong left, center and right images, but very little in between. The amp wasn't quite a smooth as the others we listened to. A steel brush on a cymbal became fuzzy and sounded more like sandpaper on steel.
What was truly impressive though was how tight the amp sounded. Kick drums had incredible in-the-chest impact. It was noticeably tighter than the Aragon amp. Truly impressive! What stands out about this amp was it's control over the speaker, and the overall incredible efficiency of the amp! The application needs to be considered carefully. This is not for a set of components in a Sound Q car, but would be great on Bass, or in an SPL vehicle. If you are building a shop demo vehicle and want to drive an array of drivers on the exterior of the vehicle, this might be a good choice.
I have since auditioned two additional Class T amplifiers, one from Alpine and one from Blaupunkt. While the Alpine was the better sounding of those two by a long shot, I was still disappointed in their ability to handle high frequencies. This is a cool technology, and it has some great power saving applications, but I have yet to hear an example that would cause me to want to use it for one of my systems.